Dry Heat vs. Oppressive Heat

"Heat Index" will Be Tough This Weekend (Source: WUSA9)

As Washingtonians endure their sixth heat wave of 2016, some may say the current heat wave is the worst.  Even if temperatures don’t reach the century mark during the next several days, the high “heat index” temperatures are what’s making the current heat wave especially dangerous.  The “heat index” is the apparent or feels-like temperature when you combine the air temperature with relative humidity.

The higher the relative humidity is, the worse high temperatures feel.  Humidity is relative to a given temperature and meteorologists use the dewpoint temperature to calculate RH.  Dewpoint temperatures above 70 degrees are indicative of an oppressive air mass.  A natural cooling mechanism is evaporation of one’s sweat.  However, whenever it’s more humid out, the body can’t effectively cool itself and this makes the heat especially dangerous.    

Such is the case during the current heat wave because heat indices the next several days are going to be above the century mark across the majority of the Mid-Atlantic Region.  Although records aren’t kept for high heat index temperatures, triple-digit heat indices are somewhat rare in the Mid-Atlantic Region.  One of the highest heat index temperatures observed in the Nation’s Capital according to the National Weather Service occurred on July 16, 1980 when an air temperature of 103 degrees and a dewpoint temperature of 78 produced an astonishing heat index of 122 degrees.

The hottest days in the Nation’s Capital are when the wind blows primarily from the west.  That’s a slightly drier wind as opposed to southerly winds that usher in more humid air.  The official weather reporting site for Washington, D.C. is National Airport and a southerly wind there tends to keep temperatures slightly cooler as the wind blows off the Potomac River.  Southerly winds are poised to keep the oppressive humidity levels in place until early next week.  The more humid conditions so common in the eastern United States during the summer months serve a purpose, too, in making triple-digit heat relatively rare.  More humid air may feel worse, but higher relative humidity also prevents the air temperature from rising as high as a drier air mass.

Unlike the arid desert Southwest where triple-digit heat is a daily occurrence during the summer, triple-digit heat occurs only every few years in the Nation’s Capital.  Prior to July 25, it hasn’t been 100 degrees in Washington, D.C. since 2012.  Minimizing time spent outdoors and drinking extra water is especially important during such extreme heat waves.  My colleagues and I on the WUSA9 weather team will keep you apprised of the latest weather conditions both on-air and online.