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2
RELATIVE HUMIDITY IN ROCKY MOUNTAIN
FORESTS OF SOUTHERN ALBERTA
IN SUMMER
by
L. B. MacHattie
FOREST FIRE RESEARCH INSTITUTE
OTTAWA
INFORMATION REPORT FF-X-1
DEPARTMENT OF FORESTRY
MARCH, 1966
RELATIVE HUMIDITY IN ROCKY MOUNTAIN
FORESTS OF SOUTHERN ALBERTA
IN SUMMER
by
L. B. MacHattie
The Kananaskis valley, looking southward from 7000 ft. on
Barrier Mt. toward Mt. Kidd.
FOREST FIRE RESEARCH INSTITUTE
DEPARTMENT OF FORESTRY
OTTAWA, 1966
RELATIVE HUMIDITY IN ROCKY MOUNTAIN FORESTS OF
SOUTHERN ALBERTA IN SUMMER
by
1
L. B. MacHattie
ABSTRACT
The significance of relative humidity in forest fire control
arises from its dominant effect on equilibrium moisture content of
woody material. Daily extremes of relative humidity abstracted from
hygrograph charts for 23 stations in a 15 by 30 mile area were analyzed
to exhibit differences between stations due to elevation, forest cover,
surface aspect and curvature. Daily minimum humidity was found
remarkably independent of cover and contour; the increase with elevation
was slight, except on an exterior slope, (facing the prairies). Nightly
maximum humidity- was more variable, frequently decreasing abruptly with
elevation just above valley bottom. At susceptible sites, (i.e. where
low night humidities were most frequent), the maximum remained below 60%
one night in ten. A coefficient of -0.7 was found for the correlation of
daily minimum humidity with daily hours of sunshine.
1Meteorologist seconded to the Dept. of Forestry, Ottawa, from the
Meteorological Branch, Dept. of Transport.
2.
INTRODUCTION
This paper reviews the dependence of forest floor flammability
on relative humidity, and reports observations of daily extreme
humidities at 23 stations in the Kananaskis and adjacent valleys
during the summer of 1960. Previous experience in this area 1954-57
contributes to the interpretation of results. The investigation arose
from earlier work by Macleod (1948). The results are discussed in
relation to other relative humidity observations reported in the
literature.
Relative humidity is significant in forestry for its dominant
influence on the equilibrium moisture content of woody material,
(MacHattie 1955). The moisture content of woody material is the major
variable influencing its flammability, (Byram 1959, p. 76); variations
in moisture content also affect its size, shape and strength. The
release of conifer seed, by the flexing of cone scales, or of pollen
by the rupturing of ripened pollen sacs, results from relative humidity variations, (Sharp and Chisman 1961, Ebell and Schmidt 1964).
The relation of relative humidity to rate of evaporation from
wet soil or transpiration from foliage in an outdoor situation is not
so direct nor decisive. Evapotranspiration per unit of land area is
more directly related to factors affecting
(1) the rate of diffusion of water vapour away from
the wet surface, and
(2) the availability of energy for latent heat of
vaporization,
than it is to relative humidity (King 1961; Sutton 1960, p. 196).
Fire Control Aspects
The flammability of the litter (Anon. 1963) layer on the forest
floor is "nil" or "very low" when its moisture content is 25% or more,
and "high" or "extreme" when its moisture content is 10% or less,
(Gisborne 1928, Wright 1932, Wright and Beall 1945). Figure 1 shows
the relationship between wood moisture content and the atmospheric
relative humidity with which it is in equilibrium. (It is assumed that
adsorption properties of litter are similar to those of wood, though
this has yet to be determined in detail). Moisture content of 25%
corresponds to 92% relative humidity, moisture content of 10% to about
50% relative humidity.
In the normal diurnal cycle, relative humidity rises to 95% at
night and drops below 50% in early afternoon in many areas of the Rocky
3.
Mountains in summer. It is apparent that humidity variations do not
have to be abnormally large to have a profound effect on the flammability of the finer fuels -- those which approach equilibrium moisture
content within two hours after a change in humidity. If the humidity
characteristic of mid-afternoon is maintained through the following
night, not only the fine fuels but also somewhat larger sized fuel may
become highly flammable, (King and Linton 1963). Figure 2 shows by
example that such humidities occur in Canada. Schroeder (1960) shows
diurnal humidity patterns in the coastal mountains of California which
are more extreme.
Adsorbed moisture content of wood as a function of relative
humidity, based on data of Kelsey (1957) and Spalt (1957).
Instead of being drawn for constant temperature, as is usually
done, these curves have been drawn for constant dewpoint -since the normal diurnal variation is less for dewpoint than
for air temperature, (at a climatological station). Summer
season dewpoint rarely rises above 60°F or drops below 20°F
in Canadian forest land.
4.
In living trees and plants the moisture content is maintained
above 30%, usually above 100%, by life processes (Wright 1932, Eyram
1959, p. 76), and meteorological factors have relatively little effect.
In dead material the moisture content is largely controlled by
meteorological factors. It is the litter lying on the forest floor
that is the critical fuel in which forest fires usually start and
spread (Davis 1959, p. 164). After a rain the litter may be saturated,
its moisture content far above 25%. The number of days it takes for
the litter to dry to the fibre saturation point, (between 25% and 30%
moisture content), is thought to depend as much on the wind speed as on
the humidity. The ultimate extent to which the top surface of the
litter dries below 25% moisture content, during a spell of clear dry
weather, should depend mainly upon the ambient relative humidity, and
slightly on the ambient temperature. This applies primarily to shaded
litter. The surface temperature of sunlit litter may rise so much
above general air temperature that the litter is effectively exposed to
a much lower relative humidity than that measured in a Stevenson screen.
Equilibrium Moisture Content
Laboratory experiments have shown that the precise value of the
adsorbed2 moisture content of wood in equilibrium with a given temperature and humidity varies somewhat from species to species (Spalt
1957); the equilibrium moisture content also depends on whether the
equilibrium was reached following a desorbing, (drying), or an
adsorbing process. This hysteresis effect introduces an uncertainty of
± 1% or 2% moisture content, if one is using relative humidity as a basis
for estimating equilibrium wood moisture content. The wider the range
over which the relative humidity varies, the larger the hysteresis error
will be. The effect of temperature on equilibrium moisture content is
to decrease the value by approximately 1% moisture content for each
300F rise in temperature, relative humidity being kept constant, (Kelsey
1957).
The figures quoted above are for softwoods, and relative
humidites in the range 20% to 85%; hardwoods are not much different. As
different a form of cellulose as cotton has a similarly shaped adsorption curve, and a similar shift of this curve with temperature (Urquhart
1960, p. 18); the numerical values of adsorbed moisture for cotton are
about two thirds of those shown in Figure 1 for wood.
2 The former distinction between adsorb and absorb is disappearing,
(Urquhart 1960). Here adsorb is used where moisture content is less than
fibre saturation.
5.
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1 50
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Flour.?
Diurnal variations of relative-humidity and temperature at Headquarters,
Kananaskis F.E.S.:
A clear weather with light winds, July
4 - 8,
1960;
B windy period with variable cloudiness, Aug 15 — 19, 1960.
The average surface wind speed and prevailing direction are shown for
twelve—hour periods, beginning at 0800 and 2000 hours.
6.
Laboratory experiments indicate that the rate at which equilibrium is approached between the adsorbed moisture content of wood and the
relative humidity of its immediate environment should vary markedly
with temperature, doubling (approximately) for each 20°F rise in temperature (Crank, 1960 p. 86). For example, if other factors are not
limiting, a piece of wood that is moved from 90% relative humidity to
40% should lose as much moisture in 1 hour if the temperature is 100°F,
as it would in 4 hours if the temperature were 60°F, or in 8 hours if
the temperature were 40°F. Further investigation is needed to determine
the degree to which this temperature effect is significant in the
drying of actual litter layers in forests.
'Then water vapour is adsorbed, in addition to the 590 calories
liberated per gram of vapour condensed to liquid, there is heat of
adsorption released. The amount of heat of adsorption depends on the
current moisture content of the wood; it varies from about 200 cal/gm
at very low moisture content to zero at high moisture content near the
fibre saturation point. Considering this and the results of some
experiments with textiles (Crank 1960) one might expect the dissipation (or absorption) of heat to be a significant factor affecting rate
of change of moisture content. On the bases of observations and
theoretical analysis, King and Linton (1963) however conclude that
heat transfer is not a limiting factor.
Following this review of factors that condition the way relative humidity affects wood moisture content, some direct observations
of moisture content and concurrent weather are presented.
Litter Moisture Content
A sample series of midday observations of litter moisture
content is shown in Figure 3 for a mature lodgepole pine stand. The
related rainfall observations are also given. Moisture content was
determined by oven-drying samples of needles and twigs from the top
surface of the litter layer. Relative humidity was observed about 4
feet above ground level by sling psychrometer, and the equilibrium
moisture content was calculated, using Figure 1. It is apparent that
litter moisture content remained well above the equilibrium value for
several days after each appreciable rainfall.
When rains are frequent, relative humidity is of minor importance in the fire control field; during a droughty period the relative
humidity is of major importance; (Hayes 1941 p. 14 agrees).
7.
