The question What is the second thermometer in the image from the Esperanza Antarctic temperature record? shows what might be a wet/dry bulb setup.
I've given one to a friend to measure humidity for fun (it's me who thinks it's fun). I'd learned about them in Earth Science class a zillion years ago, so when I happened to see one recently I grabbed it.
I understand the very basics of how it can indicate humidity; the drier the air, the faster water will evaporate into it and so the colder the wet bulb will be with respect to the dry bulb.
To get a numerical value for humidity with this set up you use the coarse table on the front (there's a finer table on the back of the box) and look up the dry bulb temperature and the dry minus wet difference to find an approximate humidity. For example If I look up 19 and 2 °C for those respectively (roughly what's shown in the image) I get about 81% relative humidity.
But suppose I hooked up a camera to a Raspberry Pi and imaged the two thermometers, processed the image and determined the two temperatures and wanted to calculate the relative humidity without interpolating the table.
Question: How is relative humidity determined from a wet and dry bulb readings? What are the steps and where's a good source for the equations and parameters involved?
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