California is suffering a severe drought, with many out-of-control wildfires. The important agricultural area of the Central Valley is irrigated by water from reservoirs along the Colorado River whose levels are at historic lows due to insufficient and early-melting snow pack. Meanwhile, the San Francisco Bay area usually has moderate temperature and high relative humidity, and is often foggy. So why don't we get rain? My guess is that there is no way to get the ground-level water vapor and cloud droplets to rise high enough to condense and make rain. The Google Earth view of the area shows very little dark surface which could act as a solar absorber to heat the air from below and make it expand and rise.
The air in this area is often moving at greater than 10 mph, so any artificially created hot spot would have to be spread out in the wind direction in order for a parcel of air to be heated for an adequate time. The wind direction at San Francisco International Airport is often in a west-to-east direction. The width of the bay at this location is about 10 miles, so if a solar absorber could be stretched across the full west-east width of the bay, a parcel of air moving parallel to the absorber at 10 mph could be heated for an hour.
I have previously described solar evaporator rafts composed of wood panels 6 inch by 18 inch with wood cross pieces apportioned to float the black absorber surfaces just below the water surface. If these are deployed upside down with sufficient cross pieces for flotation, the top surface, painted black, would remain dry. We would use neutrally buoyant synthetic fiber ropes to string the panels together, and avoid the use of metal cables which would weigh the panels down. The previously described deployment workboats carrying the rafts on submerged reels would be used. The solar absorber rafts crossing the bay can be interrupted with navigation channels for maritime traffic.
The solar input to the absorber surface would be balanced by convection heat transfer to the air passing over the absorber, plus radiation exchange with surfaces and substances remote from the air near the absorber, plus conduction losses into the water, plus evaporation of water splashed on the absorber surface. I will initially ignore the conduction losses and splashing and assume that the radiation exchange is with a surface at temperature Ta, where Ta is the ambient air temperature. The radiation is a loss. The convection heat transfer is the useful output that heats the overlying air. I will use a spread sheet to try different absorber temperatures Ts until I get a match between the solar input and the sum of the convection and radiation heat transfer.
The convection heat transfer is the useful output that heats the overlying air. I will assume that this is a layer that has the height of the atmospheric boundary layer, which is well mixed by turbulence. I believe that it is about 1000 meters on a sunny day. I will compute the temperature rise for heating at constant pressure, the volume expansion, and the resultant buoyancy force. I eventually want to figure out how far the heated parcel of moist air will rise, but I will need to learn more about atmospheric convection to get there.
