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Transactions, American Geophysical Union
Volume
35,
Number
3
June
1954
PRECIPITA TION MEASUREMENTS ON WIND-SWEPT SLOPES
Austin E. Helmers
Abstract-- Precipitation catch for three calendar years is compared for four types
of gage installation on a wind-swept south-facing slope with a 22° gradient at elevation
5500 ft. The 1950 precipitation catch by (1) weighing-recording gage with the orifice
and an Alter type wind shield sloped parallel to the ground surface, (2) unshielded non­
recording gage with orifice sloped parallel to the ground surface, (3) weighing-recording
gage with horizontal orifice and Alter type wind shield, and (4) unshielded U. S. Weather
Bureau standard gage with funnel removed was (1) 62.90, (2) 53.34, (3) 39.82 and (4) 27.64
inches, respectively. In 1951 the respective catches were (1) 61.38 (2) 52.94, (3) 35.91
.1
and (4) 25.67 inches; and in 1952 they were (1) 39.62, (2) 35.01, (3) <l7.09, and (4) 15.91
inches. Indications are that gage (1) probably did not overestimate preclpitation amounts
and that there is a relationship between wind velocity and magnitude of differences in
catch between gages.
Introduction--The accurate measurement of precipitation is quite difficult.
The problems en­
countered are increased by rugged terrain, exposure to wind, and the occurrence of a large pro­
portion of the precipitation in the form of snow.
One of the prinCipal causes of inaccuracy or variation in measuring precipitation on Wind-swept
slopes is the turbulence produced by the gage in the moisture-bearing wind stream.
Some of the
precipitation which should fall into the gage is carried over it because of an increase in pressure
and consequent upward deflection of the air stream on the windward side and a decrease in pres­
sure and marked acceleration of the air stream over the top of the gage [BROOKS,
1938a;
1952].
KREIN,
The desirability of using wind shields to minimize air turbulence over a gage orifice has been
demonstrated repeatedly.
Relatively few of these studies were concerned with snow and still fewer
included severe conditions of wind and topography.
However, in almost every case it was shown
that gage catch was materially increased by using a wind shield.
The Nipher [BROOKS,
1938b]
and Alter [ALTER,
successful of the many types devised.
suited for snow when left unattended.
1937]
types of shields have proved the most
The Nipher shield is the better of the two for rain but un­
The large flat surface collects snow which may later blow
into the gage, or accumulating snow may bridge the receiver orifice.
Slat-type Alter shields have
been most successful for snow and are widely used on snow gages in the mountainous areas of the
1940] . The
40 inches
West [BERNARD and CODD,
supporting ring approximately
shield consists of sheet-metal slats loosely attached to a
in diameter.
The apparent action of the shield is to close
toward the gage and deflect the air stream downward enough to compensate for upward deflection
caused by the gage [KREIN,
1952].
Another source of measurement error is the difference in angles at which the plane of the gage
orifice and a sloping ground surface intercept precipitation.
The error becomes large when pre­
cipitation falls at an appreciable angle from vertical into a gage with a horizontal orifice exposed
c
a steep slope.
ILTON,
1943]
Tilting the gage improves the precipitation catch on slopes [STOREY and HAM­
because it intercepts precipitation on the same plane as the ground surface.
The
catch is then multiplied by the secant of the angle between the plane of the orifice and the horizontal
to correct for slope and thus express precipitation on a horizontal basis.
The same result can be
obtained by cutting the gage orifice to the same angle as the slope, eliminating the need for slope
correction.
gage.
[1943] suggested that this type of gage be called a "stereo"
[1934], indicated that tilted gages are aerodynamically superior to
STOREY and HAMILTON
They, with PAGLIUCA
stereo gages, however.
A number of gaging sites in a precipitation gage network on the Priest River Experimental
Forest in northern Idaho are steep and exposed to winds. Gages developed for these more severe
sites combine the advantages of stereo orifices and wind shields of the Alter type.
and shield are parallel to the ground surface.
