In my last posts (some time ago) I was happy that I had found a formula for the rate of water vapor transmission through a layer of still air. I have since learned that this information was hiding in plain sight in my 1985 ASHRAE Handbook of Fundamentals. It is called permeability. It is 120 grains per hour per square foot for a vapor partial pressure gradient of one inch of mercury per inch of layer thickness. There are 7000 grains per pound. After converting all these units to metric, I got close to the vapor permeability given in that internet download.

Now I have a plausible approximation for the rate of evaporation from solar-heated evaporator rafts placed in Washington rivers like the Methow and Twisp, where devastating wildfires have raged. That's nice, but now there's the question of what happens to the water vapor. How will it spread out? Where will it go? Will it make a useful change in the wildfire threat? Will the prevailing winds keep it close to the ground or will it rise into the wild blue yonder? Answer one question and a whole bunch more show up.