A boy cools off at the Yards Park in Washington D.C., the United States, on July 21, 2019. The highest temperature reached 37 degrees Celsius in Washington D.C. on Sunday as a result of a prolonged heat wave. (Xinhua/Liu Jie)

People cool off at the Yards Park in Washington D.C., the United States, on July 21, 2019. The highest temperature reached 37 degrees Celsius in Washington D.C. on Sunday as a result of a prolonged heat wave. (Xinhua/Liu Jie)

Children cool off at Yards Park in Washington D.C., the United States, on July 21, 2019. The highest temperature reached 37 degrees Celsius in Washington D.C. on Sunday as a result of a prolonged heat wave. (Xinhua/Liu Jie)

Children cool off at the Yards Park in Washington D.C., the United States, on July 21, 2019. The highest temperature reached 37 degrees Celsius in Washington D.C. on Sunday as a result of a prolonged heat wave. (Xinhua/Liu Jie)

Ducks rest by the water near Capitol Hill in Washington D.C., the United States, on July 21, 2019. The highest temperature reached 37 degrees Celsius in Washington D.C. on Sunday as a result of a prolonged heat wave. (Xinhua/Liu Jie)

A tourist poses for photos by the water near Capitol Hill in Washington D.C., the United States, on July 21, 2019. The highest temperature reached 37 degrees Celsius in Washington D.C. on Sunday as a result of a prolonged heat wave. (Xinhua/Liu Jie)