Thursday, December 29, 2016

Rain for deserts near cold seas

Deserts near cold sea, such as the Namib Desert, appear to me to be ideal candidates for rain making. With the Namib there is a good strong sea breeze bringing in air from the sea as land warms up, and this breeze starts fairly early in the morning. Sometimes mists form and when the sun heats these mists up the water droplets evaporate, thus cooling the air. The cool air from the sea and the cooling by evaporation cause a stable situation - that is the air does not rise because of cold air beneath warmer air, or same temperature air. If the air were warmer than the surrounding air, it would rise. Looking at the air around Walvis Bay, one may note that relative humidities are high, but lack of rising air, that would cool on ascent to form rain, causes dry conditions. Because air requires contact with warm surfaces to heat up and only makes fairly superficial contact with hot ground, there is no efficient heating of the air. But if one could use huge solar air heaters in the Namib desert (there is plenty of space) one would have air rising and hopefully causing rain. Using average sorts of figures from Wikipedia for Walvis: Jan T=17.5 deg C, RH= 80%. Now if the air is cooler than surroundings, it will not rise. Say the surrounding air is at 17.5 deg C. Now heat air up to 22.5 deg C, using huge solar air heaters, and we find that the air could rise 1515 m and it only needs to rise 1060 m before clouds form (used general sort of lapse rates and Espy's equation). The high relative humidity makes for very good rain prospects. These are general figures and actual lapse rates would have to be determined, but things look good. It might be interesting to note that whilst air does not significantly heat up with solar radiation (it has to be in contact with a hot surface), clouds do heat up with sunlight. In fact clouds absorb all thermal infrared and at Earth's surface roughly 50% of "sunlight" is infrared radiation. Mist is a sort of low level cloud. So clouds or mist will heat up, water droplets will evaporate, cooling things, and so on.
See also on why Saudi Arabia is dry.
I must point out that when temperatures are low the air cannot hold much water vapour. The graph below shows the maximum number of metric tons 1 cubic kilometre of air can hold at different temperatures (number of metric tons when the air is saturated).

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