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364 Chapter 11 Hydrology: Rainfall and Runoff
Cyclonic storms are eddies in the vast planetary circulation between equator and
poles. They are generally several hundred miles in diameter, and their rotational and lateral
motions are both relatively slow. Cyclones are distinctly different from the violent whirl-
winds of tornadoes or hurricanes. In the central or low-pressure portion of cyclonic distur-
bances, moisture is precipitated from the rising air, while fair weather usually prevails in
surrounding high-pressure areas.
The storms of most importance on the North American continent as a whole originate
over the Pacific Ocean, strike the coast of the northern United States or Canada, swing
south and east over the central or northern United States, and generally escape through the
St. Lawrence Valley to the Atlantic Ocean. Storms born in the Gulf of Mexico drift north-
ward over the continent and out to sea.
The three types of precipitation—convective, orographic, and cyclonic—seldom
occur by themselves. Most precipitation in the temperate regions of Earth results from a
combination of two or more causes. Examples are (a) the rainfall of the Pacific slopes,
which accompanies cyclonic storms forced upward by the Rocky Mountains; and (b) local
thunderstorms, which are both cyclonic and convective in origin.
About a third of the water falling on continental land masses reaches the oceans as
runoff. The remainder is returned to the atmosphere by evaporation and transpiration. Of
the vaporized water, a small part is reprecipitated; but the major part is transported over the
oceans. Accordingly, the stream flow from continents represents the net water loss from
circulating air masses as they precipitate and reevaporate moisture in their course across
the land.

11.3.4 Droughts
Droughts are defined as periods when crops fail to mature for lack of rainfall or when pre-
cipitation is insufficient to support established human activities. Drought conditions pre-
vail when annual rainfall is deficient or poorly distributed in time; or when annual precipi-
tation is concentrated in a few heavy rainfalls that drain away rapidly. Droughts impose a
critical demand on (a) works designed to furnish a continuous and ample amount of water
and (b) streams expected to carry away domestic and industrial treated effluents without
nuisance. Statistical studies of past droughts suggest a strong tendency for self-perpetuation.
Some dry spells are ultimately broken only by a change in the seasons.

11.3.5 Measuring Precipitation
The cooperative observer stations of the United States measure rainfall in standard
cylindrical can gauges, 2 ft (0.60 m) high and 8 in. (200 mm) in diameter (Fig. 11.3). A
funnel-shaped inset receives and discharges the catch to a central measuring tube of such
diameter that 1 in. (25.4 mm) on a standard measuring stick inserted in the tube equals
0.1 in. (2.54 mm) of rainfall.
The official Weather Bureau stations register rainfall accumulations during short inter-
vals of time. Engineers base their designs of storm drainage systems and forecasts of flood
stages for streams on records of this kind. Continuous-recording gauges are in common
use. In the tipping-bucket gauge (Fig. 11.3b) a twin-compartment bucket is balanced in
such a way that the compartments fill, tip, and empty reciprocally. In the weighing gauge
(Fig. 11.3c), the rain concentrating from a receiving funnel is weighed on recording scales.
In both gauges, measurements are traced by movements of a pen on charts operated by
clockworks. The screen or shield shown in Fig. 11.3b keeps precipitation from being swept
past the gauge by high winds.