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Chapter 1 THE PRECIPITATION PROCESS 1.1 Steps in precipitation process 1.1.1 Knowledge of the process of precipita- tion formation is required for a thorough under- standing of the definition and derivation of prob- able maximum precipitation (PMP) to be presented in snbsequent chapters. The basic steps leading to precipitation are: (1) sufficient atmos- pheric moistnre, (2) cooling of the air, (3) con- densation of water vapor into liquid or solid form, and (-1) growth of condensation products to pre- cipitation size. 1.2 Atmospheric moisture 1.2.1 'Water, in the form of vapor, is always present in the atmosphere. For convenience the amount of water vapor is often given in terms of inches of precipitable water (TV.), which is the depth of water that would be realized over a given area if all the water vapor in the air column above that area were to be condensed and precipitated on that area without loss. There is, of course, no natural precipitation process that will completely remove all water vapor from the atmosphere. :'\{easurements of TV Pl usually made by radiosonde, mnge from a small fraction of an inch to almost 3 inches, depending on the geographical location, meteorological situation, and depth of air column. A partial listing of maximum observed TV. values for several stations in the United States is COn- tained in Weather Bureau Technical Paper No. 10 [2]. 1.2.2 Periods of no rain and clear skies are usually associated with relatively low values of TV p. Cloudiness is usually observed at relatively high values of W.. When rain is falling, W. values are usually relatively high, but some of the highest amounts of TV. ever recorded were meas- ured when no rain was falling. It follows, there- fore, that other factors must act to produce cloudiness and precipitation. 1.2.3 It is perhaps unfortunate that the term 1noisture has been so carelessly used in 'hydrome- teorology. For Some reason moisture is generally understood to refer to water vapor only. Most dictionaries, however, define moisture as applying to the liquid fol'lh. The terms "atmospheric mois- ture," "moisture contentt "moisture charge," 2 "moisture supply," etc., as used in hydrometeoro- logical reports generally refer to water vapor only. They are so used in this report except where it is made clear that water in other forms than vapor is included. By definition W., of course, refers only to water vapor. Thus, water in the form of cloud droplets, raindrops, or ice crystals is not generally included in any evaluation of the above terms (never in TV.), although water in these forms is often present in the atmosphere in rela- tively large quantities (eh. 2). 1.3 Cooling of air 1.3,1 As stated in paragraph 1.2.1, there is al- ways some ,vater va,por, or Wp, in the atmosphere. Naturally, there is an upper limit to the amount of water vapor in a given mass or volume of air. This upper limit is a function of the air tempera- ture. For practical purposes, the air may be con- sidered to be saturated when it conta.ins the maxi- mum amount of water vapor, or TV., for its tem- perature. Lowering the temperature of the air will reduce its capacity for water vapor. Conse- quently, ail' of a given temperature having less than the maximum amount of water vapor for that temperature (in other words, unsaturated air) can become saturated without the addition of moisture if it is cooled down to the temperature for which the actual amount of water vapor present would produce saturation. The temperature to which air must be cooled, at constant pressure and con- stant water-vapor content, to effect saturation is called the dewpoint. Condensation (sec. 1.4) usually occurs at or near the saturation point.. 1.3.2 Air may be cooled by several processes, but adiabatic cooling by reduction of pressure through lifting is the only natural process by which large masses of air can be cooled rapidly enough to produce appreciable precipitation. The rate and amount of precipitation depend largely on the rate and amount of cooling and the rate of inflow of moisture into the. precipitation-produc- ing mechanism to replace the vapor that is con- densed aud precipitated. 1.3.3 The lifting required for the rapid cool- ing of large air masses can be produced either by (1) horizontal convergence of the atmosphere, (2)