More Extremely Hot Days

Late July is the hottest time of year for much of the country. In the midst of this summer heat, we calculated the number of extremely hot days each year in 244 cities across the country and found that 73 percent experience more extremely hot days than they did a half-century ago. 

Texas is the state that leads the way, which is not surprising since a previous analysis showed summer is the season heating up the fastest there. On average, compared with a half-century ago, there is an additional month of temperatures above 100°F in Austin and above 95°F in Houston. Cities in the Southeast have seen surges in extreme summer heat, too. Augusta, Shreveport, and Tallahassee all have an additional three weeks worth of days above 95°F compared to 50 years ago.

Both rural and urban locations have been getting hotter as the world warms from increasing greenhouse gases in the atmosphere, but cities are further made hotter by the urban heat island effect. Concrete, bricks, and pavement absorb heat during the day and release it at night, which is why cities remain warmer than rural locations. And with the stagnant air of the summer doldrums settling in, there is not much breeze to help cool people, meaning the body does not get a chance to recover from the heat of the day which increases the risk of heat-related illnesses like heat stroke. 

METHODOLOGY: Data was gathered via the Applied Climate Information System. An “extremely hot day” is defined as the highest 5°F increment temperature that occurred at least three days in more than half of the years of the study period (since 1970). Change in number of days is based on the linear slope of the data trend.