This year we have had severe drought over much of the US and it is appropriate to consider the application of solar-heated evaporator rafts to enhance rainfall. The design that seems to work is a dark absorber plate suspended just below the water surface. Solar radiation is absorbed by the dark surface and heats the thin layer of water above the absorber plate. The water surface reaches a higher temperature compared to the temperature for an unmodified water surface where the solar radiation penetrates to many meters depth and heats a much larger volume of water. The enhanced temperature increases the rate of evaporation, resulting in greater humidity of the air passing over the raft. Depending on where this moistened air goes and what processes it undergoes, we might get to see increased precipitation where we need it. So where might we put evaporator rafts ?
To overcome my profound ignorance of climate and weather processes, I have joined the American Geophysical Union (AGU) and the American Meteorological Society (AMS), which is a copublisher of "The AMS Weather Book: The Ultimate Guide to America's Weather". The author is Jack Williams, who was weather editor for USA TODAY for 23 years before becoming outreach coordinator for AMS in 2005. From this book and many atlases and textbooks, I learn that the Bermuda High is a high pressure area that moves from its winter location near Portugal to the central Atlantic in summer. Bermuda is much closer to the US east coast than the center of this high pressure area, so why is it called the Bermuda High? Perhaps because people on Bermuda observed the seasonal change in air pressure long before there were complete maps of pressure over the entire ocean basin. Air tends to flow radially outward from the summer location of the high pressure area, but the Coriolis force turns it into a clockwise flow around the high. The southern portion of the flow crosses the Louisiana-Texas (LATEX) Gulf Coast on its way to the central US.
The LATEX Gulf Coast might be a good location for evaporator rafts. This is the location of the summer dead zone where the oxygen concentration is very low. The rafts would shade the water below the solar absorber plates and this might reduce the anoxic condition. I'm not sure where the wind goes after it crosses the LATEX coast, but our son Mike lives in southern Wisconsin and swears that he can smell the sea air coming from the Gulf. The AMS Weather Book is a delightful mix of anecdotes, career descriptions, encouragement for women and students to enter the field of meteorology, illustrations, and instruction. You have to do some poking around to get the information that you need, so it will take some time to figure out whether evaporator rafts might help with the drought. I also subscribed to the AMS bimonthly magazine "Weatherwise". You need all the folklore you can get to play in this game.
We're not confined to the seacoast. There are plenty of inland lakes and waterways that may be better evaporator locations for a large country like the US. For example, a marine magazine had an article on the low water level in inland waterways caused by the drought. There was a map of the system composed of the Upper Mississippi, Illinois, Missouri, Ohio, Tennessee, Arkansas, Lower Mississippi, and Tombigee Rivers. These extend from the Great Lakes to the Gulf. There are many inland lakes, many of them very large. There should be many ideal locations for evaporator rafts to enhance precipitation throughout the Midwest. Fresh water evaporates just as well as seawater, and we don't have to contend with ocean storms.
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