Why do weather reports sometimes say the humidity is 100 percent when there is no rain or snow falling?
Math in Meteorology
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A reading of 100 percent humidity usually means there is a high probability that rain will be or is occurring, but not always. It might be 100 percent RH because clouds are forming. If the RH near the ground is much less—for example, if a relatively dry air mass is in place—there will be no rain at the surface. This is why Doppler radar sometimes shows rain or snow in an area when none is actually reaching the ground.