Page:Handbook of Meteorology.djvu/114

 Range summits; from 10 to 30 feet may be estimated as the annual fall. The amount varies greatly from year to year; at Summit, California, 60 feet of snow fell during the winter of 1879-80.

On the Pacific Coast slope the yearly snowfall in the mountains is a matter of great importance. Since the construction of the various irrigation projects in the arid region, humanity is realizing more and more the dependence of productive lands, not only on the yearly amount of snow-fall, but on the conservation of the melting snow, as well. In the arid regions of the United States, the winter snowfall is the moisture of the summer crops.

Except at great altitudes, practically all the snow falls between the first of December and the middle of April in the zone of latitude that includes the New England States and New York. Flurries of snow occur in May as far west as the Rocky Mountains; and at elevations of 2000 feet or more they occur in June.

Sleet; Ice Storms; White Storms.—In Weather Bureau nomenclature, sleet consists of small pellets of ice, apparently formed when rain-drops are frozen in passing through a stratum of cold air next the ground. Usually the pellets are not larger than duck shot; occasionally they are the size of peas. Sleet has been reported as hail so frequently that the Weather Bureau has issued an explanatory pamphlet calling attention to the fact that the ice pellets of sleet differ materially in structure from hailstones. Ice pellets may contain enough air to give them a whitish opaque appearance; therefore they are likely to deceive observers. Sleet storms are very apt to occur in the morning, when the temperature is at its daily minimum, but this is by no means always the case. Sleet may occur when a cold wave flows under warm, moist air; it is likely to result when a warm southerly wind flows over the top of very cold surface air.

Sleet is often mixed with rain; at such times it forms an ice-coating on the ground, making a surface that is more or less pebbly. Frequently it happens that the rain-drops are not