Wednesday, March 15, 2017

Rain when sea temperatures are higher than air temperatures over land?

Some time ago I made and showed graphs indicating there was often rain when sea temperatures were higher than land air temperatures. Yesterday I came across this graph in a physical geography book showing more rain when sea temperatures were higher. I do not know if the author noticed this as well - cannot see where he/she mentions it. See
Something that will give credibility to my idea is the graph below. For my calculations I have used the following conditions: 
1) Temperature of air over the land is 26 deg C. 
2) There is a 2 m/s wind over the sea. 
3) The relative humidity of the land air is 50% and pressure = 100 kPa. 
4) The air blows from the land to the sea and at the sea surface where evaporation occurs, the air cools or warms to the temperature of the sea temperature. 
5) The variable is the sea temperature. 
a) When Tsea=14 deg C the air becomes saturated and condensation occurs (I have used a 100% RH for this on the graph), the evaporation rate from the sea surface is 0 microns of water per hour. 
b) When Tsea=16 deg C, the RH of the air over the sea surface is 92.5% (RH increases when T decreases), the evaporation rate is 55 microns per hour. 
I have put the RH and the evaporation rate in microns on the same vertical axis (not really correct, but convenient). So Tsea runs from 14 deg C to 34 deg C, the RH runs from 100% to 31.6% (see graph), the evaporation rate runs from 0 (at 14 deg C) to 1532 microns of water per hour (if the water were in a basin, water levels would drop from 0 microns per hour when T=14 deg C, to 1532 microns per hour when T=36 degC). As one can see, when sea temperatures are high relative to land temperatures, more evaporation occurs. I used an equation similar to the "Evaporation from a water surface" equation by Engineering toolbox to calculate evaporation rates (they depend on relative humidity and temperature and wind speed).

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