or eight miles above the ground. See Chart 3. The alto-cumulus clouds are smaller, more huddled together like the backs of a flock of sheep, and they are from two to three miles high; the cirro-cumulus, from four to six miles high, appear to be still smaller, and they often arrange themselves in ranks or battalions and form the beautiful mackerel sky. All these clouds are formed by the rising of currents of air in a vertical direction, the flat bases showing the level where the temperature begins to be cool enough to cause the vapor to condense, while the sides and tops outline the relative amounts of pressure, temperature and vapor which are just sufficient for the saturation to begin. From a study of these elements we may compute the vertical gradients for each 100 meters, or for each 100 feet, of elevation above the surface of the ground, and these quantities are of great value to meteorologists. The stratus or veil clouds are formed in a very different way. Instead of producing the cooling necessary to condense the moisture by raising 
Chart 1, Example of the eastward movement of the upper air above Washington, D. C., in miles per hour. The average eastward velocity at the surface is 6 miles per hour: at 6 miles high it is 70 miles per hour in clear weather. it to higher elevations, it may also be caused by the flowing of horizontal currents of air in contact with each other, as when a warm current passes over a cold one. When air from the south flows northward and air from the north moves southward into the same region, these currents generally overflow one another in two strata instead of mixing, since masses of air at different temperatures are quite reluctant to lose their individuality. This stratification of the air in horizontal sheets, flowing from the tropics and from
the polar zones, is always taking place in the atmosphere, and the stratus clouds are generally produced somewhere along these surfaces of contact. The cumulus clouds therefore indicate, as shown in Chart 1, that the air is rising vertically in certain layers, while drifting eastward, and the stratus that it is moving horizontally with a different velocity in adjacent strata. The lowest stratus clouds are elevated fog; the strato-cumulus is about two miles high; the altostratus averages four miles and the cirro-stratus six miles above the earth.
There is furthermore a stratification of the vapor contents of the atmosphere within every high cumulus cloud, which is interesting.
