I understand that when water is exposed to atmosphere, it dries quicker. Why does it make somuch a big difference though? For example, if it just rained, the whole parking lot will be dry a few hours later, but areas under an umbrella or a tree will form puddles. It will also take it soo much longer for these areas to evaporate. Is there really that much more air flow when things are not under a cover?
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A cover does two things. First, it stops sunlight from getting down to the ground. This makes the ground cooler which makes water evaporate slower. Second, it stops water vapor from rising up. If water vapor can’t pass through the cover then it will condensate on the cover and drip back down. So it’s not just about air flow, it’s about stopping the movement of sunlight and water.
When you put a cover over something you are creating a pocket of stagnant air. The air is NOT free to mix with the outside atmosphere.
Air can only hold so much water at any given temperature. Since the air is stagnant, the water will evaporate, but be trapped inside the cover increasing the water content. Eventually the water content under the cover will be so high that the net evaporation will reach zero, so that the thing you are trying to dry out is not losing any more water.
Unless you have a very good seal, the cover will allow some transfer of outside (and presumably less humid air), and the object will eventually dry out. If you do not have a cover, then natural air movement will allow fresh dry air to be in contact with the object allowing it to dry faster.
If you’re trying to dry something in a 100% humid environment, good luck with or without a cover.
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