Page 256 - NS-2 Textbook

METEOROLOGY 251

IF THE AIR COULD HOLD
[00 0 2[2 0 BOILING POINT THIS MUCH MOISTURE
OF WATER
.-.•..
••••
90 194 .•. -.-
80 176
.. ,-
0 .. • ••
70 158
AS WATER VAPOR
60 140 AT THIS TEMPERATURE GooF
50 122~ BEFORE THE MOISTURE BEGAN
TO RILL OUT AS RAIN, SNOW,
(/)40 I04W ETC.
2 :J:
(/)30 86 Z THEN-
...J W
~20 68 a:: THE RELATIVE HUMIDITY
:c
OF THE AIR WOULD
50 Lt BE 100 PERCENT
10
0
0 32° MELTING POINT
Of ICE
BUT THE AIR NOW HOLDS
-10 14 HALF THAT MUCH
MOISTURE
-20 -4
• •
-30 -22 ••••
.40 0 _40 0 •
• •• •
••
AT THE SAME TEMPERATURE 6O"F
The Celsius and corresponding Fahrenheit temperature scales.
THEN-
THE RELATIVE HUMIOITY
OF THE AIR IS NOW
hal thermometer is at the bottolll of this curve; for a mer- 50 PERCENT
cury thermometer it is at the top.
An illustration of the concept of relative humidity.
MEASURING RELATIVE HUMIDITY
AND DEW POINT
The atmosphere alvvays contains V\Tater in the form of vapor cools, and it drops when that air is heated. It fol-
vapor. Nearly 71 percent of Earth's surface is covered by lows, then, that as air rises, it cools and the water vapor
vlater. Heat causes the evaporation of millions of tons of condenses, eventually falling as some form of moisture.
water from these surfaces daily. In a process called tran- The dew poi1lt is the temperature to which air must be
spiration, additional huge amounts of water enter the air cooled-at constant pressure and constant water vapor
from the green leaves of plants. As this \Varnl, moist air content-to reach sahuation (100 percent relative hu-
rises} it expands and cools, evenhmlly reaching its satu- midity). When air is cooled to its dew point temperature,
ration level-lOa percent relative humidity. Then the small water droplets condense on objects. Dew is
vapor condenses into a liquid. The water droplets form formed. At higher altihldes, this simply means that the
into clouds, and precipitation (usually rain or snow) will air has been cooled sufficiently to cause a cloud to begin
occur. This "",rater cycle of evaporation, condensation, losing ,vater vapor. If conditions are right, these cloud
and precipitation, referred to as the hydrologic cycle, is droplets ,vill fall as rain or snmN.
continually in process. Relative humidity and dew point are measured by
We already have mentioned that the amount of water using a psychrometer. A psychrometer is simply two orcli-
vapor the ahnosphere can hold varies with the ahnos- nary thermometers mounted together on a single strip of
phere's temperahue. The relative hUlIlidity is the amount material. The bulb of one is covered by a water-soaked
of water vapor the air is aChlally holding, expressed as a V\Tick, from which the moisture is alloV\Ted to evaporate.
percentage of the amount that air, at that temperahue, can TIle moishue will evaporate until the amount of water in
hold. When the air contains all the water it can hold at a the wick equals the amount of water vapor in the sur-
given temperature, hmniclity is at the 100 percent satura- rounding ahnosphere. Since evaporation is a cooling
tion point. If it contains half of what it could hold at that process, the reading on the wet bulb will be lower than
temperature, the relative humidity is 50 percent. Since on the dry bulb-unless the humidity is 100 percent, at
warm air can hold more ""vater than cold all~ the relative which time both readings are the same. The difference
htunidity goes up when ail' with a given amount of water between the wet-bulb and dry-bulb readings is applied