WHEN MOISTURE COMES OUT OF THE AIR 



35 



holdjas much water vapor as before, and some of the 

 water vapor will condense as clouds, fog, dew, or rain. 



Water pipes and iced-water pitchers are very cold, 

 and if the air contains a large amount of moisture they 

 cool the air immediately surrounding them below the 

 dew point, and some of the moisture condenses on 

 their cold surfaces. 



Let us study Figure 57 to get a clearer picture of 

 this process as it works in nature. The pans represent 

 the water-holding capacity of air at different tempera- 

 tures. Think of the pan at the top as being about 75 

 per cent filled. If for some reason this same amount of 

 water were to be poured into the next lower pan and 



present just fills the capacity at that temperature. This 

 is the dew point for that day and any further cooling 

 will cause some of the water to come out just as it 

 runs over when poured into pan D. The water may 

 come out as dew, fog, or rain. If this takes place be- 

 low the freezing point, 32 degrees R, it will be snow 

 or sleet. 



jvVhat is the water cycle?! Moisture goes through 

 a complete cycle in nature. By reference to Figure 58 

 you can see this cycle. The cold dry air, being heavy, 

 falls toward the earth, is warmed, and as a result its 

 water-holding capacity is increased. As it comes in 

 contact with bodies of water, moisture is absorbed 



Left CIRRUS 

 Right CUMULUS 



Left STRATUS 

 Right NIMBUS 



then into the next, it would more nearly fill each one 

 because each is a little smaller than the one above it. 

 Finally a pan would be reached that would just be 

 filled by the amount of water with which we started. 

 See C in Figure 57. If the water were to be poured 

 into the next pan some would have to run over the 

 sides as at D. This is similar to what happens in na- 

 ture. At a given temperature the air can hold only a 

 certain amount of moisture, and if on a certain day at 

 that temperature it is holding 75 per cent of its ca- 

 pacity, we say the relative humidity is 75 per cent. 

 If for some reason the air is cooled, the relative hu- 

 midity will rise because the capacity has decreased. 

 This is like pouring the water from the top pan into 

 the next one below. If the cooling continues, a tem- 

 perature will be reached where the amount of water 



and the warm, light, water-laden air begins to rise. 

 As it goes higher the pressure becomes less and less 

 and it begins to expand. Now you will remember 

 from your experiment on fogs that a sudden expan- 

 sion of air cools it and as this rising air current ex- 

 pands it is soon cooled below the dew point. Some 

 of the moisture will then condense as clouds. If cool- 

 ing continues, more and more moisture condenses un- 

 til finally the droplets become too large to float longer 

 and they fall back to the earth as rain. The dry air, 

 now cool and heavy, begins again to fall toward the 

 earth or repeat this never-ending cycle. 



What causes the moisture that comes out of the au- 

 to assume different forms such as clouds, dew, fog, 

 rain, hail, and snow? Clouds and fogs are caused when 

 the atmosphere is cooled below its dew point. There 