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Figure 3
Daily noontime observations of:
observed moisture content of forest litter (solid line),
equilibrium moisture content calculated from observed
relative humidity (dashed line); and
rainfall, total noon to noon, (T=trace).
Data are from an investigation by J. S. Mactavish in a mature lodgepole
pine stand at Whitecourt, Alberta, in 1955
8.
EXPERIMENTAL AREA
Location and Topography
The observational area was on and about the Kananaskis Forest
Experiment Station, which lies along the north-south Kananaskis River
valley just inside the eastern edge of the Rocky Mountains, in southwestern Alberta. It is 40 miles west of Calgary. The headquarters
site is
miles south of the Trans-Canada highway between Calgary and
Banff, at latitude 51° 02'N, longitude 115° 021 W, elevation 4560 feet
above sea level. A contour map of the Experiment Station and surrounding area is shown in Figure 4, and a detailed map of the area of
intensive observations is shown in Figure
The location of humidity
observation stations is shown on the maps. Panoramic views from station
17 are shown in Figure
6
5.
6.
As in other parts of the Rocky Mountains, the valleys here run
north-south in the main. The Bow River valley is the only one in this
area with significant east-west segments. Hence the Bow valley is the
main route through the mountains for rail and highway, and to some
extent for the lower layers of air. In the foothills region, (beginning
about 10 miles east of the Kananaskis valley), many creeks and rivers
run eastward to the prairies from their sources on the outer slope of
the Rocky Mountains.
Soils
The soils on the Kananaskis Forest Experiment Station have been
described by Crossley (1951). He comments:
"Since the uppermost beds of rock are chiefly of limestone, it follows that such rock will predominate,
even after glacial action, in the parent material of
the soils found in the region ....
"Although the soil textures are generally heavier than
one would anticipate in such rugged country, drainage
usually is quite adequate. There is little or no pan
development to inhibit water movement through the profile, and the rocky, porous nature of the glacial till
permits of maximum water infiltration".
9.
Forest Type
The area lies in the subalpine forest region (Rowe 1959). The
main species are: lodgepole pine, (Pinus contorta var. latifolia),
whose powers of prolific regeneration following fire have resulted in
it replacing spruce over large areas; Engelnann spruce (Picea
engelmanii) and the Engelmann-white spruce hybrid complex; and at
higher altitudes alpine fir (Abies lasiocarpa). Treeline is at about
7,000 feet elevation. In the—VaTey-bottoms there are occasional stands
of poplar (Populus tremuloides and Populus balsamifera).
Nacroclimate
The summer climate of the Kananaskis area is indicated by Table
1 (Anon. 1954-63). The average frost-free period at the headquarters
station is June 27 to August 24th -- 58 days (Boughner et al 1956).
Comparative winter temperature are given in Table 2, (Anon. 1954-63).
Of incidental interest is the fact that the heaviest snowfall on record
at Kananaskis did not occur in winter, but in June (1951) when 33
inches fell in two days.
10
Contour map of the Kananaskis area showing the location of observation stations,
(identified by serial numbers corresponding to those in Table 3).
11
115°12'30"
50°59'30
115°05'
50°59'30"
--- MT. LORETTE
A
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MT.
2
McDOUGALL
5.
10
ti,
6500
11
12
14
15
6500
50°53'
115°05'
50°53'
115°12'30"
Scale in
0
Miles
2
Figure 5
Detail of the Kananaskis cross—section area -- station numbers are
the same as in Table
3.
Figure 6. Panoramic views of the Kananaskis cross-section area,
taken from Esker station (No. 17) at 1250 July 6, 1960. Upper
photo looking north; center photo looking southwest; lower photo
looking southeast.
13.
TABLE I
Climatic data for ten summers 1954-63 at Kananaskis
Forest 7xperiment Station, Headquarters site. Data
are in inches and degrees Fahrenheit.
Eonthly precipitation
Hay
Jun
Jul Aug Sep
highest 4.8
mean
2.6
lowest 0.7
9.0
3.3
1.1
6.2 6.6 3.8
2.9 2.8 1.9
0.7 0.7 1.0
Pionthly precipitation
1960
1.4
3.0
0.7 3.3 1.0
Potential evapo-transpiration A
mean
2.4
3.7
4.7 3.7 2.5
Snowfall
mean
10
T
T
T
4
highest 16
mean
11
lowest
4
20
12
7
17
10
7_7
11
5
5
5
12
8
3
10
6
No. of days with precipitation
No. of days with precipitation
1960
14
9
Monthly mean of daily max temperatures
highest 66
74
78
75
mean
70
57
66
72
69
61
Monthly mean of daily min temperatures
mean
lowest
32
40
44
41
36
30
39
41
36
32
82
-7
87
27
90
30
89
29
86
19
Extreme maximum temperature
Extreme minimum temperature
A Calculated with the nomogram of van Hylckama (1959) for Thornthwaite's
formula.
TABLE 2
January mean temperatures and their standard deviations
(in parenthesis) for the period 1954-63, OF.
Kananaskis
Calgary Ottawa
Monthly mean daily maximum temperature
27 (8)
24 (10)
19 (3)
Monthly mean daily minimum temperature
Monthly mean diurnal range
4 (8)
4 (9)
4 (4)
23
20
15
14.
INSTRUMENTS AND METHODS
The basic humidity data were abstracted from the charts of
23 hygrographs. Actually the instruments were hygrothermographs, but
only the hygrograph data are reported in this paper. To minimize the
uneasiness one must feel over relying on hair hygrograph data (MacHattie
1958), several checks were made on the perfomance of these instruments.
Calibration of Hygrographs
At the beginning of the field season all the hygrographs were
placed in a closed room together, with a fan operating to keep the air
homogeneous. Check readings with a ventilated psychrometer ensured that
the hygrographs were reading apnroximately correctly. The main objective
was to adjust the hygrographs so that they agreed with each other. The
difference in lag coefficient between psychrometer and hygrograph made
close checking between the two types of instruments difficult to do in a
significant way. The calibration period lasted about two weeks.
Throughout the summer, spot checks of psychrometer against
hygrograph were made at least once a week at each station.
At the end of the field season all hygrographs (except the
one at headquarters) were brought together and exposed under homogenous
conditions for 72 hours to humidities varying from 255 to 95 and temperatures from 45° to 75°F. All instruments were read at 19 check times
during this period. Comparison of individual readings with the group
average gave the data shown in Figure 7.
It should be explained that two types of hygrographs were used;
15 were of type A, which has a cast-iron frame and couples the recording
pen to the end of the hair bundle; 8 were of type B with aluminum frame
and the pen coupled to the midpoint of the hairs.
A significant difference was found between the readings of type
A and type B instruments. Deviations from group average shown in Figure
7 are actually deviations of individual type A instruments from the average of the 6 most consistent of the type A instruments, and deviations
of individual type B's from the average of all type Bts. The average
relation between the two groups is shown in Figure 8. In the data given
hereafter, the type B readings have been converted, (for convenience),
to the type A scale on the basis of Figure 8. It is noted that the
7
15.
specific relationship shown in Figure 8 relates to the humidity
conditions under which the instruments were calibrated. If the
calibration had been performed at other humidites the shape of the
curve would have been the same, but the cross-over points different.
To help indicate (and permit
compensation for) residual errors
in calibration and operation:
hygrographs for certain pairs of
stations were interchanged partway
through the season. Footnotes to
Table 3 specify the stations and
dates of interchange.
Exposure of Hygrographs
All hygrographs were exposed
in Stevenson screens, (see Figure
21), approximately 4.5 feet above
ground surface. (Exposure on the
ground surface would have led to
difficulties of interpretation,
considering the size of the instruments, and would have restricted the representativeness of the
data to a smaller area around
each instrument).
Observational n=etwork
Ti/hen investigating the humidity regime of a mountainous area
there are many factors one wishes
to determine: the effects of
altitude, relative elevation,
aspect, surface curvature, shape
of valley, direction of valley,
type of vegetative cover, proximity
to water areas, and the seasonal
changes in these effects, to name
some of the main ones. Many more
than 23 instruments would be
required to observe all these
factors simultaneously.
.12 1 .9 1.6
3
0'.3 1 .6
lr+12 t
Deviation (%R.H.)
Histogram of deviations of individual hygrographs from the group
average. The 22 hygrographs were
read at 12 check times during the
September comparison in the laboratory. Standard deviation was
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17.
The average of 7 type B hygrographs is plotted against the
average of 6 type A hygrogrgphs for 19 check times during the
September comparison. A 45 line is shown for comparison.
My observational plan was to space hygrographs fairly
closely on one cross-section of the Kananaskis valley to measure
variations within the valley, and to set up other instruments to
measure variations from valley to valley. The latter instruments were
located on small hillocks or benches somewhat above the valley floor.
Experience in previous years had shown that the relative humidity
exceeded 90% almost every night on the valley floor, whereas 100 feet
or more above the valley floor the nocturnal maximum humidity frequently
was much lower. These lower humidities are of particular interest in
relation to fire control. Cross-valley views of the cross-section
area are shown in Figures 9 and 10.
Descriptions of the various hygrograph stations are given in
Table 3; pictures of several are shown in Figures 13 - 23. The area
in which stations 8 - 15 were situated was burnt over in 1936. All but
two of these stations were located within stands of young lodgepole
pine which grew up after the fire.
18.
