A closer look at dangerous June heat

 

A summer's day in the nation's capital

The hottest weather of the year will soon get underway in the nation’s capital. High temperatures early next week will reach the mid to upper 90s across the DMV (DC, Maryland and Virginia). That will be similar to last year’s record-setting June heat wave that brought the nation’s capital its first triple-digit heat in nearly eight years. It’s worth taking a closer look at the terms “heat wave” and “extreme heat.” 

A ”heat wave” is loosely defined as three or more consecutive days when high temperatures reach the 90s. Extreme heat, meanwhile, can be one or two very hot days without meeting the heat wave criteria. For example, a heat wave occurred in the nation’s capital from June 7 – June 12, 2011. However, there were only two extremely hot days during that six-day stretch with highs of 99° and 102°, respectively, on June 8 and 9. A record high of 99° also occurred on June 17, 2022, but that was the only 90-degree day of the week.

June 1959 finished on an exceptionally hot note in the nation’s capital with highs of 100° on both June 29 and June 30. Making this extreme heat stand out is how it remains the only time triple-digit heat has occurred on consecutive June days in the nation’s capital. Triple-digit heat qualifies as extreme heat whether or not it occurs during a heat wave.

A similar example occurred in June 1925 when the nation’s capital experienced an eight-day heat wave from June 1 – June 8. A shorter period of extreme heat occurred within that heat wave when DC experienced five consecutive record highs from June 2 – June 6. DC’s record high of 100° on June 5, 1925, remains the earliest in the calendar year triple-digit heat has ever occurred. Helping illustrate how unusual such extreme early June heat is that all these record highs still stand a century later. 

The June 1925 heat wave is also remarkable since the nation’s capital was much less developed then. There is much more urbanization and automobile traffic now and that enhances the urban heat island effect. Overnight low temperatures remain warmer which makes heat waves more extreme. For example, DC’s average low temperature during the five-day period of June 2 – June 6, 1925, was 70.8°. By comparison, the average low temperature during the five-day period of June 22 – June 26, 2024 was 75°.

Next week’s heat wave will feature potential record-challenging heat in the DMV. Consequently, it will be important to drink plenty of water and take lots of breaks in air conditioning to prevent heat-related illness.