In cities across the U.S., heat wave seasons are getting longer and more nights are hitting temperatures that put them in heat-wave territory. Scientists say climate change and urbanization both contribute to this nighttime trend.
Note: Daily minimum temperatures were used because scientists believe nighttime temperatures may have more impact on human health. Heat waves are two or more consecutive days with minimum daily temperatures at or above the 85th percentile of July and August minimum temperatures, using 1961-1990 as a base period and calculated separately for each location — similar to a method used by a scientific study. The heat wave season is defined as the number of days between the first and last heat wave of each year. In Fayetteville, some years contained missing data; those years were omitted from the analysis.
Source: Analysis of NOAA National Climatic Data Center data. Credit: Elisabeth Gawthrop, Columbia Journalism Investigations / Center for Public Integrity