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lifting associated with horizontal convergence, (2) frontal lifting, (3) orographic lifting, (4) heating of the base of an air column, and/or (5) radia- tional nighttime cooling of cloud tops. The lifting processes were explained in paragraphs 1.3.3.- 1.3.6. The methods by which instahility may be induced thermally are not difficult to undershmd. The heat supplied by the ground to the base of an air mass by conduction acts to produce steep lapse rates in the daytime. The steepest lapse rates from this source usually occur in the afternoon when the ground is warmest. The high incidence of after- noon thundershowers is an indication of the effectiveness of this source of instability. Night- time thundershowers, on the other hand, often result from the steepening of the lapse rate in clouds by radiational cooling of the cloud tops while the bases are still receiving heat radiated from the ground. 1.4 Condensation of water vapor into liquid or solid form 1.4.1 One of the most important steps in the production of precipitation is the condensation process by which the water vapor in the atmos- phere is converted into liquid droplets or, at low temperatures, into ice crystals. The results of the process are often, but not always, visible in the form of clouds, which are nothing more th..n air- borne liquid water droplets or ice crystals, or a mixture of the two. In the United States the heavier intensities of rainfall have their origin in clouds composed of both water drops and ice crystals (par. 1.5.3). 1.4.2 Saturation does not necessarily result in condensation. Condensation nuclei are required for the conversion of water v..por into droplets. Among the more effective condensation nuclei ..re certain products of combustion and salt p..rticlea from evaporated sea spray. There are usually suf- ficient condensation nuclei in the air so that it is gener..lly assumed that condensation of water va- por takes place when the air reaches the satura- tion point. 1.5 Growth of cloud droplets aud ice crystals to _ pre<':ipitation size 1.5.1 When air is cooled to below its initial saturation or condensation temperature, and con- densation continues, the liquid droplets or ice crys- tals tend to accumulate in the resulting cloud as the temperature is lowered. The rate ..t which this excess liquid ..nd solid moisture is precipit..ted from the cloud depends on (1) the speed of the upward current producing the cooling, (2) the 4 rate of growth of the cloud droplets into raindrops heavy enough to bll through the upward current, and (3) a sufficient inflow of water vapor into the precipitation-producing area to replace the pre- cipitated moisture. 1.5.2 IVater droplets in an average cloud usually average about 0.0004 in. in radius and weigh so little that ~n upward current of only 0.5 ft./min. is sufficient to keep them from falling. Although no definite drop size can be said to m..rk the boundary between cloud and raindrops, a. radius of 0.004 in. has been generally accepted. The radius of most raiudrops re..ching the ground is usually much gre..ter th..n 0.004 in. and m..y re..ch one-eighth in. Drops larger than this tend to break into smaller drops because the surf..ce tension is insufficient to withstand the distortions the drop undergoes in falling through the air. Drops of one-eighth in. radius h..ve .. terminal velocity of about 30 ft./sec., or roughly 20 mi/hr., so that an unusu..lly strong upward current would be required to keep a drop of that size from falling. 1.5.3 Various theories have been advanced in attempts to explain the growth of cloud elements to precipitation sizes. According to Houghton [3] the two principal processes in the formation of precipitation are the ice-crystal and accretion processes, whieh may operate separately or in com~ bination. The ice-crystal process involves the presence of ice crystals in a supercooled (cooled to below freezing) water cloud. A vapor-pressure gradient from water drops to ice crystals exists because the saturation vapor pressure over water is greater than that over ice. Hence, the ice crystals grow at the expense of the water drops and, under fa vorable conditions, attain precipitation size. The ice-crystal process is operative only in super- cooled w..ter clouds and is most effective at about _150 C. (50 F.). 1.5.4 The accretion, or collision, process is based on the relative velocities of fall and the con- sequent collisions to be expected between cloud elements of different sizes. The rate of growth by accretiou depends upon the initial range of particle sizes, the size of the largest drops, the drop concentration, and the sizes of the collecting and collected drops. Studies [4] suggest that the electric field and drop charge may affect collision efficiencies and may be important factors in the release of precipitatiou from clouds. The accre- tion process operates at any temperature, and its effectiveness is different for solid and liquid particles.