Tuesday, May 23, 2017

Rain and relative humidity

The drought is much discussed in the media every day now and from my own research (drawing graphs of sea temperature related to land temperature) I have observed that very often when sea temperatures are higher than land temperatures rain occurs. This agrees with theory that says most evaporation into the air from the sea occurs when the sea is warmer than air. This is because if the air is warmer than the sea the sea cools the air above it and the relative humidity of the air becomes higher. Because less evaporation occurs into air with higher relative humidity the air will not take up so much moisture by evaporation. Why does the relative humidity of air become higher when it cools? Well imagine you are in a house and you warm the air in the house. The air expands and some of it goes out via windows, etc. Now imagine the warming of the air inside the house occurs rapidly and the air does not have time to mix with the colder air when it goes out windows, etc. Then the components of the warm air are the same and the mass of each of the components (water vapour, nitrogen, etc) is the same. This means that the mole ratio of each of the components is the same in your heated air as it was before heating. Now the pressure inside the house quickly becomes the same as the pressure outside or air would rush in or rush out. The pressure (partial pressure) of each component such as water vapour is the total pressure (which remains the same) times the mole ratio of that component (which remains the same). The partial pressure of water vapour is therefore the total pressure (which remains the same) times the mole ratio of the water vapour (which remains the same).
So the vapour pressure in the heated air inside the house is the same as the vapour pressure of the vapour before heating. Now relative humidity is the ratio of vapour pressure to the vapour pressure there would be if the air was saturated with water vapour. The latter can be found using saturation vapour tables. The latter changes (increases) with rising temperature so the ratio (vapour pressur)/(saturation vapour pressure at that temperature) decreases with rising temperature. Saying it again, looking at weather reports you will see that relative humidity usually decreases as temperature rises (unless moisture comes in), because the saturation vapour pressure increases as temperature rises, ie constant vapour pressure/increased saturation vapour pressure is a smaller ratio.
My solution is to use spray pumps over the sea (operated by wave motion) to increase evaporation over the sea and to use solar air heaters on all rooftops to get the air to rise when it comes off the sea. This will cause convectional rain. To build your own solar air heater see http://www.builditsolar.com/Experimental/PopCanVsScreen/PopCanVsScreen.htm

Note that the temperature of the Arabian Gulf waters can be well over 30 deg C, but there is little rain. In the Arabian Gulf the temperature of the water is low relative to land air temperatures during the day.

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