Chapter 15 — METEOROLOGICAL ELEMENTS 



particles falling out of a cloud but evaporating 

 before reaching the earth's surface as 

 precipitation. 



The Aerographer's Mate must learn to in- 

 fallibly recognize the various cloud types as 

 seen from the earth's surface. 



In figure 15-7 there i6 a view of all types 

 of clouds in a tier, each cloud type being shown 

 at its average height. 



Although one never sees all these types 

 at any one time in nature, quite frequently 

 two or three layers of clouds of different types 

 may be observed simultaneously. 



FOG 



Fog may be defined as a cloud on the earth's 

 surface; that is, visible condensation in the at- 

 mosphere of sufficient density to interfere with 

 marine and aerial navigation. Fog varies in 

 depth from a few feet to many hundreds of feet. 

 Its density is variable, resulting in visibilities 

 from near zero to several miles. It differs 

 from rain or mist in that its water or ice par- 

 ticles are more minute and suspended, and do 

 not fall earthward. 



The forecasting of fog is frequently a dif- 

 ficult job. In addition to knowledge of the meteor- 

 ological causes of fog formation, it is necessary 

 to have a thorough knowledge of local geography 

 and topography, for a slight air drainage may 

 be enough to prevent fog formation, or a quick 

 shift in the wind direction may cause fog to 

 cover an airport. 



The temperature to which air must be 

 cooled, at a constant pressure and a constant 

 water vapor content, in order for saturation 

 to occur is the dewpoint. This is a variable, 

 based upon the amount of water vapor present 

 in the atmosphere. The more water vapor pre- 

 sent, the higher the dewpoint. Thus, the dew- 

 point is really an index of the amount of water 

 vapor present in the air at a given pressure. 



The two manners in which the temperature 

 and dewpoint may be made to coincide are as 

 follows: 



1. By raising the dewpoint until It equals the 

 temperature. 



2. By lowering the temperature to the dew- 

 point. 



The first results from the addition of water 

 vapor to the air by evaporation from water 

 surfaces, wet ground , or rain falling through 

 the air. The second results from the cooling 

 of the air by contact with a cold surface under- 

 neath. 



Types of Fog 



There are several classifications of fog — 

 radiation fog, advection fog, upslope fog, and 

 frontal fog. 



RADIATION FOG. — Radiation fog, which gen- 

 erally occurs as ground fog, is caused by the 

 radiational cooling of the earth's surface. It 

 forms only at night and over a land surface. 

 It never forms over a water surface. This type 

 of fog usually covers a wide area. 



After sunset, the earth receives no heat 

 from the sun, but it continues to radiate heat. 

 The surface begins to cool because of this heat 

 loss. As the earth cools, the layer of air close 

 to the earth is cooled by conduction (the trans- 

 fer of heat from warmer to colder matter by 

 contact). This causes the layer near the earth 

 to be cooler than the air immediately above it, 

 a condition called an inversion. If the air be- 

 neath the inversion layer is sufficiently moist 

 and it cools to its dewpoint, fog forms. (See 

 fig. 15-8.) In case of a calm, this cooling 

 by conduction affects only a very shallow layer 

 (a few inches deep), because air is a poor con- 

 ductor of heat. Wind of low speed (3 to 5 knots) 

 causes slight, turbulent currents. This turbu- 

 lence is enough to spread the fog through 

 succeedingly deeper layers. As the nocturnal 

 cooling continues, the air temperature drops 

 further, more moisture is condensed, and the 

 fog becomes deeper and denser. 



After the sun rises, the earth is heated. 

 Radiation from the warming surface heats the 

 lower air, causing evaporation of the lower part 

 of the fog, thereby giving the appearance of 

 lifting. Before noon, heat radiated from the 

 warming surface of the earth destroys the in- 

 version, and the fog evaporates into the warmed 

 air. 



Radiation fog is common in high-pressure 

 areas where the wind speed is usually low 



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