Nine of the 10 warmest years on record for the 48 contiguous states have occurred since 1998, resulting in more intense heat waves each year. In the last decade, the number of extreme heat days rose, with rates in some states five times higher than 40 years ago. Due to what’s known as the urban heat island effect, this increase is magnified in urban areas, where over 80% of Americans live.
Extreme heat is generally defined as temperatures that are much hotter than the historical record in the region. Technical definitions vary at the county, state, and national levels.
The Environmental Protection Agency (EPA) defines extreme heat as a consecutive period of two or more days in which the recorded temperature from May through September exceeds the 90th percentile of the historical record. Government officials use the terms extreme heat and heat wave interchangeably.
In 2021, there was a median of 15 extreme heat days across all US counties. That's compared with a median of three days of extreme heat in 1979. The number of extreme heat days is increasing over time. In the 1980s, there were three years where the median for extreme heat days was 10 or more. In the 2010s, each year’s median for extreme heat days exceeded 10 days.
The recent spike in heat waves can be largely attributed to the rise in annual temperatures across the United States. Six of the 10 largest increases in average temperatures for the lower 48 states occurred in the last decade. Similar heating trends occurred in Alaska and Hawaii as well, according to state climate data.
Since the 1970s, the number of heat waves has increased in every state. Maine, Montana, and Wyoming are the three states with the coldest average temperatures between May through September, not including Alaska. These three states went from an annual median of nine extreme heat days in the 1980s to 13 in the 2010s, as measured across all counties within each state. Montana and Wyoming have both had their hottest years on record within the last decade, in 2015 and 2012, respectively.
This warming trend is stronger in the southern states. The median number of heat waves for the three hottest contiguous states, Louisiana, Florida, and Texas, rose from five days per year in the 1980s to 25 days through the 2010s, as measured across all counties within each state.
Heat waves tend to occur more frequently in urban spaces compared with surrounding rural landscapes due to the unique microclimates created by cities. This phenomenon is called the urban heat island effect.
Factors such as roads and rooftops storing heat from the sun, high density of traffic, and a lack of shade from trees to absorb heat and release moisture help create this effect.
As a result, urban areas can experience mid-afternoon temperatures that are up to 20 degrees warmer than surrounding rural areas during periods of extreme heat. As of 2010, more than 80% of Americans live in urban areas, according to Census data.
The frequency of heat waves across US cities has increased steadily over the past several decades. The average number of heat waves per year in metropolitan areas has increased from two days per year to six, according to data from the EPA. Cities included in the EPA data consist of the 50 largest metropolitan statistical areas (MSAs) with daily temperature data extending back to 1961.
The EPA also looked at heat wave seasons across the same 50 metropolitan areas. Heat wave seasons are measured by the number of days between the first and last heat waves each year.
In the 2010s, 46 of the 50 metropolitan areas in the data experienced a significant increase in the number of heat waves compared with the 1960s. Heat wave seasons lasted longer too over that time. The average heat wave season in 2021 was roughly 50 days longer than in the 1960s for these areas.
Contiguous states include all states with a common border.
Relative temperature thresholds are compared to the historical temperature at the county level.
CDC data on heat and heat-related illness does not include counties in Alaska or Hawaii.
Both the coolest and hottest states are determined by the 5-month average temperature from May through September using NCEI historical data. Region definitions are based on the Census.
A microclimate is a suite of climatic conditions affiliated with a localized area near the earth’s surface. Environmental variables – including temperature, light, wind speed, moisture, etc – can all differ from the surrounding area due to certain ecological conditions.
The CDC defines a metropolitan statistical area (MSA) as an urbanized area of 50,000 or more inhabitants.
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