Wednesday, January 31, 2018

If coastal cities need more rain...

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If coastal cities need more rain they could try heating water in greenhouses to add water vapour to the air. Here is a graph (below) using the average of six evaporation equations. You can get 20 or more times the evaporation by increasing water temperatures. 
Studies on evaporation from windy sea conditions show more rain with more evaporation of spray. 
In many cities you can evaporate over 100 000 litres every day with a 100m by 100m greenhouse and add it to the atmosphere, humidifying it and increasing chances of rain. 1kWh can evaporate about 1.5 litres and many cities can easily get about 8 kWh of solar energy on every horizontal square metre in a day.
When the air is hotter than the sea the water cools the air immediately above it and the relative humidity (RH) of this air increases and water can even condense out of the air so that "negative evaporation" occurs. If the seawater is hotter than the air the evaporation increases substantially, because the water heats the air above it, the RH of the air decreases, and the hotter air can take up moisture faster. This principle of high evaporation rate with water being hotter than air can be used to increase the humidity of the air and increase chances of rainfall. Therefore, heating seawater in greenhouses and so on can be very effective. Evaporation equations do not all give the same rate of evaporation and I use an average from six evaporation equations. Example: Wind speed=10 km/h, Air temperature above water is 32 deg C, temperature of water is 20 deg C, pressure = 1 atm. The average of the six equations is -1 mm per day so water condenses out and we could get a mist immediately above the water. Now we keep everything the same except we heat the water in a greenhouse to 37 deg C. The rate of evaporation (given by the average result of the 6 equations) is now 28 mm per day and we can increase the humidity of the air substantially, increasing chances of rain. Calculations show, if the area of greenhouses were one square kilometre, one could evaporate over 10 million litres a day in sunny areas.

Air tends to move back and forth with sea breezes and land breezes, so the humidity could accumulate every day from the greenhouse evaporation.

Here is another graph (below) where air temperature is increased by only 1 deg C. So, as the air heats more relative to the sea, so the evaporation from the sea decreases. Decreased evaporation means decreased chances of rain.

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