Chemistry Reference
In-Depth Information
TEXTBOX
71
HUMIDITY
Humidity
is a general term used to refer to water vapor in the air. The
actual amount of water vapor that the air can hold increases with the tem-
perature: the higher the temperature, the greater the amount of water
vapor that the air can hold. Various measures are used to express humid-
ity:
absolute humidity
and
relative humidity
are the two most widely used.
Absolute humidity
is the amount (by weight) of water vapor in the air at
a particular temperature and pressure and it is usually measured in grams
per cubic meter.
For most practical purposes, however, humidity is expressed as the
rel-
ative humidity
(RH), the percentage of moisture in the air at a given tem-
perature in relation to the amount of moisture that the air could hold at
that temperature before condensing as dew. Since the latter amount is
dependent on the temperature, the relative humidity is a function of both
moisture content and temperature. A value of 50% RH, for example,
means that the air holds half the water vapor it can hold at the prevailing
temperature. At 100% RH, moisture condenses and falls as rain. The rela-
tive humidity is measured with instruments known as a
hygrometers
.
17.2.
WATER AND THE HYDROSPHERE
The
hydrosphere
(the Greek prefix
hydro
means water) is the great mass of
water that surrounds the crust of the earth. Water is one of a few substances
that, at the temperatures normal on the surface of the earth (which range
between about
50 and 50°C), exists in three different states: liquid, gas, and
solid. Liquid water makes up the oceans, seas, and lakes, flows in rivers, and
underground streams. Solid water (ice) occurs in the polar masses, in gla-
ciers, and at high altitudes, and gaseous water (moisture) is part of the
atmosphere (O'Toole 1995). Liquid and solid water cover over 70% of the
surface of the earth.
Water is the most abundant substance on the surface of the earth and
the major constituent of all living organisms. Life on earth would not be pos-
sible and the biosphere would not exist without water. Water plays a key
role in the sustenance of living organisms, since it is essential for all living
processes, transporting nutrients to wherever they are required in their
bodies, and removing from them their waste products. Thus, since it plays
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