Values Tabulated
The daily extreme humidities were selected as being most significant. The values tabulated were:
Minimum -- the average relative humidity over the two
hours between 0800 and 2000 when it was
lowest;
iaxirrum -- the average relative humidity over the two
hours between 2000 and 0800 when it was
highest.
The two hour period was not necessarily continuous; it was sometimes made up of two or more segments separated by periods of less
extreme humidity. fountain Standard Time is used in this report; 0800
HST is equivalent to 0720 local mean time.
RESULTS
An overall view of humidity occurrence June 25th to September
7th, 1960, is given in Tables 4 and 5, where daily extremes of humidity are shown for 23 stations. The parallel occurrence at many stations of low values on some days and high values on other days is
evident; the cause must be related to large scale weather factors,
rather than local influences. These large scale weather factors will
be examined in another report. The present report is concerned with
what humidities occur, and what differences between stations were
found. They are presented first for daily minimum humidity.
Daily Ilinimum Relative Humidity
The frequencies of occurrence of the different daily minimum
humidities at individual stations are shown in Figure 11 for sixteen
of the stations. Tote that humidities of 20% or less occurred at all
except one of the stations.
The effect of altitude is not so large as to be immediately
Figure 9. The west side of the Kananaskis valley
in the cross-section area. Locations of Meadow,
Allan 50 and Marmot stations are shown.
Figure 10. Looking east across the Kananaskis
valley from 6500 ft. elevation in the Marmot
Creek basin, showing Mt. McDougall and McDougall
stations 48, 50 and 56.
Photo by J.S. Rowe.
P.20
O
28
25
36
26
38
27
34
28
29
59
30
25
JULY
1
240
64
3
37
4
30
22
5
6
24
19
7
8
20
25 25
9
31
44
10
23
18
49
11
51
49
48
12
13
25 26
27
26 20 18 18
14
15
28 21 39 38
16
30 22 18 20
17
26 18 15 16
18
26 20 18 18
19
20
28 23
20
40 48
52
44
21
57 55
22
21
13 11
65
59 49
23
35
27 32
24
22 24
25
27
16 17
26
22
18 21
27
26
28
29
22 22
39 32
29
27
30
30 23 20 19
31
62 64 61 56
AUGUST
1
58 52 55 48
2
60 82 64 51
3
65 68 53 51
4
82 93 97 78
5
53 60 59 45
6
55 59 57 52
7
48 49 so 45
8
37 38 40 37
9
35 36 38 32
28 26 28 25
10
33 32 33 30
11
40 38 38 34
12
13
33 — 40 32
44 — 57 59
14
37 36 39 36
15
51 47 39 38
16
17
40 38 34 35
18
31 26 26 28
19
50 63 63 54
32 30 37 34
20
21
23 20 25 19
44 37 35 32
22
23
53 48 40 42
43 43 33 41
24
53 45 35 36
25
26
56 54 58 54
42 39 35 34
27
49 43 38 36
28
54 47 45 38
29
30
32 31
57 45
31
SEPTEMBER 1
43 47
2
57 51
JUNE
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54 59 66
24
24
28
28
28
22
21
20
28
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52 50 48
so 46 48 42
46
43
48
46
47
42
23
23
26
26
49
42
22
22
24
23
58
32
22
49 56 45 46 50
so 55 35 40 42
45 so 29 31 31
25 28 37 42 25 29 29
24 30 36 37 17 28 27
31 25 32 36 27 36 37
30 22 49 52 22 32 32
32 24 53 56 25 34 35
25 22 44 so 23 30 30
56
57
51
50
36
23
44
57
52
36
22
52
32
27 20 21 24 23 24 20 21 26 27
52
LA
ia
57
55
so
LO
44 24 28 30
40 48 52 53
44 46 54 53
52 35 40 43
82 35 36 38
37 31 36 39
40
43 36 39
44 38 40
36 42 48
40 54 69
68
22
26
Le,
0
0
M
31 26
40 40
39 39
34 32
41 40
32 32
23 3 22
25 24 21
26 29 3
25 27 21
30 27 28 25 V 29 24
27 23 23 26 24 23 20
29
3 al
w
48 47 44 46 41 46 44 41 42
36 38 37 36 40 35 33 30 32 44
35 33 36 32 33 29 32 28 33 37
28 28 24
25 27 23
30 26
29 26
38
56
41
29
59
56
42
25
48
35
24
26
25 26 26 22
26
28
29 23
32 25
28 22
21 27 24
20 25 22
27 31 37
25
26
22
25
25
23
23
20
21
21
22
24
21
22
24
28
26 28
29 32 20
26 28 18 26
28 30 22 26
28 30 20 45
25 25 21
25 27 23
22 24 20
24 26 21
27 27 23
52
52
47
41 47 40 38 30
45 53 46 42 39
73
60
81
66
53
49
68
54
53
96
88
65
46
46
90
90
68
58
68
58
52
32
41 48
31 46
45 48
so
so
60
59
18 24 21
35 37 39
48 21 36 36
27 24 32 33
52 23 32 32
39 26 35 36
32 21 32 33
29 21 32 32
56 21 32 32
64 27 32 33
63 24 28 28
17 21 20 20 19 18 18 18 17 14 15 19 20 13 18 19 20 26 25
48
48
56
61
61
46
54
21 28 28
63 64 59 56 51 49 68 52 52
31 40 35 34 31 30 31 32 31 27 30 36 36 25 25 32 38 41 44
24 27 24 25 24 n 22 24 23 19 21 27 28 18 22 24 25 30 30
20 25 21 23 21 21 18 21 22 17 19 24 25 17 19 21 22 29 29
26 28 24 27 24 24 22 25 26 20 22 28 29 21 26 32 25 28 28
22 30 28 25 25 26 22 V 22 28 24 34 36 24 28 32 29 36 38
44 32 28 26 25 25 23 25 20 27 23 29 31 36 37 48 26 32 33
28 34 31 29 24 25 21 25 20 28 22 28 32 23 32 38 V 35 36
57
57
61
75
52
87
74
77
59
73
60
90
53
95
93
85
57
95
88
57
54
70
62
74
57
46
41
41
37
36
40
39
39
33
31
44
34
51 48
48 43
42 56
40 48
33 41
60
56
52
40 35
41 35
32 38
27 32 26
41 51 41
35 38 32
49 54 so
38 40 36
36
37
30
60
35
35
38
63
54
64 54 67
52 48 58
57
52
80
54 48 57
56 51 60
88 85 89
47 50 53
64
56
26
26 28
56
59
40
35
43 39
38 32
41 35
32 29
50 40
36 36
50 42
39 37
32 33
54
50
54
50
64
56
62
30
31 27 27 28
44
39
58
52
52
53
42
46
67
64
60
61
53
56
50
54
54
53
65
46
48
84
54
61
52
93
81
78
51
50
62
53
58
52
56
47
39
38
49
42 39
46 40
53
67
36
48
37
43
36 40
46 52
34 41
44 47
20 16
36
47
39
44
36
44
40
47
64
62
53
53
61
60 54 59
56
57
58
60
56
47
44
47
49
41
43
44 ,
47
49
41
43
41
45
44
40
39
40
43
43
39
40
39
4441 41
47 42
39 40
43 37
34
40
37
34
30
33
40
44
32
52
46
36
32
42
44
42
36
43
45
48
47
40
46
52
82
50
58
45
64
45
48
56
58
53
54
57
54 52 53
58
55
47 42 38
36 30 32
58 59 58
68 58 45
42 38 40
37
42
44
47 43 39
42
30
61
70
64
68
26 25 24 24 24 28
24 21
49 so
38 32
36 34
50 48
42
37
49
37
36
47 49 53 57
36 36 37 43
36 35 35 37
59
54
54
53
82
56
60
51
90
85
67
79
52
90
55
54
70
61
63
69
55
59
50
80
69 48 48 62
67 46 47 56
48 59 50 48 58
43 48 31 33 38
39 42 30 31 36
38 41 22 26 32
41 44 21 28 30
48 52 35 44 45
40 44 27 32 37
52 60 44 52 60
41 47 30 36 39
43 47 —
63 59
36 40 50 52 47
32 36 40 40 39
55 72 — 36 32
51 56 — 25 32
25
31 — 26 27
17
34 40 48 43 47 48
40 40 44 65 62 66
36 39 43 37 36 36
36 35 36 53 52 57
63
57
36 55
40 52
40 80
36 48
42 48
62
68
61
89
57
81
50
40
34
25 28 28 25 28 26 32
28 28 32 30 33 30 33
36 34 38 35 38 36 41
29 30 36 29 28 28 30
40 46 49 43 42 42 40
34 34 37 31 33 36 41
44 47 so 46 48 44 38
36 35 40 34 40 40 36
32 31 33 35 36 33 33
49 52 51
52 51 56
25 23 41
26 27 29
26 27 24 24 22 20 21 22
34
48
45
49
65
48
53 48 54
49 46 43 48 42 47 45
36 36 33 40 31 30 32
38 36 36 40 34 36 39
56 34
31 29
35 44
42 58
40 49
37 54
21 18 21
51
La_
Z
