Characterizing Climate
(a version of this exercise was written by Ricklefs for inclusion in the next edition of the text)
What we think of as climate covers many aspects of the environment. Temperature and
precipitation immediately come to mind. Being outdoors on a sunny day also reminds us that solar
radiation also can make a big difference in our perception of warmth. All of these aspects also
change with the seasons. Given such complexities, it is not surprising that ecologists have devised
many ways to characterize climate. Of course, any scheme of describing climate should consider
how climate factors affect living systems, particularly plants, which are the base of the food web
and the source of most of the energy for ecosystems.
Physical factors typically interact in their effects on living systems. For example, seasonal rainfall
promotes plant growth more strongly during warm months than during cold months. Thus, the
total precipitation or average temperature during the year would miss the important point that
plants require more water—and can use more water to support their growth—when it is hot. Most
descriptions of climate take into account this interaction between temperature and moisture. One
of the simplest depictions of climate in this way is the climograph.
A climograph shows seasonal changes in temperature and precipitation simultaneously, permitting
a visual comparison of climates at different localities. Examples of climographs for several places
are shown below. We see that the seasons at San Jose, Costa Rica bring marked variation in
rainfall but little change in temperature, just the reverse of the situation in New York City, where
pretty much the same amount of rain falls in each month, on average.
San Jose, Costa Rica
New York City
Moving from east to west across the United States (middle panel) from Cincinnati, Ohio, to
Cheyenne, Wyoming, and Winnemucca, Nevada, climate becomes more arid but temperatures
remain within the same range. Thus, the change in vegetation from deciduous, broad-leaved forest
in Ohio to short-grass prairie in Wyoming and desert shrubland in Nevada depends on the
responses of plants to water availability rather than temperature.
Cincinnati, Ohio
Cheyenne, Wyoming
Winnemucca, Nevada
Although San Diego’s weather in January resembles that of Lincoln, Nebraska in April, the
climates differ as much as their vegetation. San Diego’s Mediterranean climate, with hot, dry
summers and cool, moist winters, favors slow-growing, drought-resistant shrubs (chaparral),
whereas Lincolns’s wet summers and cold, dry winters favor the development of tall-grass prairie.
The climograph provides a useful basis for comparing localities, but it fails to combine the effects
of temperature and precipitation in a biologically meaningful way, and it does not reveal the
cumulative effects of temperature and precipitation patterns on conditions for plant growth. For
example, during extended dry periods both evaporation and transpiration (the evaporation of water
from leaves) remove water from the soil. When precipitation does not balance evaporation and
transpiration, the water deficit in the soil steadily increases, perhaps for months at a time, and
plant growth slows or stops. Soil water reflects last month’s conditions as well as today’s weather.
In 1948, the geographer C. W. Thornthwaite published a method for estimating the seasonal
availability of water in the soil from temperature and precipitation. He compared the rate at which
water is drawn from the soil by plants and by direct evaporation with the rate at which it is
restored by precipitation. The sum of evaporation and transpiration is called evapotranspiration,
and the measured amount is the actual evapotranspiration, abbreviated AET. In natural
environments, evapotranspiration is at times limited by the availability of water in the soil.
Potential evapotranspiration (PET), the amount of water that would be drawn from soil if
moisture were unlimited, can be calculated from temperature and precipitation. When
precipitation input to the soil (rainfall minus surface runoff) exceeds potential evapotranspiration
at all seasons, the soil remains saturated throughout the year, as at Brevard, North Carolina. In
Canton, Mississippi, precipitation falls below potential evapotranspiration during the warm
summer months, and so the soil is depleted of water during the late summer and early fall. In the
dry climate of Grand Junction, Colorado, soils are depleted of water most of the year and become
saturated only briefly, after heavy rains.
Questions to consider:
What kinds of adaptations would you expect of plants that must tolerate seasonal cold conditions,
as in New York City? What kinds of adaptations would you expect of plants in dry regions, such
as Winnemucca? How might chaparral shrubs and prairie grasses be differently adapted to the
special conditions of their environments?
