The cirro-cumulus, cirro-stratus, and cumulo-stratus are only modifications and combinations of the above-named principal classes.
FORMS OF ATMOSPHERIC VAPOR
Warm air is capable of holding suspended a larger quantity of moisture than cold air, and therefore the amount of vapor present in the atmosphere is subject to great variations.
WHAT CAUSES
DEW
These facts also account for the formation of dew, which is caused by the reduction of the temperature and the deposition of the moisture which the warmer atmosphere of the day had held in suspension. Dews will hence be usually most abundant when cool nights succeed warm days, and on a clear night than when the skies are obscured by clouds, because a cloudless sky is usually much colder than a beclouded one. It is also essential for the copious formation of dew, that the ground or other substance on which it is deposited should be much cooler than the superincumbent air; for if the ground be warm it will impart its temperature to the air near its surface and dew will not be formed.
FORMATION OF MISTS
AND FOGS
When the ground or water is warmer than the air, mists and fogs are frequently formed; and since water and marshy surfaces cool less rapidly than dry land, mists and fogs are of more common occurrence in low, damp situations than in dry, elevated districts. They are formed by the condensation of the vapor, or, in other terms, its transformation into the minute globules of water, which instead of descending to the earth in the form of dew, remain suspended above the land or the water.
RAIN, HAIL AND
SNOW
Clouds are formed by the condensation of vapor at considerable but various elevations in the atmosphere. Vapor is always invisible, clouds, therefore, are not vapor but water, and consist of a fine watery powder, the size of each particle being exceedingly minute; and consequently they are so light that clouds formed of an accumulation of such particles are readily borne forward by the winds. Clouds are sometimes suddenly formed and as suddenly disappear, probably owing to sudden and partial changes of temperature. When a considerable difference of temperature prevails in the aerial currents which may come in contact with the local atmosphere, a further condensation takes place, and the particles of this fine watery powder unite into drops, and, becoming heavier, fall to the earth in the form of rain, hail or snow.