It’s the Heat and the Dew Point!

by Tom Ross

Well, the pattern changed on a dime over the late spring. We got a good dose of much-needed rainfall in April and May. Most places across Western North Carolina received between six–nine inches of precipitation in April alone, and that has turned our deficit of precipitation for the year into a small surplus. We will have to monitor the situation over the next several months and see where we go.

June on average has high temperatures in the low 80s with an average low temperature within a few degrees of 60. June, July, and August comprise our “Meteorological Summer,” which is the warmest three-month period on average during a given year. We should also notice an increase in the dew point temperatures, which is the measure we use to denote how “muggy” or “uncomfortable” it feels outside. This increase in the dew point will be most noticeable in July and August.

Dew Point

The humidity comfort level is the actual dew point temperature, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly. The chance that a given day will be uncomfortable or muggy in Asheville increases very rapidly during June, rising from 12% to 40% over the course of the month. For reference, on July 26, the muggiest day of the year, there are muggy conditions 59% of the time.

Weatherspark

The humidity information above was taken from an internet site called “WeatherSpark.” This site offers a unique way to graph, plot and analyze the weather data for the Asheville area. See their site online for more info at weatherspark.com.

Unfortunately, the relative humidity is relative to the air temperature. As the temperature changes, so does the relative humidity, and it is not a good method for measuring comfort at the warmest and coolest time of the day. So when you hear someone say “It’s not the heat, it’s the humidity” they are actually half right. The best measure for comfort is the actual dew point temperature.

Turning to the rest of summer, the long range forecast from NOAA’s Climate Prediction Center (cpc.noaa.gov) for the June through August period predicts slightly above normal for temperatures and about normal for precipitation.

However, as you know, precipitation is a tricky forecast here in the mountains during the summer, due to the spotty nature of thunderstorms that drench some areas while other spots remain dry.

Meteorologist Tom Ross managed the Climate Database Modernization Program at the National Climatic Data Center.

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