Geoscience Reference
In-Depth Information
RAINFALL
0
EXTREMELY MOIST
(4.0 or more)
10
20
VERY MOIST
(3.0 to 3.9)
UNUSUALLY MOIST
(2.0 to 2.9)
30
40
50
NEAR NORMAL
(-1.9 to +1.9)
40
30
MODERATE DROUGHT
(-2.0 to -2.9)
20
SEVERE DROUGHT
(-3.0 to -3.9)
10
EXTREME DROUGHT
(-4.0 or less)
0
OCT
NOV
1992
DEC
JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
DROUGHT
1993
Figure 4.8
Percentage of the continental USA affected by wet spells or drought, based on the Palmer
Index (see scale on right), during the period October 1992 to August 1993.
Sources: US Climate Analysis Center and Lott (1994). Reprinted from Weatherby permission of the Royal Meteorological Society.
Crown copyright ©.
balance that exists between these variables. When-
ever the balance between one or more of them is
disturbed beyond a certain limit, condensation
may result.
The most common circumstances favoring
condensation are those producing a drop in air
temperature; namely contact cooling, radiative
cooling, mixing of air masses of different temper-
atures and dynamic cooling of the atmosphere.
Contact cooling occurs within warm, moist air
passing over a cold land surface. On a clear
winter's night, strong radiation will cool the
surface very quickly. This surface cooling grad-
ually extends to the moist lower air, reducing the
temperature to a point where condensation occurs
in the form of dew, fog or frost, depending on the
D CONDENSATION
Condensation is the direct cause of all the various
forms of precipitation. It occurs as a result of
changes in air volume, temperature, pressure
or humidity. Four mechanisms may lead to
condensation: (1) the air is cooled to dew-point
but its volume remains constant; (2) the volume
of the air is increased without addition of heat;
this cooling occurs because adiabatic expansion
causes energy to be consumed through work (see
Chapter 5); (3) a joint change of temperature and
volume reduces the moisture-holding capacity of
the air below its existing moisture content; or (4)
evaporation adds moisture to the air. The key to
understanding condensation lies in the fine
