– How can humidity in the air be measured as a percentage? How much humidity is 100%?
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It’s not quite that air has a fixed amount of water it can hold. At the point where liquid water touches air, there are two things going on; some of the liquid molecules have enough energy to escape and become gas, and some gas water molecules hit the surface of the water and go back to being a liquid. The hotter the environment, the more likely the liquid molecules are to evaporate. The more water there is in the air, the more likely a gas water molecule will hit the surface and go back to being a liquid.
Because the rates of those processes are difficult to measure, we measure the vapor pressure as a stand-in for the rate of evaporation, and the partial pressure of water as a stand-in for the rate of condensation. Relative humidity is how those two pressures relate to each other; a partial pressure of 10% means that the partial pressure of water in the air is 10% the vapor pressure of water at that temperature.
Over 100% humidity means that water is condensing faster than it is evaporating. This is when fog happens; the water in the air starts to condense with itself to form small water droplets. However, this condensation does take time, so the atmosphere does spend some time over 100%
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