Chapter 15— METEOROLOGICAL ELEMENTS 



70 percent when there was an abundance of hy- 

 groscopic nuclei. 



The condensation which results when all three 

 mentioned conditions are fulfilled is in the form 

 of clouds, fogs, or haze. Fogs are merely 

 clouds on the surface of the earth. Fogs re- 

 ceive treatment in a separate section of this 

 chapter. 



In our industrial cities in which byproducts 

 of combustion are profuse, the distinction be- 

 tween smoke, fog, and haze is not easily dis- 

 cernible. A combination of smoke and fog gives 

 rise to the existence of the so-called 'smogs' 

 which are characteristic of these industrial areas. 



Little is known about the properties of sub- 

 limation nuclei, although it is believed that 

 they are essential for sublimation to occur at all. 

 It is assumed that sublimation nuclei are very 

 small and very rare, possibly of a quartz or 

 meteoric dust origin. All cirriform clouds are 

 composed of ice crystals and are believed to be 

 formed as a result of direct sublimation. Sub- 

 limation is the process whereby water vapor 

 changes into ice without passing through the 

 liquid stage. 



In the atmosphere, water clouds, water and 

 ice crystal clouds, and pure ice crystal clouds 

 may exist at the same time. 



Next under consideration is the cooling pro- 

 cess which may induce condensation. There are 

 several processes by which the air is cooled — 

 convective cooling by expansion, mechanical 

 cooling by expansion, and radiational cooling. 

 Any of the three methods may be working in 

 conjunction with another method, thus making 

 it even more effective. The methods are as 

 follows: 



1. Convective cooling. The ascent of a 

 limited mass of air through the atmosphere due 

 to surface heating is called thermal convection. 

 If a sample of air is heated, it rises (being less 

 dense than the surrounding air) and decreases 

 in temperature at the dry adiabatic lapse rate 

 until the temperature and dewpoint are the 

 same. This is the saturation point at which 

 condensation begins. As the parcel continues to 

 rise, it cools at a lesser rate, the moist/sat- 

 uration adiabatic lapse rate. (An adiabatic pro- 

 cess is one in which no heat is added to or taken 

 away from the mass of air by exchange with its 



environment during the process.) The parcel of 

 air continues to rise until the surrounding air 

 has a temperature equal to, or higher than, the 

 parcel of air. Then convection ceases. Cumuli- 

 form clouds are formed by this means with their 

 bases at the altitude of saturation and their tops 

 at the point where the temperature of the 

 surrounding air is the same as, or greater than, 

 the temperature of the parcel of air. 



2. Mechanical cooling. In this section, oro- 

 graphic and frontal processes for cloud for- 

 mation are discussed as follows: 



a. Orographic. If air is comparatively 

 moist and is lifted over mountains or hills, 

 clouds may often be formed. The type of cloud 

 depends upon the lapse rate (the rate of de- 

 crease in temperature with increase in height 

 unless otherwise specified) of the air. If the 

 lapse rate is weak (that is, a small rate of 

 cooling with an increase in altitude), the clouds 

 formed are of the stratiform type. If the lapse 

 rate of the air is steep (that is, a large rate 

 of cooling with increasing altitude), the clouds 

 formed are of the cumuliform type. 



b. In the previous chapter, you learned 

 that, at frontal surfaces, the warmer, less 

 dense air is forced to rise along the surfaces 

 of th e colder air masses. The lifted air under- 

 goes the same type of adiabatic cooling as air 

 lifted orographically. Likewise, the type of 

 clouds which results, depends on the lapse rate 

 and moisture of the warm air and the amount 

 of lifting. The latter is determined by the slope 

 of the front; if the slope is very shallow, the air 

 may not be lifted to its saturation point and 

 clouds will not form. When the slope is steep, 

 as with a fast-moving cold front, and the warm 

 air is unstable, clouds are of the cumuliform type 

 with great vertical extent. 



3. Radiational cooli'ng. At night the earth 

 radiates long-wave radiation, thereby cooling 

 rapidly. The air in contact with the surface is 

 not heated by the outgoing radiation but, instead, 

 is cooled by contact with the cold surface. This 

 contact cooling lowers the temperature of the 

 air near the surface, causing a surface inversion. 

 If the temperature of the air is cooled to its 

 dewpoint, fog and/or low clouds form. Clouds 

 formed in this manner dissipate during the day 

 due to surface heating. 



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