WRAL WeatherCenter Blog

It’s not the Heat, it’s the Humidity, I mean Dewpoint . . .

Posted June 2, 2006 10:22 a.m. EDT

It seems like we have made a quick transition from Spring to Summer, but is wasn’t so much a change in temperature as it was a change in the moisture in the air. Our daytime highs were running on the warm side, but with dewpoints in the upper 40s to lo 50s, a daytime high in the mid to upper 80s isn’t too uncomfortable. Dewpoint, versus Humidity, is a more precise measurement of moisture in the air. Relative humidity, tells us how much water there is in the air as a percentage of the maximum possible amount. The dewpoint is the quantity meteorologists actually measure to determine the relative humidity. OK, so what is dewpoint? Well, if we cool air without changing its moisture content, eventually we will reach a temperature at which the air can no longer hold the moisture it contains. Then water will have to condense out of the air, forming dew or fog. The dewpoint is this critical temperature at which condensation occurs.

How does the amount of moisture in the air impact how uncomfortable it feels outside? Well, the reason why more humid air feels warmer to us is that we keep cool by evaporating moisture from our skin (perspiration). Evaporation is a cooling process and the higher the humidity, the less evaporation that occurs and the warmer we feel.

Also, with less moisture in the air we will see a much more significant cool down at night. Spring and Fall, when the air is drier, we see a much wider range in temperatures from daytime highs to nighttime lows.

So, as we look for different ways to say “Hazy, Hot & Humid” this summer, remember, it’s not the heat, it’s the dewpoint!