Areas of the Lower 48 States With Hot Daily Low Temperatures, 1910–2008. EPA / Climate Change Indicators in the United States

This chart shows the percentage of the land area of the lower 48 states with summer daily low temperatures well above normal. The bars represent individual years, while the line is a smoothed nine-year moving average. Heat waves occurred with high frequency in the 1930s, and these remain the most severe heat waves in the U.S. historical record. Many years of intense drought (the “Dust Bowl”) contributed to these heat waves by depleting soil moisture and reducing the moderating effects of evaporation. Like the heat wave index, the percentage of the United States affected by heat waves has also risen steadily since the 1970s. The recent period of increasing heat is distinguished by a rise in extremely high nighttime temperatures.

Climate Change Indicators in the United States [pdf]