Science for beginners . Clouds.—The lowest clouds and allthe denser clouds are like fog. Clouds are formed by the con-densing of water vapor in the air. They are interesting andoften beautiful. Clouds are the messengers of the air. Some-times they tell us of sunny days to come; often they warn usof approaching storms. What they tell depends on how muchone knows of them. The foregoing six pages introduce clouds and show howto study them for yourself. Figure 127 shows the principal classes of clouds, arrangedaccording to their height. The accompanying table gives thechief divisions and a number
Science for beginners . Clouds.—The lowest clouds and allthe denser clouds are like fog. Clouds are formed by the con-densing of water vapor in the air. They are interesting andoften beautiful. Clouds are the messengers of the air. Some-times they tell us of sunny days to come; often they warn usof approaching storms. What they tell depends on how muchone knows of them. The foregoing six pages introduce clouds and show howto study them for yourself. Figure 127 shows the principal classes of clouds, arrangedaccording to their height. The accompanying table gives thechief divisions and a number of their combinations. Somecloud forms are further illustrated in Figures 124 to 133. In recording cloudiness in weather records, a day with %oor less of cloudiness is called clear; %0 to %o is called partlycloudy; and %0 or more is cloudy. See Weather Record,page 154, and study the governments Weather Reports. 186. Cumulus Clouds; How They Are Formed.—Cloudsmay be formed in several ways. The most important is that. Fig. 134.—Rising air currents, due to warming of earths A no cloud is B a small cloud resultsAt (7 a stronger current builds a much higher cloud. which builds the cumulus. Cumuli are formed by the cool-ing that takes place in ascending currents of rather warm andmoist air. Air is warmed most next to the earths surface 176 THE WEATHER (Art. 165). Often, especially in summer, this lower air be-comes considerably warmer, and therefore considerablylighter, than the air above. Then, for a brief time, there iscool heavier air above, and warm lighter air below it next tothe earth (Fig. 134). Soon this warming air below breaks up-ward through the overlying colder heavier air, and ascends inbroad streams or masses here and there, while the colderheavier air settles downward between. The ascending streamsof warm air continue to rise as long as they are warmer thanthe air around them at the same -height. As they rise the pres-sure on them becomes les
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