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33 29 29
45 42 42
45 42 43
43 37 37
52 45 48
42 35 34
4
5
6
7
44
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38
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51
3
27
26
I—
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0
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0
0
McDOUGALL 50
z
0 0
z
McDOUGALL 48
0
DATE
HEADQUARTERS
TABLE 4 — Daily Minimum Humidity (%) — Average Of The Lowest Two Hours 0800-2000
50 52 56
65
51 52 53
40 45 44 45
43 59 54 56
49 50 51 46
37 57 64 54
29 36 42 41 41
50 51 61 41 41 42
26 52 63 36 34 40
17 18 22 26 30 31
44 40 46 64 58 67
30 25 34 54 52 61
29 28 33
P.21
JUNE
JULY
25
26
27
28
29
30
1
9
10
11
AUGUST
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
SEPTEMBER 1
2
2
4
5
6
7
79
64
81
87
94 100
88
89
89
91
93
92
91
91
96
98
96
95
95
90
94
96
97
90
85
94
94
86
81
90
95
91
93
65
94
67
61 89
77 90 88 94
74 92 96 90
75 94 100 94
68 85 57 82
70 76
81
94 92
96 95
97 100
87
75
73
69
49
0
WI
2
<
J
J
<
u_
0
..o
100 100 100 100
86 92 77 70
82 89 78 79
82 89 80 87
100 100 99
95
100 100 99
93
100 100 99
95
100 100 100
95
89 100 93
90
87 90 98 93
97 100 100 95
91 98 100
91
98 98 100 93
97 100 97
77
73
95
94
89
95
86
81
95
96
90
53
57
93 100
99 92 95 100 100 93 100
95 100 100 95 100 100 100 97 100
69 100 80 56 51 61 69 55 93
98 100 100 98100 100 100 100
96
64 too 78 57 54 55 61 51 97
91
98
91
94
83 100
93 100
97 78
93 100
93 100
53 44 48
83
86 100 100
54 ao 78
92 100100
70 75
94 100
83 88
59 84
65 83
98 90
96 100
96 100
94 91
96
62
99
100
93
93
90
80
100 98
100 99
100 99
100 100
100 97
95
83
95
91
80
88
66 79 73
72 67 58
68 65 58
88 100 94
96 100 100
92 100 99
81 84 74
87 100
86 94
97
93
94 100 100 100 100
42 85 72 59 51
49 100 92 100 78
92 91
100 93
100 93
100 90
95 98
98 96
97 100
96 85
69 85
100 91
78
97
29 37
97
(14
93
el
98
100
100
100
100
95 100
99 100
96
96
97
95
98
92
95
89
86 100 97
92 100 100
77
78 78 60
61 49 44 36
72 52 53 44
63 58 77 100
86
96
96
73 100
73 100
74 73 70 68
63 68 69 63 78 68 48 62 58
76 66 57 44 58 88 70 83 73
88 97 100 94
89 96 82 58
85
96
84 100
85 100
83 100
100 95
100 99
100 100
100 99
97 99 99 81 59
57 57 59 78 as 96
68 65 56 97 93 96
98 100
98 100
86 100
75
77 66
60 75 68
90
96
94
95
95
0
-I 0
▪
J
98
95
91
72 100
89 100
95 100
96 97
95 98
94 96
94 95
97 98
95 100
94 98
90 92
0
0
99
98
96
97
91
95
91
88
86
77 es 67 78 63
68 72 57 72 56
49 79 62 59 43 50 44
98 93 97 86 78 95 82
87 75 75 83 76 89 80
100 93 83 74 77 97 88
95 75 60 80 80 100 86
0
0
M
_I 1_1 M
0
97
95
96
97
95
97
72 67
96
98
03
-J
W
tn
100
100
100
100
100
100
90
82
vl
W
L11
98
98
98
97
89
60
59
le
0
98
98
98
94
94
100 100
100 100
100 96
97 84
89
cc
w
69 76 61 70 66 60
85 83 60 80 74 62
94 93 74 90 86 77
91
96
93
98 100 90
97 100 100 89
95 100 100 94
97 100 100 90
96
95 94 79
97 100 100 89
98
94
97
97
98 93
67 56 56 45
65 48 48 44
97
75 100
77 98
68 95
66
86 96 99 89
72 94 93 89
84 78 98 90
90 100 100 89
68
100 82
97
100 94
64 67 90 87
97 100 100 92 100
75
78 72
96 75
87 75
87
82
0
0
2
86
91
78
■
-■
U0
M
97
90 100
77 96
90 82
93 94
89 54 100 100
93
91
90
89
94 95 100 86
93 95 100 89
96 96 99 90
95 98 100
91
91 97 100
90
<
Ill
M
100 100 100
92 100 95
65 82 76
93
98
95 100
90
97
81
80
85
88
95 99 100
96 97 100
95 95 100
95 97 100
0
ELK HOLLOW
98
“I
Ul
Z Z
2
«<
_i w J
J -.1 J
< <
<
McDOUGALL 56
91
M
Mc DOUGA LL 50
cc
<
M
I—
0
55 52
44 50
2
3
4
5
6
7
8
co
CO
71
0
J
McDOUGA LL 48
DATE
HEADQUARTERS
TABLE 5 — Nightly Maximum Humidity (%) —Average of the highest two hours 2000-0800, recorded opposite the latter date.
74 100
70 100
58
85
87
91
88
91
96
95
93
92
95
95
95
93
83
100
100
100
100
100
100
100
100
100
100
100
100
100
89 100
79 100
100
100
100
100
97
100
95 93 80 100
84 82 62 83
75 89 53 77
99 97 87
99 100 100
98 94 87
98 97 98
98 98 93
98 90 75
99 95
99 73
96 68
94 70
91 81
77
90
77
82 92 81 88 82 81
83
62
86 88 75 75 83 70
70
54
100 100 94 100 100 100 100 100 95 98 94 99
99 99 91 100 96 95 100 100 100 98 96 100
99 100 91 100 96 96
93 95 98 100 100 98
94 91 76 96 86 78
98 93 93 99 89 86
78 72 56 73 85 65 93 70 56 95 77 64
82 78 63 84 93 81
95 86 79 98 67 52
91 86 70 85 89 72 100 83 91 99 70 51
96 82 63 85 94 81
96 93 82 100 75 58
77 74 53 89 90 74 81 57 56 98 63 46
76 69 51 72 82 64 az 75 90 93 58 46
96 100 88 100 98 98 100 99 100 98 100 100
100 100 93 100 100 100 100 98 97 97 97 100
96 95 83 95 95 85 100 95 94 94 72 43
90 97 88 96 86 82
95 95 99 88 87 67
52 53 45 50 77 48 44 98 89 93 81 74
86 76 57 77 82 67 78 72 49 100 83 77
78 69 48 73 82 69 89 70 56 97 63 49
64 57 36 59 63 55 87 60 42 88 72 44
87 87
80 81 77
96 89 81 93 85 86
90 83 95 91 95 89
93 96 91 97 94 100
88 75 83 91 98 85 100 86 81 95 68 56
61 77 78 90 82 75 86 60 72 94 94 90
95 83 88 100 97 94
96 97 93 98 97 100
100 95 100 100 99 99
98 99 98 98 98 98
100 95 100 100 97 97 100 98 96 98 96 99
100 95 100 100 96 97 100 96 96 99 96 95
100 94 100 100 93 96
98 96 96 95 95 92
100 95 100 100 95 95 100 97 97 99 97 100
100 96 100 100 95 97 100 97 98 92 94 99
99 93 97 100 97 95 100 96 97 99 100 93
97 90 94 100 96 90
99 93 95 99 98 84
94 81 83 90 96 81
97 89 91 98 85 47
82 74 82 81 93 74
81 81 70 97 68 44
78 57
98
90
98 93 88
98 90 83
88 60 67
98 100 100 100
89 100 83 89
100 87 86
90 67 73
97 100 81 60
96 100 100 100
93 99 89 77
97 94 85 86
67 91 72 77
55 52 54 58 59 56 57 50 56 73 60
54 50 51 54 57 54 51 44 52 87 60
89 100
97 91 97 100 89
98 100 100 97 100
100
97 100 100 96 100
99
73 so 70 76 75
87
59 86 66 50 54
48
72 80 67 70 73
89 100
90 77 86
88
88
84 83 90
98 100 100 98 100
48 46 46 51 53
66 65
61 64 69
58 55 61 59 63
93 100 100
99 100, 97
98 100 97
65 85 97
46 61 83
55 70 54
72 67
85 77
89 79
100
99 100
77 70
97 100
99
100 90 100
82 100 96 100
65 100 98 100
97 98 100 100
48 40 46 48 52 74 54 66
72 52 56 77 61 88 81 93
60 52 60 91 63 90 81 83
92 93 72
89 100 92
78 95 92
95 100 100
75 98
85 80
95 100 100
92 100 100 93 94
93 100
97 84 85
95 100 100 100 98
94 100 100 74 95
78 1 oa
85 64 73
93
97 98 95
98
98
86
99
67 55 46
42 40 41 48 46
42 36 40 39 44 58 52 64
88 100
92 100
95
98 98 93
96
97
95
96 100
83
98
98 100
97 100
70 81
98 100
95
95
99 97 100 97 99
98 100 100 75 77
97
96
98 99 99 93
97 100 78 59
60 53 99 70 90
97 100 100 98 100
97 100
22.
apparent. The shape of the histograms is similar for Allan 50 and 60,
for Ilk 61 and Lookout 68; humidities are however slightly higher at
the upper levels. Numerical data for the mean variation of humidity
with elevation arc Liven later in Table 7.
