. Practical physics. n contact with them isin a state of saturation, but also to lower the whole body ofair near the earth below this temperature, then the condensa-tion takes place not only on the solid objects but also on dustparticles suspended in the atmosphere. This constitutes a fog. 211. The formation of clouds, rain, sleet, hail, and the cooling of the atmosphere takes place at some dis-tance above the earths surface, as when a warm current ofair enters a cold region, if the resultant temperature is belowthat at which the amount of moisture already in the air issufficient to
. Practical physics. n contact with them isin a state of saturation, but also to lower the whole body ofair near the earth below this temperature, then the condensa-tion takes place not only on the solid objects but also on dustparticles suspended in the atmosphere. This constitutes a fog. 211. The formation of clouds, rain, sleet, hail, and the cooling of the atmosphere takes place at some dis-tance above the earths surface, as when a warm current ofair enters a cold region, if the resultant temperature is belowthat at which the amount of moisture already in the air issufficient to produce saturation, this excessive moisture im-mediately condenses about floating dust particles and forms aclo\id. If the cooling is sufficient to free a considerable amountof moisture, the drops become large and fall as rain. If thisfalling rain freezes before it reaches the ground, it is calledsleet. If the temperature at which condensation begins is be-low freezing, the condensing moisture forms into HYCtEO:\LETHY 175 When the violent air cnrrciits A\hich accompany thunder-storms carry the condensed moisture up and down severaltimes through alternate regions of snow and rain, hailstonesare formed. 212. The dew point. Tlie temperature to wJi/ch tJie atmospheremust he cooled in order that condensation of the tiater vaporuithin it may hegin is called tlie deiv p)oint. Thistemperature may be found by partly tilling withwater a brightly polished vessel of 200 or 300cubic centimeters capacity and dropping into itlittle pieces of ice, stirring thoroughly at thesame time with a dew point is the temperatureindicated by the thermometer atthe instant a film of moisture ap-pears upon the polished surface. In winter the dew point is usually „ ,_, , . ^ i. ^ ^ Fh;. 1<4. Apparatus tor deter- below freezing, and it will tliere- mining dew point fore be necessary to add salt to the ice and water in order to make the film appear. The experi-ment may be perf
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectphysics, bookyear1922
