AEROGRAPHER'S MATE 3 & 2 



Spray and Blowing Spray 



Spray consists of water droplets torn by 

 the wind from a substantial body of water, 

 generally from the crests of waves, and carried 

 up a short distance into the air. Blowing spray 

 occurs when the spray is raised in such quan- 

 tities as to reduce the visibility at eye level 

 (6 feet on shore, 33 feet at sea) to 6 miles or 

 less. 



Precipitation Theory 



Precipitation is an outgrowth of condensa- 

 tion. The condensation theory is discussed in 

 this chapter under "Cloud Formation." The 

 main concern at the present time is with the 

 growth of condensed water droplets and their 

 subsequent descent to the earth's surface. 



Several theories have been formulated in 

 regard to the growth of raindrops, and all of 

 them have some validity. The theories which are 

 most widely accepted today are treated here in 

 combined form. 



Raindrops grow in size primarily because 

 water exists in all three phases in the atmosphere 

 (see chapter 3), and the air is supersaturated 

 at times (especially with respect to ice) due 

 to adiabatic expansion and radiational cooling. 

 This means that ice crystals coexist with liquid 

 water droplets in the same cloud. The difference 

 in the vapor pressures between the water drop- 

 lets and the ice crystals causes water droplets 

 to evaporate and then to sublimate directly onto 

 the ice crystals. Condensation alone does not 

 cause droplets of water to grow in size. The 

 turbulence in clouds permits this droplet growth 

 process and aids it. After the droplets become 

 larger, they start to descend and are tossed 

 up again in turbulent updrafts within the cloud. 

 The repetition of ascension and descension 

 causes the ice crystals to grow larger (by water 

 vapor sublimating onto the ice crystals) until 

 finally they are heavy enough to fall out of the 

 cloud as some form of precipitation. It is be- 

 lieved that most precipitation in the midlatitudes 

 starts as ice crystals and that most liquid pre- 

 cipitation results from melting during descent 

 through a stratum of warmer air. It is generally 

 believed that most rain in the Tropics forms 

 without going through the ice phase. 



In addition to the above process of droplet 

 growth, simple ACCRETION is important. The 

 droplets colliding with other smaller ones simply 



hold the smaller ones, and the droplets thereby 

 accumulate more layers. During the growing 

 process, the droplets which form the outer 

 layers are frozen onto the larger droplets. This 

 process of accretion is thought to be especially 

 effective in the growth of hail. There are, to 

 be sure, other factors which explain in part 

 the growth of precipitation, but the aforemen- 

 tioned processes are the primary ones. 



CLOUDS 



A cloud is a visible mass of minute water 

 droplets, or ice particles, suspended in the at- 

 mosphere. It differs from fog in that it does not 

 reach to the surface of the earth. Clouds are 

 a direct expression of the physical processes 

 which are taking place in the atmosphere. An 

 accurate description of both type and amount 

 plays an important part in the analysis of the 

 weather and in forecasting the changes which 

 are taking place in the weather. 



Cloud Formation 



To be able to thoroughly understand clouds, 

 the Aerographer's Mate must know the physical 

 processes which form clouds. 



Three conditions must be met before clouds 

 can form as a result of condensation — presence 

 of sufficient moisture, a cooling process, and 

 hygroscopic or sublimation nuclei in the atmos- 

 phere. Moisture is supplied to the atmosphere 

 by evaporation and is distributed horizontally and 

 vertically by the winds and vertical currents. 

 The first task is to consider the hygroscopic 

 and sublimation nuclei. 



Hygroscopic nuclei are particles of any na- 

 ture on which condensation of atmospheric mois- 

 ture occurs as a liquid. It can be said that hy- 

 groscopic nuclei have an affinity for water or that 

 they readily absorb and retain water. The most 

 effective hygroscopic nuclei are the products of 

 combustion (sulfuric and nitric acids) and salt 

 sprays. Some dust particles are also hygroscopic, 

 but not effectively so. As has been stated, the 

 presence of hygroscopic nuclei is a must. Water 

 vapor does not readily condense without their 

 presence. Air has been supersaturated in labora- 

 tories to over 400 percent before condensation 

 began when there were no hygroscopic nuclei 

 present. On the other hand, condensation has 

 been induced with relative humidities of only 



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