It is remarkable in many of these histograms that the mode,
(the class with the greatest frequency), is so close to the lowest
humidity class. The Moose histograms are exceptional in showing a
fairly symmetrical distribution. A suggested. explanation for this is
given on page 49.
Differences Between Stations in Daily Minimum Humidity
For a two-month period June 17 to August 18, there were five
stations operating in the low zone of the valley cross-section
(elevations 4700-5000 ft). All five stations used type A instruments.
Comparison of these stations indicates the uniformity of conditions
in ear13, afternoon.
Figure 12 shows histograms of the amount the individual
stations differed in daily minimum humidity from the average of the
five stations. Deviations of these five instruments during the laboratory calibration in September are also shown for comparison. It
is seen that the deviations in the laboratory (a = l.910), were almost
as large as at the field stations, (r = 2.50).
One may conclude that daily minimum humidities were the same
at all five stations in the low zone of the valley to within the
limits of accuracy of hygrograph measurement.
The contrast in surroundings of Meadow, McDougall 50 and
Moraine is shown in Figures 14 - 16, (and in Table 3).
None of the above five stations was in a mature stand. The
only nearby low level station with an overhead crown canopy was Rock
F (Figures 19 and 22). Daily minimum humidities at Rock F are
compared in Figure 17 with:
(1) Meadow, to show the difference between open
field and forest stations on the relatively
flat valley floor; and
(2) Esker, to show the difference between inside
the forest and just above it.
23.
20
Allan 60
20
McD.56
20
15
15
15
10
10
10
5
Lookout
68(58 d
5
0
10 20 30 40 50 60 70 80 90
20
0
10 20 30 40 50 60 70 80 90
20
Allan 55
15
0
15
10
10
5
5
0
0
0
20
Allan 50
10
10 20 30 40
0 60
s 8s 90
20
McD.48
10 0 30
20
15
15
15
10
10
10
5
5
5
0
0
0 20 30 40 50 60 70 80 90
20
8 90
20
McD.50
15
10 20 30 40 5' 60 0 8s 90
10 20 30 40 50 60 7
Moose 56
0
10 20 30 40 50 60 70 80 90
10 20 30 40 50 60 0 80 90
20
20
15
15
15
10
10
10
Esker 50
Moose 51
5
0
0
0
10 20 30 40 50 60 70 80 90
10 20 30 40 50 60 70 8' 90
20
20
15
15
15
a 10
10
10
5
5
5
20
Rock F 50
a
Elbow 53
0
0
10 20 30 40 50 60 70 80 90 100
0
10 20 30 40 50 60 70 80 90
10 20 30 40 50 60 70 80 90
Humidity(y)
20
Sibbald 49
15
10
Figure 11
0
10 20 30 40 50 60 70 80 90
Histograms of daily minimum relative humidity June 25 to September
5.
24.
40 —
40
30
30
20
20
10
0
10
01
-41
6
t_1
r16
t 01
16
61
1 1
0 6
-12
'0
6'
6 101 61
Deviation (%R.H.)
Allan 50
Meadow
McD 48
McD.50
Moraine
Lab.
Figure 12
Deviations in daily minimum humidity at individual
stations from the average of the five stations June
17 - August 15. Deviations of the same hygrographs
from the group average while together in the laboratory are also shown for comparison.
Esker station was located on top of an esker, (covered with sparse
grasses and related low vegetation), at the same level as the top of
the 60 ft lodgepole pine stand which bordered it on the south (Figure
6). Rock F was situated in the densest part of that 60 ft stand.
To the north of Esker 15 ft pine made up most of the forest cover.
It is seen that there is no appreciable difference in mean daily
minimum humidity between Meadow, Esker and Rock F.
Also in Figure 17, three upper slope stations are
compared. Allan F - Allan 60 represents the difference between mature
forest and a young stand that does not shade the Stevenson screen
appreciably; these stations are close together on the same slope.
Marmot-Allan F represents a difference due to ground curvature -Marmot being in a concave area close to Marmot Creek. The humidity
difference between the mature forest stations Marmot-Allan F shows
less variability than the difference between either of these and
Allan 60.
25.
Humidity variations related to larger topographic features
are shown in Figure 18. Using the average of the five low level
stations as a reference, histograms of daily differences in minimum
humidity of other stations from this reference are shown for:
(1) Upper slope stations of the Kananaskis cross-section;
(2) Low level stations in other parts of the Kananaskis valley;
(3) Low level stations in other valleys than the Kananaskis.
Although the mean differences show no consistent relation to
distance, the standard deviation of the differences generally
increases with increasing distance between the stations being
compared. Figure 20 shows that the standard deviation increases
roughly 1% RH per 4 miles of distance between stations.
Effect of Rain on Humidity Differences Between Stations
One might expect the larger inter-station differences in
daily minimum relative humidity to occur on rainy days when, for
instance, different amounts of rain may fall at different stations,
or rain occur at one station and not at another.
In Figure 18, days when rain fell, (at one or more of the
stations being compared), are distinguished from non-rain days.
It is seen that the larger differences between upper level and low
level stations on the Kananaskis cross-section occurred on rainy
days. For stations in different valleys the association of larger
differences with rain is less pronounced but still apparent. For
the three stations elsewhere in the Kananaskis valley there is
little difference between rain days and non-rain days. Data given
below suggest that where there are larger differences on rain days,
these may be not so much due to rain as to differences in sunshine
at the stations being compared.
Direct Effect of Rain on Humidity
As a direct measure of the effect of rain on the humidity at
an individual station, the data for Figure 2L were obtained. Using
all stations where recording rain gauges were operating, (as well as
hygrographs), days were selected on which rain began between 1000
Figure 13. Headquarters station (No. 4) looking westward.
Figure 14. Meadow station (No. 11) looking north.
I
Figure 15. McDougall 50 station (No. 13) looking
south.
I
Figure 16. Moraine station (No. 15) looking north.
Guy wires are attached to the anemometer mast at 10,
15 and 25 ft.
28.
12
2 -6 0 6 12
Rock F-Meodow
Between
Period June 25-Sept. 5
Mean Difference 0%
Standard Deviation 5 %
Figure 17
0
-6 0 6
-6 0 6 12 18 24
.6 0 6 12
Difference n Doily Minimum R.H.
Marmot-Allan
F. Marmot-Allan 60
Allan F -Allan 60
Rock F-Esker
July 23-Sept. 5
July 23-Sept. 5
July 23-Sept. S
June 25-Sept. 5
3%
3%
0%
0%
6%
4%
6%
4%
Histograms of differences between stations in daily minimum humidity.
and 1600 hours. The amount of humidity increase due to rain, was
taken to be the difference between:
(1) The maximum humidity between the time rain began and 1700
hours, and
(2) the minimum humidity between 0800 hours and the time
rain began.
Figure 21i, is a histogram showing the frequencies with
which various humidity increases occurred. The average amount of
increase is seen to be about 40 percentage points.
To indicate to what extent this increase was limited
by saturation, the cases where the humidity rose to 90% or higher are
distinguished from the others in Figure 21i.. It is seen that the
humidity rise was limited by approach to saturation only in cases
where the rise was greater than 30 percentage points; the humidity
did not rise as high as 90% RH in half of the cases.
Besides the immediate effect of rain on humidity, a
more persistent effect due to wetting of the soil and subsequent
increased evapotranspiration might be expected. To determine how
much this affected daily minimum humidity, the period June 26 - Sep
5 was divided into periods which were either predominantly sunny or
predominantly rainy. (Figure 25 shows the distribution of rain during
the summer). Days following heavy rains were included in the rainy
periods. Days when rain did not begin till late afternoon (e.g.
August 19) were included in the sunny periods. Table 6 shows the
successive sunny and rainy periods and gives mean daily minimum
29.
30
m=0
20
r
20
-4
10
r
-2
4
10
12 -6 0 6 12
2 6 0 6 12
Allan 55
McD 56
m 1
20
0-
m 1
c= 4
20
6
6 0 6 12 18
Allan 60
30
rn - 3
v g
20
0
10
4
10
MTh
6 0 6 12 18
H0
-6
10
0
6
Esker
20
m-3
7
12
4
8
-12
-6
Elk
0 6 12 18
Hill
m=8
v.,2
10
11111
111111
11111
NV
6 0 6 12 8
an
6 0 6 12 18 2 4
Pigeon 46
Sibbald
20
rrt- 7
v= 7
=9
10
,X0t9
. X4‘NX%
Cia 1■V
12 -6 0 6 12 18
Elbow
_
-18
12
Vtialelai0
6 0 6 12 8
Moose 51
Deviation (°/.R.F1.1
Figure 18
Deviations in daily minimum humidity from the average of the five low
level stations are shown for several stations. The observational period
used began June 17th for all stations, except
June 18 for Sibbald and Elbow,
June 23 for Moose 51,
June 25 for Elk Hill, and
July 7 for Pigeon 46;
it ended August 18th for all stations. Days when rain occurred between
0800 and 1800 are distinguished by hatching. Means (m) and standard
deviations (or) are given, in % R.H.
Figure 19. Crown canopy seen with 60° wide angle lens looking
up from the Stevenson screen at: (a) Rock F, (b) Marmot,
(c) Allan F.
