Heat waves in the United States: definitions, patterns and trends

Abstract

High temperatures and heat waves are related but not synonymous concepts. Heat waves, generally understood to be acute periods of extreme warmth, are relevant to a wide range of stakeholders because of the impacts that these events have on human health and activities and on natural environments. Perhaps because of the diversity of communities engaged in heat wave monitoring and research, there is no single, standard definition of a heat wave. Experts differ in which threshold values (absolute versus relative), duration and ancillary variables to incorporate into heat wave definitions. While there is value in this diversity of perspectives, the lack of a unified index can cause confusion when discussing patterns, trends, and impacts. Here, we use data from the North American Land Data Assimilation System to examine patterns and trends in 15 previously published heat wave indices for the period 1979-2011 across the Continental United States. Over this period the Southeast region saw the highest number of heat wave days for the majority of indices considered. Positive trends (increases in number of heat wave days per year) were greatest in the Southeast and Great Plains regions, where more than 12 % of the land area experienced significant increases in the number of heat wave days per year for the majority of heat wave indices. Significant negative trends were relatively rare, but were found in portions of the Southwest, Northwest, and Great Plains.

Fig. 1

Regional division of the Continental United States (CONUS)

Fig. 2

a Average daily T_max (°C) over the time period 1979–2011. b Standard deviation of T_max over the time period 1979–2011. c 95th percentile of T_max(°C)

Fig. 3

1979–2011, annual average number of heat wave days. Note the varying scales. Results for HI01–HI15 are shown by a–o, respectively

Fig. 4

Trends in the number of annual heat wave days, over the period 1979–2011. White areas indicate results below 95 % significance. Units are days/year

Fig. 5

Average 95 % significant trends in the number of annual heat wave days, over the period 1979–2011, divided by region. The value printed in each cell is the trend value (in days/year). The color of the cell represents positive (red) and negative (blue) trends. The shades of red and blue represent the landmass percentage covered by this significant trend given by the scale bar. Regions are: Northwest (NW), Southwest (SW), Great Plains (GP), Midwest (MW), Southeast (SE) and Northeast (NE)