Man's physical universe



In the tropics there are no well-defined seasons as far as temperature

is concerned; but the rainfall does vary, and wet and dry seasons are

recognized. In the arctic and antarctic zones, summers are short and

winters are long, whereas spring and autumn are of brief duration.

Near the poles, the chief difference between the seasons is in the amount

of light received ; here one would refer to the light and dark seasons. In

the temperate zones the four common seasons of varying temperature

and length of day (i.e., winter, spring, summer, and autumn) are well

known. This change in the seasons is one of the major factors in producing

changes in w^eather and will be discussed in a later paragraph.

The unequal distribution of the sun's heat between the equator and

the poles is the cause of the most widespread variations in climate.

The equatorial regions receive more radiant energy from the sun than

do the polar regions ; it is always warm at the equator and always cold

at the poles.

It Takes Time to Heat the Earth's Surface.

On June 21 or 22 the sun reaches its most northerly position and

turns southward again. In spite of this fact the hottest month in the

United States is July. The earth loses heat by the process of radiating

it to interplanetary space; and the rate of radiation is the greater the

higher the temperature.

Even after the days begin to shorten, more heat continues to be

received from the sun than is lost by radiation, because the earth has

not yet warmed up to the temperature where the rate of loss is as great

as the rate of gain; but as the temperature increases, while the days

keep on shortening, the rate of loss finally becomes as large as the

gain, and the temperature ceases to increase.

Because of the continual redistribution of heat by winds, ocean

currents, etc. and because of the different thermal properties of different

kinds of surfaces, the duration of the lag of highest and lowest temperatures

behind the longest and shortest days is widely different at

different places on the earth. In southern Arizona and New Mexico

the highest normal daily temperatures come only 10 days after the

longest day. Around the Gulf of Mexico and the Atlantic coast the lag

is about 40 days. In the central states the lag is about 30 days, and in

San Francisco it is 100 days, although at Sacramento, less than 100

miles inland, the lag is 37 days.

Weather Is Caused by Atmospheric Circulation.

"Weather" differs from "climate" in that weather represents the temporary

condition of the atmosphere at a given place and time, while climate

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