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362 Chapter 11 Hydrology: Rainfall and Runoff

Solution 1 (U.S. Customary System):
1. When condensation starts, T c D c , and
T c 60 5.5 10 3 H c
D c 54 1.1 10 3 H c
6 4.4 10 3 H c
Hence, H c 1,360 ft
Air cools at the dry adiabatic rate below this elevation and at the retarded adiabatic rate
above it. When H c 1,360 ft, T c D c 52.5 F
2. The temperature at the top of the mountain is
T t Temperature 60 1,360 5.5 10 3 (4,000 1,360) 3.2 10 3
44 F
3. If the descending air warms at the dry adiabatic rate, the temperature on the plain becomes
T p Temperature 44 3,000 5.5 10 3
60.5 F

Solution 2 (SI System):
1. When condensation starts, T c D c , and
T c 15.55 10 10 3 H c
D c 12.22 2 10 3 H c
15.55 12.22 (10 2) 10 3 H c
Hence, H c 416 m
Air cools at the dry adiabatic rate below this elevation and at the retarded adiabatic rate
above it. When H c 416 m, T c D c 11.4 C
2. The temperature at the top of the mountain (1219 m) is T t
T t 15.55 10 10 3 H c 5.83 10 3 (H t H c )
3
15.55 (10 10 ) 416 5.83 10 3 (1,219 416)
6.7 C
3. If the descending air warms at the dry adiabatic rate, the temperature on the plain becomes
T p T t 10 10 3 H p
6.7 10 10 3 (914)
15.8 C

11.3.3 Cyclonic Precipitation

Cyclonic precipitation is associated with unequal heating of Earth’s surface and buildup of
pressure differences that drive air from points of higher to points of lower pressure. Major
temperature effects are (a) temperature differences between equator and poles, producing
so-called planetary circulation; and (b) unequal heating of land and water masses, forming
secondary areas of high and low pressure on sea and land and consequent atmospheric
circulation.
Differences in Earth’s relative rotary speed between equator and poles deflect
tropical air currents moving toward the poles. This explains the general easterly direction