Figure 20. The standard deviation of
the difference in daily minimum humidity
between stations, as a function of the
distance between the stations, (data
from Figure 18). The regression line
was drawn from inspection.
Figure 22.
Figure 21.
Figure 21. Allan 60 station
(No. 8) looking southwest.
Figure 22. Rock F station
(No. 16) looking south.
Figure 23. Marmot station
(No. 6) looking southeast.
Figure 23.
32.
humidities for each period. It also gives mean daily number of hours
of bright sunshine observed at the Meadow station for each period.
Except for the July 4 - 30 sunny period there is remarkably
little difference in minimum humidity between the sev=1 rainy and
sunny periods. This suggests that rain in itself is not a major
factor determining daily minimum humidity.
The difference between Rock F and Meadow was also examined tc see if it was different during rainy spells than sunny spells,
using the same periods as in Table 6. There was no appreciable difference. The average for 36 sunn2, days showed Rock F daily minimum
humidity 0.85 higher than Meadow; for the 36 rainy days Rock F averaged
0.2 higher than Meadow.
Sunshine and Daily Minimum Humidity
10
When daily minimum
relative humidity was plotted
against daily hours of bright
sunshine at Meadow station (Figure
26), a definite correlation was
apparent -- correlation coefficient
r = -0.72.
0
10
20
30
40
50
60
Humidity Rise (%)
Figure 2A
A slightly different
Histogram of the amount of humidity
sunshine parameter was used with
rise following the beginning of
Headquarters data. The intensity
rain (in the daytime). Results
of sunshine received on a horizon- from all stations are combined.
tal surface had been recorded on
Hatching marks the cases when the
an actinograph. The number of
humidity reached 905 or higher.
hours per day that the actinograph
exceeded a reading corresponding
to approximately 0.35 langleys/min.
was used as the sunshine parameter.
Daily minimum relative humidity is plotted against this parameter in
Figure 27 and shows a similar correlation coefficient, (r = -0.74).
Thus in each case the variation in sunshine accounts for half the
variance in daily minimum relative humidity.
Days when rain fell are marked in Figures 26 and 27 so that
the smallness of the effect of rainfall on daily minimum humidity is
apparent.
33.
Variation of Daily Extreme Humidities with Elevation
Data are given in Table 7 to exhibit the variation of humidity with elevation. They are arranged as averages of successive 20day periods so that the sizes of random and systematic variations are
indicated.
For valleys within the mountains proper there is no significant variation of daily minimum humidity with elevation up to 1000
ft above valley bottom. Above that, as indicated by the single station
Allan 60, there appears to be an increase in relative humidity. For
slopes on the outer face of the mountains, represented by noose, there
is an increase of humidity in the first 700 ft above valley bottom.
The variation of nightly maximum humidity with elevation
follows a different pattern. The data in Table 7 agree with theory in
showing the sharpest variation near the valley bottom. Note that the
change in maximum humidity in the first 70 ft of elevation (Meadow to
McDougall 4E) is almost twice as large as in the next 210 ft (McDougall
48-50). Above that there is generally a slight further decrease in
nightly maximum humidity with elevation -- except on the Allan slope.
Nightly Maximum Relative Humidity
Histograms of nightly maximum humidities at individual
stations are given in Figure 28 for sixteen stations. Since hair
hygrographs are relatively inaccurate near saturation, the distribution of occurrences between the 95% and 100% classes is of little
significance. Attention is directed to the frequencies in classes
of less than 90% humidity.
The number of nights at each station that humidity
remained less than 75% is given in Figure 28. For the two stations
Meadow )47 and Allan 60 the frequency was lowest, averaging 1 night in
11. At most of the stations the frequency was about 1 night in 6.
At the four stations McDougall 56, Moose 56, Esker 50 and Lookout 68,
the humidity remained below 75% one night in three, approximately.
Inter-Station Differences in Night Maximum Humidity
Some nights nearly all stations approach saturation and hence
there is little variation from station to station. On the few nights
34.
35.
TABLE
6
Sunny period and rainy period averages of daily
minimum relative humidity; also of daily hours
of sunshine at Headox
Jun 26
- 29
Jun 30
-Jul 3
Jul 4
- 30
Jul 31
-Aug 14
Aug 15
- 19
Aug 20
-Sep 5
Sunny
Rainy
Sunny
Rainy
Sunny
Rainy
Headquarters
Sibbald
42
48
Allan 60
Allan 55
Allan 50
Meadow
McDougall 48
McDougall 50
McDougall 56
46
43
42
Esker
Elbow
Noose 51
Elk Hill
50
28
38
45
54
38
45
37
43
54
46
46
43
32
28
28
29
42
29
29
48
42
42
140
39
40
42
47
42
40
41
40
40
43
38
40
40
46
44
26
57
50
46
47
46
44
50
41
42
39
33
46
41
41
30
48
41
36
49
46
49
45
37
32
50
48
41
45
38
45
5.7
11.0
5.8
7.7
5.8
42
40
45
Hours of Sunshine 8.0
when most stations remain far below saturation the differences
between stations are most pronounced. The majority of nights fall
between these categories.
For the set of stations whose minimum humidities were
compared in Figure 17, histograms of nightly differences in
maximum humidity are given in Figure 29.
It is apparent that inter-station differences in maximum
humidity are greater than in minimum humidity, especially for the
36.
nights when maximum humidity is less than 93%. Note that 5% classes are
used in Figure 29, whereas 3% classes were used for differences between
stations in minimum humidity.
The consistently lower maxima at Allan F than at Allan
60 are noteworthy.
To portray the, inter-station differences for the general
network the following procedure was used. The stations were separated
into two groups on the basis of relative elevation. The 15 stations
300 ft or less above valley bottom formed the low level group. The
other 8 stations, all 700 ft or more above valley bottom elevation,
formed the upper level group. Night maximum humidities were rounded off
to 5% classes centred on 80%, 75%, 70%, etc. All maxima over 82% were
placed in the 85% class.
For each group of stations the average class of the
station maximum humidities was found for each night June 25 - Sep 5.
Only nights when the station average class was 75% or lower were
considered further. For the low level group the distribution of such
station average classes is given in Table 8.
The average distribution of stations by classes is
given in Table 9 for
(1)the 10 nights when the station average class was
75%, and
(2)the 6 nights when the station average class was
65%.
Comparable data for the upper level group of 8 stations
is given in Tables 10 and 11.
One might expect the distribution of stations in Tables
and 11 to decrease exponentially toward lower humidity. It is note9
worthy that for nights with a station average class of 65%, the distribution follows an approximate normal curve about a mean of 65%9.in Table
11, and (between 75% and 45%) about a mean of 60% in Table
Effect of Rain on Night Maximum Humidity
On each of the 11 nights when .01 in or more of rain
was recorded at Meadow station (2000 - 0800, June 25 - Sep 5) the
maximum relative humidity was 95% or 100%. For the 14 such rainnights at Headquarters, maximum humidity was 85% one night and 90% or
•
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I
I
I
i
I
I
2
3
4
5
6
7
8
9
10
11
12
13
Sunshine (hours)
Figure 26
Daily minimum humidity is plotted against daily hours of bright sunshine
(recorded by a Campbell-Stokes instrument) for the period Jun 8 - Sep 7
at Meadow station. Days with rain (2000-2000) are marked. Correlation
coefficient is -0.72.
39.
80
•
LEGEND
Rain
70
No Rain
.01"to .09
0
0
.10"or more
•
•
60
0
•
0
0
0
Min imum
R Hi% )
O
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0
0
0
0
0
0
0
0
10
2
3
4
5
6
7
8
9
10
I
11
12
13
Sunshine(hour0
Figure 27
Daily minimum humidity is plotted against sunshine hours (when radiation
was more intense than one-third langley per minute on a horizontal surface)
at Headquarters station. Days with rain (2000-2000) are marked.
Correlation coefficient is -0.74.
40.
more on the others. Appatently .01 in of rain at night is sufficient
to raise the humidity to near saturation.
When averages for sunny and rainy periods were calculated,
using the same periods as in Table 6, the rainy periods had slightly
higher average night maximum humidities, (Table 12). But when the
periods were grouped together, it was found that there was no difference between the sunny and rainy averages at two of the stations
(Headquarters and Meadow), and only 2 percentage points difference at
two other stations (McDougall 50 and Elbow). When the longest sunny
30) was compared with the longest rainy period (Aug.
period (July
20 - Sep 5) it was found that the rainy period had the lower average
maximum night humidity at more than half of the stations. This
presumably was the result of cloud cover diminishing net outgoing
radiation.
4-
It is concluded that while rain is falling it has a large
immediate effect on humidity, but after rain stops falling its persistent effect over the following 12 to 36 hours is so small it may easily
be masked by other factors.
The average nightly differences in maximum humidity between
mature forest (Rock F) and open field (Meadow) are also shown in Table
12 for the sunny spells and rainy spells. Maximum humidity is seen to
be lower in the forest than in the field for all periods; the amount of
the difference was less during the rainy spells than during sunny spells.
DISCUSSION
Forest-Meadow Comparison
In the literature daytime relative humidity has generally
been reported to be higher within a forest than outside it. Jemison
(1934) reports observations in two densities of forest (presumably
coniferous) and a clear-cut area in northern Idaho. Averaging daily
observations at 1630 for July and August 1931-33 inclusive he obtained:
27.3% in the clear-cut area;
29.0% in a thinned forest (half the canopy removed)
35.8% under dense virgin timber.
