Man's physical universe

xanabras

270 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME

It Is Important to Observe the Clouds in Weather-forecasting.

The best known clouds are the fair-weather cumulus clouds.

These

snow-white billowy piles which project upward into the sky have dark,

flat bases. In "muggy" weather these clouds become thunderheads.

Stratus clouds are low, flat, foglike clouds of wide extent. Another

type is the nimbus, from which precipitation takes place.

are very high, wispy, rapidly moving clouds of fine ice crystals.

Cirrus clouds

They

often appear as the advance guard of a cyclone, having outstripped the

Fig. 89. Weather map retouched to show lines of equal pressure and equal

temperature. Compare this map with the one shown in Fig. 88.

storm that is following. As the storm gets nearer, these clouds form

slightly lower, extensive, white veil-like layers called cirro-stratus clouds.

The velo cloud of southern California is the very thick high fog that

drifts in from the sea. As it reaches the land, the lower portions are

evaporated by the heat from the land, leaving the higher portion to be

dissipated later by the heat of the sun.

The Zuiii Indians had an adage: "When the sun is in his house it

will rain soon." Large, nearly colorless rings around the sun or moon,

known as halos, are produced by the refraction of light by the ice

crystals of very tenuous cirrus clouds. The larger the rings appear to

be, the closer is the approach of the storm, because the ring, which in

reality is always of the same diameter, gives the appearance of an

increase in diameter as the clouds come closer to the earth when the

storm is approaching.

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