Man's physical universe

xanabras

CLIMATE AND WEATHER 255

refers to the general or average condition of the atmosphere.

The circulation

of the atmosphere is one of the primary factors in bringing about

weather changes.

The so-called general or planetary circulation of the atmosphere,

brought about by the unequal heating of the earth's surface at different

latitudes and modified by the rotation of the earth, is not as simple as it

seems, because many factors alter this general picture. For example,

the unequal heating of land and water areas during the day and the

unequal cooling at night cause disturbances. In the temperate zones

in the summer the atmosphere over the land areas becomes heated and

the pressure becomes less; the atmosphere over the ocean has a correspondingly

low temperature and high pressure. During the winter

season these conditions are reversed. Mountain ranges also greatly

influence the currents in the atmosphere. The preceding effects are

especially marked in the northern hemisphere because of the preponderance

of land there.

Most important of all, however, are the frequent storms of middle

latitudes which continually disturb the general circulation. The

interlatitudinal circulation of air between the heated equatorial regions

and cold polar regions does not take place in a regular, unvarying

manner but largely by way of great disturbances which give rise to the

irregular weather changes of the temperate zones.

Water Is Capable of Existing in the Atmosphere in Vapor Form in

Amounts Depending Only on the Temperature.

Water continually evaporates from living organisms and from moist

soil as well as from the surface of bodies of water. Evaporation is more

rapid when winds are blowing because they carry off the vapor and

keep the space unsaturated; evaporation is especially rapid when a hot,

dry wind is blowing because the tendency for water to evaporate

(vapor pressure) increases with rising temperatures. The maximum

amount of water vapor that can exist in a given volume increases with

rise in temperature. The more humid the air, i.e., the more water

vapor that is already present, the less rapidly will more water evaporate

into this space.

The ratio of the amount of water vapor present to the maximum amount

possible at the existing temperature, expressed as a percentage, is called the

relative humidity. If the air is saturated, the relative humidity is 100

per cent. Under such conditions wet clothes would not dry at all. On

a "good drying day" clothes dry quickly, and the relative humidity is

low, about 30 to 40 per cent or less; that is, there is present only 30 to

40 per cent of the water vapor that could exist in the same volume at

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