25
25
20
20
McD 56
24
0 20 0 40 50 60 0 0 90 10
0 80 •0 0s
25
25
25
20
20
20
15
15
26
25
25 -
25 -
20 -
Allan 50
20 -
20
McD.48
15
15 -
15 75
10-
12
10-
5-
10
5
1111.1
0
I
I
II
I
I
I
1
fi711
1 I I
10
10 20 30 40 50 60 70 80 90 100
20 30 40 50 60 70 80 90 100
25
25 -
20
20 -
49
"us
20
30
40
60
15 -
10
1 0-
Moose 51
15
12
10-
1-1
40
50
60
70
80
90
1111111
Fr—FL
111111 .11
16
20
Rock F 50
1
37
25
25 -
25-
I
0 20 30 40 50 60 70 80 90 100
10 20 30 40 50 60 70 80 90 100
100
100
29
6
30
90
20
15
20
80
25
Meadow 47
10
70
1
20
20
HO 46
15
15 -
10
10-
12
0
5
0 20 30 40 50 60 70 80 90
d
100
lo
20 36 40' 50
od 76 80 96 lob
10
20
30
•0
50
60
70
80
90
100
Maximum R.M .
Figure 28
Histograms of nightly maximum relative
humidity Jun 25 — Sep 5, 1960.
25
20
10
4
42.
Note that the difference in humidity between full canopy and half canopy
is much greater than between half canopy and clearcut. It is felt the
same would be true for many other forest areas.
Geiger (1950, p. 332) comments that high humidity is the
most characteristic feature of the microclimate of the trunk space.
Molga (1962) gives the humidities shown in Table 13 (assumed to be
monthly means for sites in Poland).
Pasak (1962), using observations from near Prague, derived
regression equations relating forest humidity (Hp) in spruce plantations, (with and without broadleaved undergrowth), and in an oak-pine
stand to humidity observed in the open (Ho) -- in the middle of the
forest nursery. All observations were taken at 1400 hours. The three
equations are similar and are approximated by the following single
equation
HF = 75% + .75 (Ho - 68%)
In contrast to Jemison's and Pasak's results for mature
conifer stands, my Kananaskis observations show early afternoon humidity to be as low in a mature pine stand and in several young lodgepole
pine stands as it is in the open field. It is recognized that with a
shade-intolerant species like lodgepole pine the crown canopy will not
be as dense as with spruce. Contributing reasons for the humidity not
being higher in the Kananaskis forests are thought to be:
(1) transpiration reduced by lack of soil moisture
(cf Table 1);
(2) convection mixing transpired moisture with the
surroundings before appreciable gradients of
vapour pressure develop;
(3)
the relative paucity of lesser vegation in the
mature stand (which in turn is related to soil
dryness).
Eisl (1962) observations in coastal British Columbia forests
show significant differences in daytime humidity between sites with
ample soil moisture and drier sites.
The Kananaskis nighttime observations showed slightly lower
nightly maximum humidities in forest than in open field.
Variation with Elevation: Daily Minimum Humidity
From adiabatic theory, Cramer (1961) indicates that in a
X13.
TABLE
8.
Frequencies by classes of station-average
maximum humidity, for the low level group
of stations, June 25 - Sep 5.
Max. humidity,
(average of 15 stations)
75% 70% 65% 60%
No. of nights this max
occurred out of 73 nights
10
0
6
2
55% 50%
1
1
layer that is thoroughly mixed, the relative humidity should increase
with elevation; when the humidity is near 20% the adiabatic rate of
increase is 3% per thousand feet, for humidity near 40% the rate is
6% per thousand feet, and near 60% the rate is 10% per thousand feet.
The amount of increase per thousand feet is seen to be approximately
17th the humidity.
Geiger (1950, p. 250) quotes results from Gross Arber (in
Bavaria) for clear days in spring showing an increase in daily
minimum humidity of 8% (38-46) in the first 1000 ft, which is
slightly greater than the adiabatic rate. The increase he reports
for the next 1600 ft is only 4%, which is only half the adiabatic
rate.
Baumgartner's May 1955 observations on Gross Falkenstein
(Geiger 1961, p. 456) show an increase from 55% to 60% in the first
thousand feet (about half the adiabatic rate). In the next 1250
ft of elevation the humidity increased from 60% to 75%, which is
almost exactly the adiabatic rate.
Hayes (1941) gives charts for the median day of August
based on observations on pairs of adjacent north and south slopes
at 4 elevations 2300 ft to 5500 ft in northern Idaho 1935-38. He
found an increase of less than 2% per thousand feet over the first
2000 ft of elevation -- the relative humidity was about 30%. This
is less than half the adiabatic rate. For the next thousand feet
of elevation the humidity increase was roughly adiabatic.
44.
TABLE 9
Distribution of the 15 low level stations
by class of night maximum humidity
85% 80% 75% 70% 65% 60% 55% 50% 45%
Humidity Class
For the
10 nights
station
average
was 75%
Proportion
of
For the
station-nights. 6 nights
in each class station
average
was 65%
.36 .15 .14 .13 .07 .09 .04 .01 .01
.19 .07 .05 .13 .11 .16 .13 .11 .05
Approximate calculations from Tanner's (1963) midafternoon
data for April-July 1955 indicate the following:
(1)In the Great Smoky Mountains (Tennessee) under forest
cover, increase of relative humidity with elevation is
less than half the adiabatic rate between 2000 and
6000 ft above sea level.
(2)In the Chiricahua
8400 ft above sea
adiabatic rate in
adiabatic rate in
Mountains
level the
June (the
July (the
of Arizona between 5400 and
increase is at half the
dry season), but at the full
wet season).
The Kananaskis data in Table 7 shows:
(1) in the first 800 ft above valley bottom:
(1)less than 4 adiabatic rate on Allan and
McDougall slopes in the cross-section area;
(2) one half the adiabatic rate on Elk and
Pigeon slopes;
(3)double the adiabatic rate on the Moose
slope.
(2) higher than 800 ft above valley bottom the single observation
on Allan slope shows the full adiabatic rate.
45.
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TABLE 10
Frequencies by classes of station-average
maximum humidity, for the upper level group
of stations, Jun 25 - Sep 5.
Max humidity
(average of 8 stations)
No. of nights this max
occurred out of 73 nights
75% 70% 65% 60% 55% 50% 45%
13
5
6
3
1
In summary, it appears that only in rare cases is the increase
of relative humidity with elevation more rapid than the adiabatic rate;
usually it is less. Varations from the adiabatic rate may be influenced
by many factors, but are mainly attributed to the observations, made
within a few feet of the ground surface on a mountain slope, not being
representative of humidity in the free air at the same elevation.
Variation with Elevation: Nightly Maximum Humidity
At night radiational cooling of the surface and the air next
it, with subsequent settling of the cool air into the valley bottom,
usually leads to a stable stratification of air. In this stratified
zone the temperature increases and the relative humidity usually
decreases with elevation.
Geiger (1950, p. 250) quotes observations on Gross Arber
for 25 clear spring days showing a decrease of maximum night humidity
with elevation amounting to 15% in the first 1000 ft. There is a
47.
TABLE 11
Distribution of the 8 upper level stations
by class of night maximum humidity
Humidity Class
85% 80% 75% 70% 65% 60% 55% 50% 45% 40% 35%
For the 13
nights when .42 .11 .19 .04 .08 .06 .05 .03 .02
average class
-
-
-
-
75%
Proportion,
of
For the 6
stationnights when .04 .07 .11 .18 .20
.13 .18 .07 .02
nights in average class
each class
65%
For the 5
nights when .03 .02 .03 .13 .13 .08 .18
.16 .18 .03 .03
average class
< 65%
further decrease of 7% in the next 1600 ft. On Gross Falkenstein in
Nay (Geiger 1961, p. 456) there is a decrease of 10% in the first 1000
ft and an increase of 2% in the next 1250 ft of elevation.
Hayes (1941) reports 33% decrease on a north slope and 45%
decrease on a south slope in the first 1000 ft above valley bottom.
(In the first 400 ft the decrease is 20% on the north slope and 30%
on the south slope). Above 1000 ft humidity decreases slowly with
elevation.
The Kananaskis data in Table 7 indicate a sharp decrease in
humidity of 11% in the first 250 - 300 ft above valley bottom. Above
that the decrease is more gradual, (less than 1% per 100 ft) -- except
on the Allan slope where there is a slight increase.
The cross-sectional shape of a valley must have considerable
148
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effect on the humidity profile at night, since the low zone in which
the temperature increases with elevation would normally be shallower
in a U-shaped than in a V-shaped valley section.
The low level stations at which observations were taken in
1960 should not be considered typical, in the sense of being areally
representative, for night humidity. Most of the stations were sited
on knolls or benches somewhat above the valley bottom and thus were
biased towards low night humidity compared with the average valley
bottom condition.
Exterior and Interior Valleys
The shapes of the histograms of daily minimum humidity shown
in Figure 11 are notably different for the two Moose slope stations
from those for the Kananaskis cross-section. The histograms for
Moose are nearly symmetrical; the others are peaked toward the low
humidity classes.
