In chapter 3, the average world surface
temperatures are represented on two world charts for
January and July in figures 3-2A and 3-2B. These are
mean charts and are not meant to be an accurate
portrayal of the temperatures on any one particular day.
Note that in general the temperatures decrease from low
to high latitudes.
LAND AND WATER DISTRIBUTION
Land heats and cools about four times faster than
water. Therefore, the location of continents and oceans
greatly influences Earths pattern of air temperature as
well as the sources and direction of movement of air
Influence on Air Temperature
characteristics of the land or water that is on their
windward side. In latitudes of prevailing westerly
winds, for example, west coasts of continents have
oceanic temperatures and east coasts have continental
temperatures. These temperatures are determined by
the wind flow.
Since the upper layer of the ocean is nearly always
in a state of mixing, heat losses or heat gains occurring
at the surface are distributed throughout a large volume
of water. This mixing process sharply reduces air
temperature contrasts between day and night and
between winter and summer over oceanic areas.
Over land, there is almost no redistribution of heat
by turbulence; also, the effect of conduction is
negligible. Thus strong seasonal and diurnal contrasts
exist in the interiors of continents. During the winter, a
large part of the incident solar radiation is reflected
back toward space by the snow cover that extends over
large portions of the northern continents. For this
reason, the northern continents serve as source regions
for dry polar air.
The large temperature difference between the land
and water surfaces, which reverses between the two
seasons, determines the seasonal weather patterns to a
In chapter 3, figures 3-2A and 3-2B, the isotherms
over the Northern Hemisphere are more closely spaced
and parallel in winter than in summer. In the Southern
Hemisphere, the temperature gradient does not have as
great a seasonal change as it does in the Northern
Hemisphere. These conditions are due to the unequal
distribution of land and water on the two hemispheres.
Since the Southern Hemisphere has less land and more
water surface than the Northern Hemisphere, the
change due to the greater water surface is less with
consequently more nearly uniform isotherms. Also, the
continents of the Southern Hemisphere taper toward the
poles and do not extend as far poleward as do those in
the Northern Hemisphere.
The nature of the surface affects local heat
distribution. Color, texture, and vegetation influence
the rate of heating and cooling. Generally, a dry surface
will heat and cool faster than a moist surface. For
instance, plowed fields, sandy beaches, and paved
roads become hotter than surrounding meadows and
wooded areas during the day. During the night,
however, the situation is reversed.
The distribution of water vapor and clouds is
another important factor influencing air temperature.
Although areas with a high percentage of cloud cover
have a high degree of reflectivity, the energy, which is
not reflected, is easily trapped in the lower layers due to
the greenhouse effect. Thus, areas of high moisture
content have relatively high temperature.
Influence on Air Circulation
The higher mean temperature of the Northern
Hemisphere is an effect not only of its higher
percentage of land, but also of the fact that its oceans
are also warmer than those in the Southern Hemisphere
are. This is partly due to the movement of warm
equatorial waters from the Southern Hemisphere into
the Northern Hemisphere caused by the southeast
trades crossing the equator. Another factor conducive to
higher mean temperatures in the Northern Hemisphere
is the partial protection of its oceans from cold polar
waters and arctic ice by land barriers. There is no such
barrier between the Antarctic region and the southern
Climates over land may vary radically within very
short distances because of the elevation and variations
extremely important role in determining the climate of
The height of an area above sea level exerts a
considerable influence on its climate. For instance, the
climate at the equator in the high Andes of South
America is quite different from that found a few feet
above sea level at the same latitude. All climatic values
are affected by surface elevation.