Humidity can often be a misunderstood concept due to the different ways in which it is measured. The most familiar of these measurements is probably relative humidity. Although it is most familiar, that does not mean it is the most understood. In fact, it’s probably the least understood.
Relative humidity is measured as a percentage and its name is appropriate because it truly is a relative measurement. Saying the humidity is 75% outside tells you nothing about how humid it actually feels. The percentage is relative to the current temperature. For example, a cold winter morning could have 100% relative humidity and the temperature could be 35 degrees. This just means the temperature of the air has dropped to the point of saturation. Now, on a warm summer night the humidity can also be 100% and actually feel incredibly humid. Let’s say the temperature in this situation is 75 degrees. In both situations, the relative humidity is 100%, but without knowing the temperature you have no idea what this humidity actually feels like. Clearly the cold winter night feels much less humid than the warm summer night, although both measurements of humidity are the same.
As you can see relative humidity and temperature are inversely related. Dew point is an actual measurement of the current moisture content of the air, not relative to the current temperature of the air.
So the next time someone says "the humidity is awful this afternoon," you will know that they are likely referring to the dew point even if they don't know it! Humidity is relative - dew point is not!
--William Churchill, MWN Intern
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