PRECIPIRATION) OF MOISTURE.

 

 

GHAPTER, i

Precipitation of Moisture.

  

262. Evaporation—F rom every water surface,
and even from masses of ice and snow, there is
constantly arising, at all temperatures, an invisible
vapor of water. Water vapor is about three-fifths
as heavy as air. It diffuses readily through the
air, and is borne by the winds to all parts of the
earth. This giving off of vapor from the surface
of water is called evaporation. It is evaporation
which dries the wet earth, when the moisture is
unable either to pass off the earth’s surface by
drainage, or to soak through the porous strata.

About one-half, by weight, of the vapor of the atmo-
sphere is within a little over a mile above the mean sea
level.

263. The Rapidity of Evaporation is influ-
enced by the following circumstances:

(1.) The temperature of the atmosphere. The
capacity of the air for absorbing moisture in-
creases with an increase of temperature. Warm
air can retain more vapor than cold air.

(2.) The extent of surface exposed. Evapora-
tion takes place only’from the surface; therefore,
the greater the surface, the greater the evapora-
tion.

(3.) The quantity of vapor already in the air.
Dry air absorbs moisture more rapidly than moist
air. All evaporation ceases when the air is com-
pletely saturated.

(4.) The renewal of the air. During very calm
weather, the air in contact with a water surface
becomes saturated, and so prevents further evapo-
ration. Gentle breezes, by renewing the air, in-
crease the rapidity of evaporation.

(5.) Pressure on the surface. A diminished
atmospheric pressure increases the rapidity of
evaporation.

264, The Dew Point.— When the air contains
as much vapor as it is capable of holding, it is
said to be at tts dew point.

The quantity of moisture necessary to saturate a given
quantity of air and bring it to the dew point, varies with

 

SeCTLO Nee
MOISTURE OF THE ATMOSPHERE.

2038300

 

 

the temperature. Cold air requires less moisture to satu-
rate it than air which is warmer, and, therefore, may feel
damper than warm air, which may contain more vapor.
We thus distinguish between the actual humidity, or the

. amount actually present in a given volume of air, and

the relative humidity, or the relation between the amount
present and that required to saturate the air at the given
temperature.

The humidity of the air is determined by means of an
instrument called a hygrometer.

Weight in grains of aqueous vapor in ONE cubic foot of
SATURATED AIR at different temperatures. (Silliman.)

 

Temperature, Fahr. Weight in Grains. Approximate Values,

 

0° 0.545 0.6
10° 0.841 0.9
20° 1.298 1.3
30° 1.969 2.0
40° 2.862 2.9
50° 4.089 4.1
60° 5.756 5.8
70° 7.992 8.0
80° 10.949 11.0
90° 14.810 15.0

100° 19.790 20.0

 

No matter how much aqueous vapor a given
quantity of air contains, if its temperature be
lowered, tt will grow relatively moister until, if the
fall of temperature be sufficient, its dew point is
reached; and as soon as the temperature falls
below the dew point, a deposition of moisture
will begin, either in the liquid or solid state.

265. Precipitations.—The invisible vapor may
be precipitated from the atmosphere and become
visible, either as dew, mist, fog, cloud, rain, sleet,
hail, or snow. These are called precipitations.

Law of Precipitations.

In order that any precipitation may occur, the
air must be cooled below the temperature of its dew
point.

266. Distribution of Precipitations—The quan-
tity of moisture in the air depends on its tempera-
ture, and its vicinity to the sea.

The amount of precipitation regularly decreases
as we pass from the equator to the poles, and from
the coasts of the continents toward the interior.

267. Dew.—If, during a warm day, a dry glass
be filled with cold water, the outside of the glass
will soon become covered with small drops of
water, derived entirely from the air. The air