Geoscience Reference
In-Depth Information
Köppen Climate Classification
2.
Seasonality
—Seasonality is a critical variable in the
character and distribution of global climates for two
reasons related to Sun angle: (a) the number of hours
of sunlight in a day changes in many places over the
course of the year, and (b) insolation and temperature
can be highly variable between the winter and summer
seasons. In places like the tropical regions, however,
very little annual change occurs with respect to these
variables, which is reflected in the relatively consistent
temperature and precipitation characteristics of tropical
climates.
3.
Air mass circulation
—As discussed in Chapter 6, the
atmosphere flows in predictable ways, with distinct pres-
sure systems associated with specific zones on Earth.
The unique distribution of these pressure systems results
in regions with heavy or persistent precipitation, such as
those associated with the Intertropical Convergence Zone
(ITCZ), whereas others (such as those near the Subtropi-
cal High [STH] Pressure System) are relatively dry. Still
other regions have distinct weather-producing systems,
such as midlatitude cyclones that result in highly variable
weather from day to day.
4.
Maritime vs. continental relationships
—Proximity
to a large body of water can have a big impact on
temperature and the direction of airflow, as we saw
in Chapter 7. In the interior of continents the annual
temperature range can fluctuate a great deal, whereas
it is moderated along ocean coasts. Large bodies of
water can also be great sources of atmospheric water
vapor through the process of evaporation, and airflow
can transport water vapor inland where it falls as
precipitation. Evaporation of large amounts of water
vapor is usually associated with warm oceans, where-
as cold oceans can have a drying effect on adjacent
continental locations.
5.
Topographic effects
—Topography can dramatically
influence atmospheric processes in a region, as shown
in Chapters 6 and 7. For example, the windward side
of mountain ranges is often a place of heavy precipita-
tion because air cools adiabatically there. In contrast,
rain shadows commonly form in the leeward side of
mountain ranges due to the descent of air and associ-
ated adiabatic warming. Similarly, the topography of
a region can also influence the flow of air, resulting in
Chinook winds and cool-air drainages that otherwise
would not occur.
All these variables interact with one another in a holistic
way to influence climate. As you study the next section on the
geography of global climates, remember to look for the ways in
which these factors interact to form a regional climate pattern.
To assist you with this process, you will frequently be prompted
with questions that are intended to make you think about the
interrelationship of variables. If you have trouble seeing how
any particular variable fits into the picture, review parts of the
preceding chapters.
The purpose of climate classification is to identify certain char-
acteristics, such as temperature and precipitation, which have
observable regional patterns. Given the discussions in the previ-
ous chapters, you may already be thinking of some distinctive
regional climate patterns. For example, tropical regions tend
to be wet due to the presence of the ITCZ, whereas latitudes
around 30° N and S are drier due to the influence of the STH
Pressure System. Although this pair of climate zones is easy
to visualize, it is often difficult to classify many other climate
zones because their geography depends on a number of vari-
ables. It is also difficult to classify climate zones because they
form a spatial continuum; in other words, sharp breaks from
one climate region to the next rarely occur.
Despite these difficulties, a variety of helpful climate clas-
sification systems have been devised. One such system is the
Thornthwaite system, which was developed by the American
climatologist C. Warren Thornthwaite. This system focuses on
the local scale and is used frequently in the United States to as-
sess soil moisture characteristics as they relate to vegetation and
agriculture. It is based on the concept of
potential evapotrans-
piration (potential ET)
, which is an estimate of the amount of
water used by plants with an unlimited water supply. Potential
ET increases with increasing temperature, daylight length, and
wind strength and decreases when humidity increases. Can you
determine why these changes take place? Think about factors
such as insolation and the vapor gradient. In contrast,
actual
evapotranspiration (actual ET)
reflects the actual amount of
water used by plants.
The Thornthwaite system identifies three basic cli-
mate zones on Earth. Low-latitude climates have potential
ET
>
130 cm (51 in.), whereas middle-latitude climates are
those with potential ET that ranges between 130 cm and 52.5 cm
(20.5 in.). High-latitude climates are those with potential ET
that is
<
52.5 cm. The Thornthwaite system can also be subdi-
vided on the basis of the length of time, and by what amount,
actual ET falls below potential ET. In this context, moist cli-
mates are those with a surplus or small deficit (15 cm; 6 in.) of
water, whereas dry climates have a yearly deficit
>
15 cm.
Although the Thornthwaite climate system is used fre-
quently in the United States, it is not commonly used elsewhere
in the world. Instead, the most widely used classification sys-
tem is the Köppen (pronounced
Kepun
) Climate Classification
System, which was developed by the German botanist and cli-
matologist Wladimir Köppen, who recognized the relationship
Potential evapotranspiration (potential ET)
A measure
of the maximum possible water loss from a given land area
assuming sufficient water is available.
Actual evapotranspiration (actual ET)
The quantity of water
actually removed from a given land area by evaporation and
transpiration.
