d hence rain.
If, however, the air is not near the saturation point, a fall in
temperature will not necessarily produce bad weather.

The measurement of humidity is of far wider importance than the mere
forecasting of local weather conditions. The close relation between
humidity and health has led many institutions, such as hospitals,
schools, and factories, to regulate the humidity of the atmosphere as
carefully as they do the temperature. Too great humidity is
enervating, and not conducive to either mental or physical exertion;
on the other hand, too dry air is equally harmful. In summer the
humidity conditions cannot be well regulated, but in winter, when
houses are artificially heated, the humidity of a room can be
increased by placing pans of water near the registers or on radiators.

30. Heat Needed to Melt Substances. If a spoon is placed in a vessel
of hot water for a few seconds and then removed, it will be warmer
than before it was placed in the hot water. If a lump of melting ice
is placed in the vessel of hot water and then removed, the ice will
not be warmer than before, but there will be less of it. The heat of
the water has been used in melting the ice, not in changing its
temperature.

If, on a bitter cold day, a pail of snow is brought into a warm room
and a thermometer is placed in the snow, the temperature rises
gradually until 32 deg. F. is reached, when it becomes stationary, and the
snow begins to melt. If the pail is put on the fire, the temperature
still remains 32 deg.F., but the snow melts more rapidly. As soon as all
the snow is completely melted, however, the temperature begins to rise
and rises steadily until the water boils, when it again becomes
stationary and remains so during the passage of water into vapor.

We see that heat must be supplied to ice at 0 deg. C. or 32 deg. F. in order
to change it into water, and further, that the temperature of the
mixture does not rise so long as any ice is present, no matter how
much heat is supplied. The amount of heat necessary to melt 1 gram of
ice is easily calculated. (See Laboratory Manual.)

Heat must be supplied to ice to melt it. On the other hand, water, in
freezing, loses heat, and the amount of heat lost by freezing water is
exactly equal to the amount of heat absorbed by melting ice.

The number of units of heat required to melt a unit mass of ice is
called the _heat of fusion_ of water.

31. Climate. Water, in freezing, loses hea