Both the orifice
Stereo orifices are used because some of the gages
471
472
AUSTIN E. HELMERS
[Trans. AGU, V. 35 - 3]
are of the weighing-recording type and not adapted to tilting. PAGLIUCA [1934] suggested slanting
the gage and shield parallel to the slope for measuring snow, and HAYES and KITTREDGE [1949]
used a tilted gage with Nipher shield for rain. The Priest River gage is thus essentially like the
previously suggested forms but differs in using the slat-type shield which is better adapted to snow.
As this combination is relatively uncommon, a test installation was operated for three years begin­
ning in the fall of 1949 to compare the performance of this gage with three other types of gages.
Early results of the test were reported by HELMERS [1952]. The concluding results are described
below.
Gage installations and resultsuThe gage
installation (Fig. 1) was located at an elevation
of 5500 ft on a windy south-facing slope with a
220 gradient, and included: (A) recording gage
with stereo orifice and tilted Shield, (B) un­
shielded non-recording gage with stereo orifice,
(C) recording gage with horizontal orifice and
shield, and (D) unshielded standard gage (8 by
24 inches non-recording) with horizontal orifice.
It is estimated that 50 pct or more of the
annual precipitation at the test site occurs as
snow. Gage orifices were all about ten feet above
the ground surface to keep the gage bases just
above the maximum snow-pack depth. Precipi­
tation catches were usually measured weekly
during precipitation periods but less frequently
during 1952 and during the summer months of
light precipitation.
A Recording 90� will? stertro "ri�t;e omf
T,/1'ed shu,l&!
909/1
t: Reccrt:l"'g gogt! ,..if" horuonflll orfl'lCt! 17",,1
shield
1) lJns!fI,ltled hOrlz"nl'o! orffit;1f sfonliord
11 I./nrh/�ltI� sft!�"
Fig. 1--Skeich of precipitation gage
test installation
Table 1 summarizes the annual catches for the three years of record. The differences in
catch between gages are expected to vary between years due to differences in wind and in the pro­
portion of preCipitation falling as snow. In spite of these possible sources of variation in catch,
the four gages ranked about the same in magnitude of catch dudng the two years of nearly equal
preCipitation. Precipitation for 1952 was roughly 40 pet less than for the previous two years and
catches by the non-standard gages were from 10 to 30 pct higher as compared with the standard
gage.
Table 1--Summary of 1950-1952 annual precipitation catch by four types of gage installation
Gage
Installation
Measured precipitation
B
C
D
Recording gage with stereo
orifice and tilted shield
Unshielded stereo gage
Recording gage with horizontal
orifice and shield
Unshielded horizontal orifice
standard gage
I
I
I
1952
1950
inch
inch
inch
pct
pet
pct
62.90
53.34
61.38
52.94
39.62
35.01
228
193
239
206
249
220
39.82
35.91
27.09
144
140
170
27.64
25.67
15.91
100
100
100
1950
A
I
Comparison with
standard gage
1951
The rank in magnitude of annual catch was observed for practically
(Fig. 2). The principal exceptions were due to heavy snow storms when
partially or completely bridged with snow. Other occasional exceptions
ing errors or to very small storms, the total precipitation of which was
of the gage weighing devices.
1951
1952
every measurement period
both stereo orifices became
were attributable to weigh­
near the limits of accuracy
The observed tendency of stereo gages to bridge earlier than the horizontal orifice gages tends
to support the previously noted [STOREY and HAMILTON, 1943; PAGLIUCA, 1934] aerodynamic in­
feriority of stereo gages.
Gage catch was compared with snow-pack water equivalent to determine, if possible which gage
was most efficient. These comparisons were not wholly successful, however. Rain and wind effects
60
f--
40
f--
A
B
C
lJ
I
1lV
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STEHO DIlJFICI � IINSHIII.#ID
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i---I---' -
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�50
VV'
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�
� ,
U 41
Iy
, j,
,
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,
I,
,
G
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,
,
,
�
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JAN. 'II. MIlA.
