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360 Chapter 11 Hydrology: Rainfall and Runoff
wells. It is comforting to know that subsurface water stored at useful depths of less than
2,500 ft (762 m) equals the total recharge of the ground during 16 average years.

11.2.5 Evaporation
The city of Boston made careful measurements of evaporation before 1885, and many
thousands of observations have been accumulated since the turn of the last century. Today
the Weather Bureau collects information from several hundred land pans, but monthly
measurements of Lake Mead (in Nevada and Arizona) evaporation comprise one of the few
regular reservoir studies.

11.3 PRECIPITATION
Atmospheric moisture precipitates in large amounts as rain, snow, hail, or sleet; it condenses
in small amounts as dew, frost, and rime. The most important causes of precipitation are ex-
ternal and dynamic, or internal, cooling; dynamic cooling implies the reduction in the
temperature of the atmosphere accompanying its expansion as air masses rise or are driven
to high altitudes. The observed drop in temperature is called the lapse rate. Within the tro-
posphere, up to 7 miles (11.26 km) above ground level in the middle latitudes, the lapse rate
is about 3 F in 1,000 ft (5.5 C in 1,000 m); but it is quite variable within the first 2 or 3 miles
(3.22 or 4.83 km) and may, at times, be negative. An increase in temperature with altitude,
or negative lapse rate, is called an inversion. Adiabatic expansion cools ascending air by
about 5.5 F in 1,000 ft (10 C in 1,000 m) if no moisture is precipitated; but if the dew point
is reached, the latent heat of evaporation is released, and the rate of the resultant retarded or
wet adiabatic rate of cooling drops to about 3.2 F in 1,000 ft (5.8 C in 1,000 m).
Air is stable and will not ascend by convection when the lapse rate is lower than both
the wet and dry adiabatic rates of cooling. Otherwise, its temperature would become less
and its density more than the temperature of its surroundings as it is moved into the higher
altitude. If the lapse rate is greater than the dry adiabatic rate, rising air becomes warmer
and lighter than the air along its upward path. Hence, it continues to ascend and remain un-
stable. If the lapse rate lies between the wet and dry adiabatic rates, the air remains stable
when moisture is not condensing but becomes unstable as soon as precipitation sets in.
This conditional stability is one of the requirements for successful rainfall stimulation. Dry
ice or silver iodide may then provide the nuclei that trigger precipitation and convert stable
into unstable air. Nevertheless, rainfall can be heavy only in the presence of a continuing
supply of moisture. In other words, cloud seeding becomes favorable only when atmos-
pheric conditions are already conducive to natural precipitation. Accordingly, seeding ap-
pears to hold out some, but not much, hope for rainmaking.
Moist air is moved upward principally by (a) convective currents to cause convective
rainfalls, (b) hills and mountains to produce orographic rainfalls, and (c) cyclonic circula-
tion to generate cyclonic rainfalls.

11.3.1 Convective Precipitation
Convective precipitation is exemplified by tropical rainstorms. Air masses near Earth’s
surface absorb heat during the day, expand, and take up increasing amounts of water vapor
with a specific gravity near 0.6 relative to dry air. The air mass becomes lighter; almost
exclusively vertical currents are induced and they carry the mass to higher altitudes where
it is exposed to colder surroundings and expands under reduced pressure. Under both
external and dynamic cooling, water vapor is condensed, and precipitation follows.