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11.3 Precipitation 361
11.3.2 Orographic Precipitation

Precipitation is orographic when horizontal currents of moist air strike hills or mountain
ranges that deflect the currents upward. In North America the rainfalls of the Pacific
Northwest and the Southern Appalachians furnish examples of this type of precipitation.
The condensation as a function of temperature and elevation can be modeled by the
following equations:
-3
T = 60 - 5.5 * 10 H (U.S. Customary Units) (11.1)
c
c

-3
D = 54 - 1.1 * 10 H (U.S. Customary Units) (11.2)

c
c
Where T c is the air temperature in F, D c is the due point temperature in F, and H c is the
elevation in ft.
T = 15.55 - 10 * 10 -3 H (SI Units) (11.3)
c

c
D = 12.22 - 2 * 10 -3 H (SI Units) (11.4)
c

c
Where T c is the air temperature in C, D c is the due point temperature in C, and H c is the
elevation in m.
The condensation begins when the air temperature, T c , equals the dew point tem-
perature, D c .
Air cools at the dry adiabatic rate below the elevation H c and at the retarded adiabatic
rate above it. The temperature at the top of the mountain above the elevation H c is
T = 60 - 5.5 * 10 -3 H - 3.2 * 10 -3 (H - H ) (U.S. Customary Units) (11.5)
c
t
t
c
Since the descending air warms at the dry adiabatic rate, the temperature on the plain
becomes
-3
T = T + 5.5 * 10 ¢H (U.S. Customary Units) (11.6)
p
t
p
Where T t is the air temperature above H c (such as H t ) in F, H t is the elevation above H c in
ft, T p is the air temperature below H c (such as H p ) in F and H p is the elevation difference
below H t in ft.
The equivalent equations using the SI Units are
-3
T = 15.55 - 10 * 10 H - 5.83 * 10 -3 (H - H ) (SI Units) (11.7)
c
T = T + 10 * 10 ¢H (SI Units) (11.8)
t
c
t
-3
p
t
p

Where T t is the air temperature above H c (such as H t ) in C, H t is the elevation above H c in
m, T p is the air temperature below H c (such as H p ) in C and H p is the elevation difference
below H t in m.
The above equations (11.1) through (11.8) can be applied to coastal areas where the
dew point is lowered at a rate of about 1.1 F in 1,000 ft (2 C in 1,000 m).
EXAMPLE 11.1 CONDENSATION AS A FUNCTION OF TEMPERATURE AND ELEVATION
Coastal air with a temperature of 60 F (15.6 C) and a dew point of 54 F (12.2 C) is forced over a
mountain range that rises 4,000 ft (1,219 m) above sea level. The air then descends to a plain 3,000 ft
(914 m) below. If the dew point is lowered at a rate of 1.1 F in 1,000 ft (2 C in 1,000 m), find:
1. The height at which condensation will begin
2. The temperature at the mountain top
3. The temperature on the plain beyond the mountain assuming that condensed moisture
precipitates before the air starts downward.