A Staff Report from the Straight Dope Science Advisory Board

What's the "dew point" all about?

November 1, 2001

Dear Straight Dope:

What's up with dew points? For years we were conditioned to say, "It ain't the heat, it's the humidity." Now it's more like "It ain't the humidity, it's the dew point." Were we wrong all along?

Not to worry, Bernie. The saying "It's not the heat, it's the humidity" is as valid as ever. Some people think dew point is more precise, though. Here's what the USA Today Web site has to say on the subject (www.usatoday.com/weather/whumdef. htm):

Humidity is the amount of water vapor in the air and can be described in different ways, including "relative humidity," which is the term used most often in weather information meant for the public. Relative humidity is the amount of water vapor in the air compared with the amount of vapor needed to make the air saturated at the air's current temperature.

The dew point temperature gives a much better estimate of the amount of moisture actually present in the air, which is very important in determining precipitation amounts and even how comfortable you feel. Let's compare very cold, 10-degrees Fahrenheit air with a relative humidity of 100% with 72-degree tropical air, also with 100% relative humidity. In both cases, the relative humidity is 100%, but the cold air's dew point is 10 degrees and the warm air's dew point is 72 degrees. The higher the dew point temperature, the more moisture in the air.

Well, duh. Does this mean high humidity is more pleasant at lower temperatures? We beg to differ. In our opinion, dew point isn't really an indication of how comfortable things are--the relative humidity tells you that. Rather, it indicates the likelihood of a change in the weather. At the dew-point temperature, the relative humidity is 100% and atmospheric water vapor--suspended humidity--condenses and makes fog, clouds, dew, rain, or some other form of precipitation. What you need to pay attention to is the difference between the dew-point temperature and the current and expected temperatures. If it's a steady 70 degrees F now and the dew point is 55 degrees, chances are the weather is going to be stable. On the other hand, if the dew point is 68 and there's a cold front coming in, it's a pretty sure bet something is going to happen. Exactly what depends on a lot of factors. But the dew point gives you an idea of how close you are to the edge.

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