04'.'1. MAY
..._...
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M/tI
.
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A:
��!
Vt:::=r=:::
j:::
:.=: VI
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l--vdVI--I--I--
Gage catch consistently exceeded water equiv­
alent gain in 1951. From February 2 to March 12
the gage accumulated 3.60 inches of precipitation
but the snOW pack gained only 2.4 inches water
equivalent. The 1951 period, however, was windy
as compared to the 1950 period as shown by
Table 2.
I..---'
l-
20
V
.J-...3'
��'X
D V
.r-V
Vt=:
r-V
�t:;:=
""�}-- V--
�20
,
�
�
D
I-----
"'"
�
�
on the snow pack sometimes eliminated a sound
basis for comparison. Apparent water equivalent
gains were reliable only when there were no losses
due to percolating rain or snow-melt water and
when there was no wind erosion of snow. One of
the more reliable comparisons of gage catch with
snow-pack water equivalent gain includes the
period from February 9 to March 8, 1950. Pre­
cipitation catch by the stereo gage with tilted wind
shield was about 6.05 inches. The snow-pack wa­
ter equivalent gain was 7.9 inches during the same
period. March 23 to April 5, 1950, was another
period of little wind erosion. The gage received
about 2.8 inches of precipitation while the snow
pack gained 3.2 inches of water. The gage orifice
was about six feet above the snow surface on Feb­
ruary 9 and March 23, and four feet on March 8
and April 5. These comparisons indicate that the
most efficient gage of the four tested is probably
the most accurate.
,,�
I
C"'1ENR
50
.....--
C
Table 2--Comparison of daily wind travel during
February 9 to March 8, 1950, and February 12
to March 12. 1951
lMJ
�
D
ocr.
sIn
J5.(J!
'
NOv.
orc-
Fig. 2--Cumulative annual precipitation catch
for three years by four types of gage instal­
lation on a wind-swept slope.
S
.
J
i: 1
.
�
--
.
!
I
o
February 12 to
March 12,1951
mile
Less than 100
100 to 200
Over 200
No record
no. days
no. days
16
5
0
6
27
8
16
4
0
28
During windier periods some wind-blown snow
is very likely deposited in the gages at the same
time that the snow pack is eroded. The gage ori­
fice was about six feet above the snow surface dur­
ing the 1951 period above.
.---
.
�:--;--
�
�
�
�
February 9 to
March 8,1950
-
I:
�-I
it
Daily wind
travel
Total
:-,-- �
(\
�
473
PRECIPITATION MEASUREMENTS ON WIND-SWEPT SLOPES
[Hydrology]
/'l
2
J
-I
S
! I
MEAN W#K�Y WINO V&lOCITY
9
(mIl»
Wind was recorded from a three-cup ane­
mometer. 'This type of anemometer is unsuited
to winter conditions and was often immobilized
by snow, ice, and rime, resulting in fragmentary
records. Available data, however, tend to support
the observation of PAGLIUCA [1934] and others
that the effeC tiveness of wind ?hields increases with
,
,
increasmg wmd velocity. A significant (r = 0.39)
relationship was found between mean weekly wind
velocity and the ratio of the catch of the stereotilted shield gage to that of the unshielded stand­
ard gage with horizontal orifice (Fig. 3). The precipitation catch ratio of the first gage to the sec­
ond (assuming the ratio is a linear. function of mean weekly wind velocity, mph) was
Fig. 3 --Relationship of the precipitation catch
ratio (catch by stereo gage with tilted shield
to catch by unshielded standard gage) to mean
weekly wind velocity
catch ratio
=
1.78
+
0.24
x
according to the best wind records available.
