Tuesday, April 18, 2017

Rain to the desert using hot rocks at night

The desert air is very hot and has a low relative humidity. But the desert air holds a lot of moisture because hot air does hold a lot of moisture even if the relative humidity is fairly low. When desert air cools down the relative humidity increases and at night is often high. At night, if we could cause rain, the rain would not evaporate as it is falling, because the air is cool and relative humidity is high.
Here is my idea for rain in deserts: With rocky areas, paint the rocks black (or so that they are dark). If there are no rocks then build  concrete or brick structures and paint them black so that they heat up during the day. Stone, brick and concrete have high volumetric heat capacities (a small volume can hold a lot of heat). At night the hot stone, brick or concrete will heat up air that has a high relative humidity. The air will rise and convectional rain will occur. The area painted black will have to be large, but a massive amount of air can be heated from a relatively small volume of rock. In fact rock has a heat capacity of about 0.8 kJ per kilogram per 1 deg C temperature increase. One cubic metre of rock weighs very roughly 2700 kg, so a cubic metre of rock has can store  of 2700x0.8=2160 kJ per cubic metre if its temperature is raised 1 deg C. One cubic metre of air can store about 1.2 kJ of energy if it is raised 1 deg C.
Air: 1.2 kJ per cubic metre per deg C
Stone: 2160 kJ per cubic metre per deg C (very roughly).
A variation could be to reflect sun, using mirrors, onto ordinary rocks, or rocks painted black, so the rocks heat up more.
Desert air can get very cold at night.
EXAMPLE: The desert air has temperature 40 deg C and at this temperature the relative humidity (RH) is 10%.
Night approaches and the temperature falls. As it falls RH increases
T             RH
35           13.1%
30           17.4%
25           23.3%
20           31.6%
15           43.3%
10          60.1%
5            84.6%
2.6         100%

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