Why You Should Prefer Dew Points Over Relative Humidity

By: The Washington Post - Capital Weather Gang Email
By: The Washington Post - Capital Weather Gang Email

During July you’re likely to hear someone say, “it’s 90 degrees with 90 percent humidity out there today.”

When a meteorologist or a dedicated weather weenie hears the “90-degree heat with 90 percent humidity” refrain he or she winces and wants to say something like, “It never happens here. Let me teach you a little about what ‘humidity’ means.”
Here’s the gist of what a weather weenie acquaintance could tell you:
First, “humidity” refers to the amount of water vapor—the invisible gaseous form of water—in the air. Meteorologists have various ways of describing it.

Someone without much weather knowledge is usually referring to the relative humidity when he says “humidity.” It’s the most commonly reported measurement of humidity, although for most people most of the time it’s the least useful.

If you think relative humidity (RH) indicates the amount of water vapor in the air, then you’d think Washington’s air became less humid between 7:52 am. and 1:52 p.m. on the Fourth of July.
Anyone who went outdoors on July 4th realized that the air did not become less humid as the temperature warmed up.

Why dew point counts
Dew point is the temperature at which water vapor in the air will begin condensing to form dew on the grass, or mist and fog in the air. If the air is rising and cooling, its water vapor begins condensing into cloud drops when the air cools to the dew point. When the dew point and temperature are the same meteorologists say the air is saturated. That is, water vapor begins condensing out of the air if the air cools any further.

Relative humidity tells you how close the combination of temperature and humidity are to saturation, but it does not tell you the overall amount of moisture in the air. The 82 percent relative humidity at 7:52 a.m. tells you that the air was 82 percent on the way to condensation.
By 1:52 p.m. the air was only 65 percent along the road to condensation, because while the air contained just a little more water vapor than earlier, the temperature had increased.

Dew point is a good guide to comfort
Dew point is an indicator of the total amount of moisture in the air. Most people find dew points of less than 60 degrees comfortable. When the dew point rises above 65 degrees people will begin feeling that the air is “sticky.” As the dew points climb through the 70s the humidity bothers more and more people who are outdoors. The dew point at which the humidity begins to bother you is determined by whether you’re in the sun or the shade, if there is or isn’t a breeze, how long you’ve been outside, and how acclimated you are to hot, humid weather.

High dew points are uncomfortable because the air’s humidity slows the evaporation of perspiration from your body, which is nature’s way of cooling you off. The National Weather Service calculates the heat index, which doesn’t measure the actual air temperature, but rather what the humidity makes the air “feel like.” It’s also the best guide to the danger of heat illnesses. On July 4 the National Airport heat index was 96 degrees at 3:52 p.m.

Rising temperatures and dew points also mean your car, home, and office air conditioners have to work harder to cool the air and condense out more water vapor.

In the last few years more people have learned to use dew point to gauge their likely summer comfort thanks to the efforts of folks like Capital Weather Gang writers and broadcast meteorologists who regularly explain it.

A close look at ‘90-degrees and 90 percent humidity’
To have a 90 percent relative humidity with a 90-degree temperature the dew point would have to be 86.67 degrees. As far as anyone knows that has never happened and it’s not likely to happen in the Washington area. In a July 20, 2011 Capital Weather Gang article Jason Samenow wrote: “The best we can say is we know that we’ve had a dew point as high as 82.”

Places around the warm Gulf of Mexico are more likely to see dew points in the 80s. The Midwest also reports high dew points from time to time during the summer when air from over the Gulf of Mexico moves into the region. Here water vapor coming from large fields of corn and other crops is believed to help account for the high dew points by adding more moisture to the Gulf air. This water vapor comes from water the plants transport to their leaves where it evaporates. This water vapor source along with evaporation from other places such as lakes and rivers is called evapotranspiration.


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