Monday, August 12, 2013

How humid is it? Why dew point is a superior measure of humidity

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.
This brings us to the superior measurement of moisture in the air, known as dew point. Dew point will tell you exactly how humid the air is regardless of the temperature. It is a measure of "absolute humidity," or the actual moisture content in the air.  In our previous example, since the relative humidity was 100%, the dew points were equal to the temperatures, meaning the air was saturated in both cases. But a dew point of 35 degrees is very different than a dew point of 75 degrees! Just like higher temperatures equal hotter conditions, the higher your dew point is the more humid it feels, because the amount of moisture in the air is higher. A dew point in the 70’s, which is very uncomfortable to most people, can be common on a southern summer day. The dew point may not change much during the day, remaining in the 70’s. Our example demonstrated exactly how useless relative humidity can be when you want to know how moist the air actually is. This is because relative humidity gets close to 100% just about every night, while it gets lower during the day as the temperature rises - even if the dewpoint does not change! In other words, relative humidity is always changing.

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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