The explanation suggested to account for this relates to
the source from which replacement air is drawn into a valley when
solar heating sets up convection currents which carry warm air up
out of the valley. For interior valleys the replacement air must
be drawn mainly from ridgetop level or higher, since low level
reservoirs of air are scarce. As the replacement air descends into
the valley, adiabatic warming results in low relative humidity.
For valleys on the outer slopes of the Rocky Mountains,
replacement air may be drawn from the adjacent foothills and plains
area, which air will usually ascend towards the valley in question
thus becoming more humid enroute. At other times, with crossmountain winds, the dynamics of the atmospheric flow pattern may
bring descending air into these valleys, resulting in low relative
humidity. Exterior valleys may thus receive air from a wider
variety of sources than interior valleys, some of the sources being
more humid and others less humid than the air originally in the
valley. This would account for the broad range of Moose slope
minimum humidities.
For interior valleys the source is almost always the
relatively dry air aloft. Hence the minimum to which the humidity drops on a given day depends largely on the vigour of convection, which in turn depends on the amount of sunshine, (Figure
26), among other factors. Figure 30 adds some confirmation, by
showing that the histogram of daily hours of sunshine is skewed
similarly to that for daily minimum humidity at Meadow (Figure 11).
o.
5
TABLE 13
Relative humidity (after dolga)
Apr.
May June
July Aug Sept.
Evergreen forest
78
79
83
82
83
92
Deciduous forest
72
78
82
81
83
90
Open field
72
74
80
75
77
82
SUMMARY & CONCLUSIONS
The relative humidity regime in the forest and meadow lands
of the Kananaskis area in summer exhibits the following characteristics.
The daily minimum humidity is remarkably uniform over the
valley bottom area and for 500 to 1000 ft elevation up the valley sides.
Differences in site related to humps and hollows, or slope of the ground
surface, proximity to the river, and presence or absence of forest cover,
caused no appreciable difference in daily minimum humidity, (observed
4.5 ft above ground surface). Differences of t 5% between stations were
commonly observed; they were not consistent, as site differences would
be, and so were considered random variations related to horizontal
separation of the stations and slight instrumental inaccuracies. The
standard deviation of the difference in daily minimum humidity between
stations (which was about 4% for stations close together) increased in
general with distance between the stations at the rate of roughly 1%
RH per L miles.
A difference was found between exterior and interior valleys
in the statistical distribution of daily minimum humidities. The distribution was symmetrical for exterior valleys; it was peaked towards low
humidity for interior valleys.
The variation from station to station is much greater for
51.
nightly maximum than for daily minimum
humidity.. A sharp decrease of maximum
humidity with elevation frequently occurs
just above the valley bottom, with
decreases of 5% to 20% in the first 100 ft
of elevation. Occasionally differences of
40% occur between stations less than a
mile apart, when one of the stations is in
the cool stratified air on the valley floor,
(or some other concave area protected from
wind). At susceptible sites, (which were
always somewhat above valley floor elevation),
night maximum humidity remained below 75%
one night in three, on the average.
15—
10
J
E
5
0
........ .
5
Although the effect of rain on
relative humidity was large while it was
falling, it was negligible the next day.
That is, no persistent effect due to
wetting of the soil was observed. A
signification correlation was found
between daily minimum humidity and daily
hours of sunshine.
10
15
Sunshine (hours)
Figure 30
Histogram of daily hours of
bright sunshine Jun 25 - Sep 5,
1960 at Meadow station.
With respect to fire hazard rating and fire control, these results
emphasize the importance of nighttime relative humidity, (as also did Macleod
1948). Night humidity may be as low as mid-afternoon humidity, and it is
frequently more variable in space and time. A fire in the valley bottom which
shows promise of damping down at night may suddenly accelerate if it spreads
a short distance upslope into a less humid (and probably windier) zone.
Observation stations for relative humidity will give much the same
mid-afternoon reading regardless of site, but for night humidity stations
should be located on knolls, ridges, or the outer parts of benches 100 ft or
more above the valley bottom in order to be representative of conditions in
the more flammable zone of forest on the slopes.
52.
REFERENCES
Anon. 1954-63. Monthly Record, (Meteorological Observations in Canada).
Queen's Printer, Ottawa.
Anon. 1963.
Glossary of Forest Fire Control Terms. Canada,
National Research Council Pub. No. 7312.
Boughner, C.C., R.W. Longley and M.K. Thomas. 1956. Climatic
Summaries for Selected Meteorological Stations in Canada,
Vol. 3. Queen's Printer, Ottawa.
Byram, G.M. 1959. Combustion of Forest Fuels. In: Forest Fire
Control and Use, by Kenneth P. Davis. McGraw-Hill Book
Co.
Cramer, O.P. 1961. Adjustment of Relative Humidity and Temperature
for Differences in Elevation. U.S. Forest Service.
Pacific NW Research Paper 43.
Crank, J. 1960. Rate of Change of Moisture Content. In: J.W.S.
Hearle & R.H. Peters (Editors) -- Moisture in Textiles.
Butterworth's Scientific Publications, London.
Crossley, D.I. 1951. The Soils on the Kananaskis Forest Experiment
Station in the Sub-alpine Forest Region in Alberta.
Canada, Dept. of Forestry, Silvic. Res. Note 100.
Davis, K.P. 1959. Forest Fire Control and Use. McGraw-Hill Book Co.
Ebell, L.F. and R.L. Schmidt. 1964. Meteorological Factors Affecting
Conifer Pollen Dispersal on Vancouver Island. Canada,
Dept. of Forestry Publication 1036.
Eis, S. 1962. Statistical Analysis of Several Methods for Estimation
of Forest Habitats and Tree Growth near Vancouver, B.C.
Univ. of British Columbia Forestry Bull.
4.
Geiger, R. 1950. Climate near the Ground. Translation of M.N.
Stewart et al. Harvard Univ. Press. 494 pp.
Geiger, R. 1961. Das Klima der bodennahen Luftschicht, 4th ed.,
Friedr. Vieweg & Sohn. Braunschweig. 646 pp.
Gisborne, H.T. 1928. Measuring Forest Fire Danger in Northern Idaho.
U.S. Dept. of Agric. Misc. Pub. 29.
53.
Hayes, G.L. 1941. Influence of Altitude and Aspect on Daily
Variations in Factors of Forest-Fire Danger. U.S.
Dept. of Agric. Circ. 591.
van Hylckama, T.E.A. 1959. A Nomogram to Determine Monthly Potential
Evapotranspiration. Monthly Weather Rev. 87: 107-110.
Jemison, G.M. 1934. The Significance of the Effect of Stand Density
upon the Weather Beneath the Canopy. J. Forestry 32:
446-451.
Kelsey, Kathleen E. 1957. The Sorption of Water Vapour by Wood.
Austral. J. Applied Science 8: 42-54.
King, A.R. and M. Linton. 1963. Moisture Variation in Forest Fuels:
The rate of Response to Climate Changes. Austral. J.
Applied Sci. 14: 38-49.
King, K.M. 1961. Evaporation from Land Surfaces. Canada, N.R.C. Assoc.
Comm. on Geodesy & Geophysics, Proc. of Hydrology
Symposium No. 2 pp. 55-80.
NacHattiel L.B. 1955. Ramifications of Relative Humidity in Forestry.
Royal Meteorological Society Canadian Branch, VdL 6, No. 5,
14 pp.
MacHattie, L.B. 1958. Accuracy of the Hair Hygrograph. Canada,
Dept. of Transport, Meteorol. Branch Circ. 3022.
Macleod, J.C. 1948. The Effect of Night Weather on Forest Fire Danger.
Canada, Dept. of Forestry, Forest Fire Research Note 14.
Molga, M. 1962. Agricultural Meteorology Part II. Translation
Available from Office of Technical Services, U.S. Dept.
Commerce.
Pasak, V. 1962. Air Humidity Inside Forest Stands. Lesnictvi 8:
719-734 (English Summary pp. 732-733).
Rowe, J.S.
1959. Forest Regions of Canada. Canada, Dept. of
Forestry, Bull. 123.
Schroeder, M.J. 1960. Humidity Patterns at Middle Elevations in
the Coastal Mountains of Southern California. U.S.
Forest Service, Pacific SW Research Note 165.
Sharp, W.M. and H.H. Chisman. 1961. Flowering and Fruiting in
the White Oaks. I. Staminate Flowering Through Pollen
Dispersal. Ecology 42: 365-372.
514.
Spalt„ H.A. 1957. The Sorption of Water Vapour by Domestic and
Tropical Woods. Forest Products Journal 7: 331-335.
Sutton,
0.G.
1960. Understanding Weather. Penguin Books Ltd.
Tanner, J.T. 1963. Mountain Temperatures in the Southeastern and
Southwestern United States During Late Spring and Early
Summer. J. Appl. Meteor. 2: 473-483
Urquhart, A.R. 1960. Sorption Isotherms, In: J.W.S. Hearle and R.H.
Peters (Editors), Moisture in Textiles. Butterworth's
Scientific Publications, London.
Wright, J.G. 1932. Forest-Fire Hazard Research as Developed and
Conducted at the Petawawa Forest Experiment Station.
Canada, Dept. of Forestry, Forest-Fire Hazard Paper 2.
Wright, J.G. and H.W. Beall. 1945. The Application of Meteorology
to Forest Fire Protection. Imperial Forestry Bureau,
Oxford, Technical Communication No. 4.