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Vapor condensed in air having a temperature below thirty-two degrees Fahrenheit freezes, or passes to a crystalline form, producing snow. Snowflakes occur in a great variety of forms, which usually present the outline of either a regular hexagon or a six-pointed star.

Their size depends upon the temperature and the relative humidity of the air through which they fall, for, like raindrops, they increase by successive additions from the vapors with which they come in contact in descending. Thus in mild weather they are much larger than in very cold weather.

PICTORIAL CHART OF THE CLOUDS, SHOWING THEIR FORMS AND POSITION

1. Cirrus (sir´rus).—Small curl-like clouds, usually high in the heavens. 2. Cirro-stratus (sir-ro-strā´tus).—Intermediate between the cirrus and stratus. 3. Cirro-cumulus (sir-ro-kū´mu-lŭs).—Resembling the scales of mackerel. 4. Alto-cumulus (al´tō-kū´mu-lus).—High cumulus clouds. 5. Alto-stratus (ăltō-strā´tūs).—High stratus clouds. 6. Strato-cumulus (strā´to-kū´mu-lŭs).—Forms of cumulus and stratus combined. 7. Nimbus (nim´būs).—A rain cloud. 8. Cumulus (kū´mū-lus).—A conical heap of clouds. 9. Cumulo-stratus (kū´mu-lo-stra´tŭs).—Intermediate between the cumulus and the stratus. 10. Stratus (strā´tŭs).—Arranged in a horizontal band or layer. 11. Fracto-stratus (frăk´tō-strā´tŭs).—Broken forms of stratus. 12. Fracto-cumulus (frăk´to-kū´mu-lus).—Broken forms of cumulus.

THE BEAUTIFUL CRYSTAL-FORMS OF SNOWFLAKES

1–3. Six-rayed stars. 4–13, 18–25. Combinations of six-rayed stars with decorated flat surfaces. 14, 16, 17. Combinations of stars and columns. 15. A true pyramid.

When the lower air is warm enough partially to melt the crystals, they form minute balls. When raindrops, formed in the upper air, fall through a cold current, they are often frozen, producing sleet instead of snow.