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.
[Illustration: =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.]
[Illustration: =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.