Page:Handbook of Meteorology.djvu/113

. Normal crystals are six-sided or six-pointed; the angles are usually 60° or 120°. The snowflakes of ordinary storms consist of tangled masses of broken crystals. They are at their best when the temperature is not higher than 25° F and the air is still. The flakes should be caught on black cloth. If a microscope is used, it must be used in a place where the temperature is below the freezing point. If photo-micrographs are made, a low power objective—2-inch or 4-inch—gives excellent results.

Occasionally the snowflakes take the forms of soft pellets—the graupeln of the German meteorologist. At other times they are half-melted, but retain traces of crystallization. The presence of slowly falling snow crystals during fairly clear weather is common in many localities. It is the greatly-dreaded poguenib of the far-western Indian, who associates it with pneumonia.

Snowfalls have been recorded in every state in the Union. They occur occasionally along the Gulf Coast between Pensacola and Brownsville. Snow has fallen in Florida as far south as Fort Myers. A line drawn from Savannah through San Antonio, El Paso, and Yuma to San Francisco marks roughly the limit south of which snow seldom falls. South of the thirty-fifth parallel, snow rarely lies on the ground more than a day or two. At New Orleans a measureable snowfall occurs about once in fifteen years. It is about as frequent in the city of Los Angeles, although the mountain summits in the vicinity occasionally are snow-clad.

In the vicinity of the Great Lakes the ground is covered most of the winter. In the New England and Middle Atlantic states the annual snowfall is 7 to 8 feet. It decreases toward the west, being about 2 feet in North Dakota. In the basin region of the Rocky Mountain States snowfalls occur at long intervals only; in the plateau region they may be expected yearly on the range summits. The heaviest snowfalls occur along the northern Rocky Mountain and the Sierra Nevada