Exercise: Plot the monthly temperature and precipitation for Windsor, Ontario through the year,
and then as a function of each other in the form of a climograph (data for your vicinity and other
sites around the globe are available from http://www.worldclimate.com). To which of the
climographs above is the local climate most similar?
Website: Go http://www-cger.nies.go.jp/grid-e/gridtxt/tateishi.html, which shows global monthly
actual evapotranspiration at different seasons of the year. Notice seasonal shifts and differences
between regions. Where are the areas of highest AET, and the lowest AET? What aspects of
climate are responsible for these differences? Which areas have high AET throughout the year;
which are continuously low? What kind of vegetation do you think might occur in each of these
areas?
Thornthwaite’s approach to the calculation of potential evapotranspiration was based primarily on
empirical measurements to which he fitted the following equation for PET expressed in
millimeters per day (mm/month):
when T > 0
PET  16  (10  T / I ) a
and
PET = 0
when T ≤ 0
where
T = mean temperature (º C),
a = 0.49 + 0.079i + 7.71×10-5i2 + 6.75×10-7i3,
i = (0.2×T)1.514, a monthly heat index, and
I = the sum of the 12 monthly values of i.
Because the value of PET calculated by this equation is for a 12-hour day and 30-day month, the
value has to be adjusted by the average day length (DL, hours) and number of days per month (M)
according to PET (adjusted) = PET((DL/12)×(days/30).
These days, climatologists use much more sophisticated equations including solar radiation and
water vapor pressure of the atmosphere to calculate potential evapotranspiration, however
Thornthwaite’s equation provides a reasonable approximation.
Go to the Excel spreadsheat labeled “Thornthwaite's calculation of PET” and fill in the monthly
values for average monthly temperature (from www.worldclimate.com) and day length for your
area, which you can obtain from the web or from data/calculations shown below). Thornthwaite’s
PET will automatically be calculated. You can also compare these monthly values to your average
precipitation and determine months during which soil water is depleted and recharged.
If you are unable to locate daylength on the web, there are two approaches to getting an
approximate daylength for sites at differing latitudes. The simple way does an interpolation on
latitudes in the table below, and provides a value for each month (the table is for day lengths on
the 21st of each month).
Latitude (ºN)
Month
0º
10º
20º
30º
40º
50º
60º
70º
80º
J
12.07
11.39
11.07
10.33
9.49
8.48
7.08
2.00
0
F
12.07
11.52
11.36
11.18
10.58
10.28
9.44
6.30
0
M
12.07
12.07
12.07
12.09
12.11
12.13
12.18
12.30
13.15
A
12.07
12.24
12.42
13.04
13.30
14.07
15.05
19.00
24.00
M
12.07
12.37
13.09
13.47
14.34
15.40
17.35
24.00
24.00
J
12.07
12.43
13.21
14.07
15.01
16.23
18.53
24.00
24.00
J
12.07
12.37
13.10
13.48
14.36
15.44
17.41
24.00
24.00
A
12.07
12.24
12.42
13.04
13.32
14.09
15.09
20.00
24.00
S
12.07
12.08
12.08
12.10
12.13
12.17
12.23
12.45
13.15
O
12.07
11.51
11.35
11.17
10.55
10.26
9.41
7.30
2.00
N
12.07
11.38
11.07
10.32
9.48
8.47
7.07
2.30
0
D
12.07
11.32
10.55
10.12
9.20
8.04
5.52
0
0
The more complex way is to use a formula that calculates daylength for any Julian day (days
counted from 1 on January 1 of a year to 365 on December 31 of the year (except, of course, for
leap years) and latitude. The calculation formulae given below are from Forsthe, et al. (1995).
They calculate daylength in three steps:
1) Calculate the revolution angle  (in radians, based on the elliptical orbit of the earth and its
tilt) from the Julian day J:
 = 0.2163108 + 2 tan-1[0.9671396 tan (0.00860  {J – 186})]
2) Calculate the declination angle  (in radians, the angle at solar noon between the sun and
the equator) from the orbit and revolution angle:
 = sin-1[0.39795 cos ]
3) Calculate daylength D (plus twilight, with differing definitions of what is included in
twilight embedded in a coefficient p) using the declination angle and latitude L. Looking at
what is required to perform these calculations (partly due to including the elliptical orbit of
the earth around the sun), you may find it simpler to use the table. The p in the equation is 0
if sunrise and sunset are defined as the time when the center of the sun is even with the
horizon. Its value is 0.26667 if sunrise and sunset times are defined as when the top of the
sun is even with the horizon.
p
L


sin

sin
sin



24
180
D  24 
cos 1  180

L



cos
cos
180


Now compare the graphical estimates of actual evapotranspiration to the calculated potential
evapotranspiration. During which seasons/months is there a water balance in deficit? Why is there
no measure of rainfall in the calculation of PET?
Reference:
Forsythe, W.C., E.J. Rykiel Jr., R.S. Stahl, H. Wu and R.M. Schoolfield. 1995. A model
comparison for daylength as a function of latitude and day of year. Ecol. Model. 80:87-95.