mean weekly wind velocity, mph
474
AUSTIN E. HELMERS
[ T r a n s . A G U , V . 35 - 3]
S u m m a r y — P r e c i p i t a t i o n c a t c h f o r t h r e e c a l e n d a r y e a r s i s c o m p a r e d f o r f o u r t y p e s of gage
i n s t a l l a t i o n s on a w i n d - s w e p t s o u t h - f a c i n g s l o p e at 5500 f t e l e v a t i o n . P r e c i p i t a t i o n c a t c h by a
s t a n d a r d e i g h t - i n c h g a g e w i t h a h o r i z o n t a l o r i f i c e w a s c o n s i s t e n t l y l o w . T h e c a t c h of a standard
g a g e w a s i n c r e a s e d by adding a h o r i z o n t a l l y m o u n t e d A l t e r t y p e w i n d s h i e l d . C u t t i n g the gage ori­
f i c e t o an a n g l e e q u a l t o the s l o p e g r a d i e n t ( " s t e r e o - o r i f i c e " ) r e s u l t e d i n a g r e a t e r c a t c h , however
than a d d i n g a h o r i z o n t a l l y mounted w i n d s h i e l d . T h e g r e a t e s t c a t c h w a s o b t a i n e d b y a s t e r e o gage '
w i t h a w i n d s h i e l d t i l t e d p a r a l l e l t o the s l o p e g r a d i e n t .
P r e c i p i t a t i o n c a t c h by the s t e r e o g a g e w i t h t h e t i l t e d w i n d s h i e l d a g r e e d w e l l w i t h snow-pack
w a t e r e q u i v a l e n t g a i n d u r i n g p e r i o d s w h e n w i n d d i d n o t e r o d e s n o w c o v e r o r d e p o s i t drifting snow
i n t o the g a g e s . C o n c l u s i v e c o m p a r i s o n s w e r e not a v a i l a b l e , h o w e v e r . T h e s t e r e o g a g e with tilted
s h i e l d i s p r e s u m e d t o b e the m o s t a c c u r a t e of the g a g e s t e s t e d .
T h e e f f e c t i v e n e s s o f the w i n d s h i e l d i n c r e a s e d w i t h i n c r e a s i n g w i n d v e l o c i t y .
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A L T E R , J. C , S h i e l d e d s t o r a g e p r e c i p i t a t i o n g a g e s , M o n . W e a . R e v . , v . 6 5 , p p . 2 6 2 - 2 6 5 , 1937.
B E R N A R D , M E R R I L L , and A S H T O N R . C O D D , P r o g r e s s r e p o r t o n m o u n t a i n s n o w f a l l p r o g r a m of
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' ~"
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H E L M E R S , A U S T I N E . , P r e c i p i t a t i o n m e a s u r e m e n t s o n w i n d - s w e p t s l o p e s , N o r t h w e s t Sci v 26
pp. 65-68, 1952.
' * '
K R E I N , T E D , M o d e l t e s t i n g of p r e c i p i t a t i o n g a g e s , I d a h o E n g . , v . 2 9 , p p . 1 0 - 1 1 , 3 0 , 1952.
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G e o p h y s . U n i o n , p t . 2 , p p . 3 8 5 - 3 9 0 , 1934.
S T O R E Y , H . C , and E . L . H A M I L T O N , A c o m p a r a t i v e s t u d y of r a i n g a g e s , T r a n s . A m e r . Geophys
U n i o n , p t . 1, p p . 1 3 3 - 1 4 1 , 1 9 4 3 .
I n t e r mountain F o r e s t and R a n g e E x p e r i m e n t S t a t i o n ,
U . S. F o r e s t S e r v i c e , D e p a r t m e n t of A g r i c u l t u r e ,
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( C o m m u n i c a t e d m a n u s c r i p t r e c e i v e d July 6 , 1953; o p e n f o r
f o r m a l d i s c u s s i o n until N o v e m b e r 1, 1 9 5 4 . )