Wednesday, July 4, 2018

Heat waves and decreasing air density by increasing RH

If there is a heat wave and you are near the sea there is something one could do. During heat waves relative humidity (RH) is usually low and this gives an ideal opportunity to add water to the atmosphere. If you add water by evaporative cooling you cool the air and then the air cannot hold so much moisture. If you add moisture by heating a part of the sea surface the air stays hot and the RH and water content of the air can increase a lot. You can heat a part of the sea surface by having floating black mats in the sea. Now when you add water vapour to the air the air becomes less dense. When the air is hot and dry you can add so much water vapour to the air that it becomes a lot less dense than the surrounding air and will rise, probably punching its way through the heat wave dome and creating clouds and possibly causing convectional rain. Example: Assume P=1 atmosphere (101.325 kPa). Assume the air temperature is T=38 deg C and RH=20%. Then the density of the air is1129.0 grams per cubic metre. Now increase humidity so that RH=80% with T still at 38 deg C.The density of the air is now 1112.2 grams per cubic metre. If you heated the original air with T=38 deg C and RH=20% to 42.71 deg C its RH would fall to 15.56% and it would have density of 1112.2 grams per cubic metre. This is the same density as the air with T=38 deg C and RH=80%. So increasing the humidity from 20% to 80% has the same effect on density as heating the air from 38 deg C to 42.71 deg C.

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