The masses of more or less condensed vapor in the upper air currents are what are known as clouds. Their various forms and appearance are shown in the cut on the —— page.
The cirrus and cirro-cumulus clouds are the highest, are mostly in the altitudes of perpetual frost, and are supposed often to consist of minute ice crystals. In temperate latitudes they are usually formed in, and move with, the upper air current, or return-trade from the tropical regions.
The cumulus clouds are characteristic of the tropics, and of the summer days in middle latitudes, their height depending upon the relative humidity of the air. They are formed by local ascending currents, which carry a large amount of vapor into the cooler upper air. There the vapors are condensed, and are gradually heaped up into those heavy masses of sharply defined clouds, which look like vast snowy mountains. Their base is horizontal, and marks the height at which the dew point is reached and condensation begins.
The accumulation of vapors is often so great that these clouds form a column several thousand feet high. In this case the difference in the temperature and the electrical conditions of the upper and lower portions is such that electrical discharges take place, accompanied by condensation of a portion of the cloud, forming a thunderstorm.
Stratus clouds are most frequently seen in the morning or evening, and are always low. They are formed by the descent of the higher clouds and vapors of midday into the lower air as the temperature decreases. They are more frequent in winter and summer than in the intermediate seasons.
The nimbus cloud is more dense and heavy than the others, which may all be transformed into the nimbus by
