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How is it that sometimes the relative humidity can be 100%, and yet there not be any precipitation; even fog? Is it that coalescence can't occur until a suitable cold temperature is arrived at, i,e. dew point? Thanks, Doug P.
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Relative humidity is one of the hardest things to understand in weather for sure. Coalescence doesn't have to do with RH though so we aren't going to get into that today.
When the RH is 0% you have totally dry air, at 100% the air is saturated. Air rises, it cools and condenses and forms clouds and rain. When this happens, the RH is 100% but the air that the rain is falling into, the air might not have an RH of 100%.
To start clouds forming you need the RH at 100% but remember, that's where the clouds are, where the rain is forming. Below those clouds, where we live, the RH is less than that.
So it can be raining outside when the RH is less than 100%. Once the layer of air is saturated, the RH should move up to 100%. You can also have high humidities on clear nights and temperatures cool down and get close to or equal the dew point. This would be another example of it not raining when the RH gets really high.
Humidity measures how much water vapor is in the air, not how much water vapor and liquid are together in the air.
Answered by KSL Meteorologist Dina Freedman.
