The earth is much cooler than the sun, and the wave length of the earth's radiations is mu

ch longer than that of sunrays. These longer heat waves cannot pass through the atmosphere as easily as the short waves coming from the sun. Similarly, radio stations that transmit programs to distant countries broadcast in short waves, which can travel all around the world. Stations that broadcast local programs send out longer waves, which can be received more easily, but only in a limited area.

(83) Heat waves, rising from the earth, meet resistance in the atmosphere from the water vapor there. Many if the waves are stopped by the water vapor can not get back out into space. The atmosphere, when it is warmed up, also adds some heat back to the earth's surface.

When the sun's rays strike water surface, some of the water becomes absorbed by the air as water vapor. The warmer the air is, the more vapor it can hold. When the air has absorbed its maximum amount of vapor, it is said to be saturated. If the air is then cooled, some of the vapor will condense and clouds will form. (84)These clouds, will contain drops of water at temperatures above freezing, or ice crystals at temperatures below freezing.

Clouds can greatly affect the temperature of the earth's surface. When there are many clouds in the sky, all of the sun's rays can not reach the earth. The cloudy day, then, will be cooler than the cloudless day. Clouds also prevent the earth from cooling off rapidly at night. For this reason, countries such as the British Isles, which are often covered by clouds, have a relatively constant temperature. The weather in these cloudy areas is neither very hot in summer nor very cold in winter. On the other hand, places such as deserts, which have few or no clouds, have very sharp variations in temperature—between night and day as well as between summer and winter.

Radio stations that transmit programs to distant countries send out waves that resemble______.

A．heat waves rising from the earth

B．waves absorbed by water vapor

C．waves sent out by radio stations that transmit local programs

D．the rays sent out from the sun