Friday, April 26, 2019

Rain from dry and wet air layers

When wind blows up a mountain the air cools at about 9.8 deg C for every 1000 m rise (the dry adiabatic lapse rate for air that is not saturated). However, if the air is moist, one might have pockets where condensation occurs and this releases heat of condensation, keeping the air warmer. 
What sometimes happens is that one has a layer of moist air under a layer of drier air and as it blows up the mountain the top dry layer cools at 9.8 deg C for every 1000 m rise and the moist layer below might cool at an average of, say, 8 deg C per 1000m m rise.
So the bottom moist layer is continuously getting warmer relative to the top dry layer. This causes the bottom layer to continuously get less dense than the top layer and so the bottom rises by convection as well as by being blown up the mountain. The bottom layer could rise high enough by this method for rain to form, even if the mountains are low.
It does not take much to heat moist air, but it does take a lot of heat to evaporate water to make air moist. So if one could heat the air just above the sea, where it is moist, by infrared radiation (evaporation of spray would also occur), then one could have more rain even with low mountains. One could do this using solar power to heat dark objects on land and then reflect the infrared heat from the dark objects to the air above the sea surface. When the wind blows to land rain could result even with low mountains.
From
https://journals.ametsoc.org/doi/pdf/10.1175/1520-0442%281991%29004<1023%3AHPOTO>2.0.CO%3B2 
it appears that the RH is usually greatest at about 940 mb (about 600 m above the ocean). If one could increase RH to about 90% in the first 500 m or so above the ocean the above method could work well. 


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