What is Extreme Heat?

Extreme heat refers to high temperatures that are significantly hotter and/or humid than average in a given area. Average temperatures and humidity levels vary significantly from place to place; therefore the definition of extreme heat depends on what is considered average for a particular location.

Extreme heat is dangerous because it can lead to heat stroke, dehydration, and other serious health problems. Heat has been the most deadly extreme weather phenomenon in the U.S. over the last 30 years. Certain types of people are more prone to being injured by extreme heat. These people include the elderly, young children, people with mental illnesses, and people with chronic diseases.

Heat illness is caused when the body heats up significantly faster than it can cool down through sweating. If the body heats up too much without being able to cool down, then it can result in damage to the internal organs, and in extreme cases, death.

What Causes Extreme Heat?

Extreme heat is caused by heatwaves. Heatwaves occur when systems of high atmospheric pressure, also called anticyclones, move into an area and become trapped for two or more days. When this happens, air from the upper levels of the atmosphere is pulled toward the ground and becomes compressed, hotter, and dryer. This layer of sinking air acts as a heat dome, trapping heat on the ground that has been previously absorbed by the landscape. The high pressure system also drives out cooler air currents and clouds, allowing the sun to bake the landscape and increase temperatures to extreme levels.

Heatwaves can last anywhere from several days to several weeks and they most commonly occur in the summertime. In the northern hemisphere, the effects of solar radiation build cumulatively during the longest days of the year. This causes temperatures to peak in July and August several weeks after the summer solstice. However, the geography and climate of some regions of the U.S. cause them to differ significantly from this average; monsoon season in New Mexico and Arizona causes hot temperatures to peak in June and the effects of the marine layer along the Pacific coastline typically delays the hottest temperatures to September or October.

Recent Heatwaves in the U.S.

Many of the worst instances of extreme heat in U.S. history are from the dust bowl era of the 1930s due to the effects of poor land practices and years of intense drought. However, heatwaves in the U.S. in the last few decades have been particularly severe due to effects of climate change. Here are some notable recent examples of extreme heat:

  • Summer 2021: Heat dome caused 7 Western states to reach all time record high temperatures, hundreds of deaths
  • 2012: Considered the worst drought since the 1930s, effects lasted for months and caused 34.8 billion in economic disruption
  • Summer 1998: Estimated 5,000 dead of the effects of heat and 4.4 billion in economic damage, 11 states declared natural disasters
  • Summer-Fall 1980: Heatwave concentrated in the Central and Eastern U.S. with an estimated death toll of 10,000

Which Areas are the Most Affected By Increased Heat Risks?

Heatwaves occur across America, but the Western and Southern states are typically the most heavily affected by dangerous heat. In addition, the impacts of heatwaves are frequently felt more heavily in densely populated urban areas such as large cities. Buildings and asphalt absorb and radiate heat significantly more than plants and trees which makes urban areas particularly dangerous.

Since 1979, many cities have seen an increase in the amount of days that feel 90 degrees or hotter. This indicates a heightened chance of severe heatwaves in these areas. Ten U.S. cities that have seen the most dramatic increases in hot weather are:

How is Climate Change Increasing Heat Risk Across the U.S.?

Since the 1960s, heatwaves have become significantly more frequent, more intense, longer, and have occurred across a wider seasonal range. The accumulation of greenhouse gases in the atmosphere due to global warming is responsible for this increase. These gases absorb sunlight and solar radiation that have bounced off of the earth which traps in heat and leads to an overall warming of the earth’s atmosphere. Temperatures are projected to increase by at least 5 degrees Fahrenheit in most places by mid century and 10 degrees by the end of the century.

Heat - Projected increase by state in number of hot days - days with extreme heat

What are the Effects of Extreme Heat?

Heat and heatwaves are the leading cause of weather-related deaths in America. They can cause devastating harm to farming and industry and can result in disruptions in infrastructure and electricity. They also exacerbate the effects of other damaging natural disasters like drought and wildfire by creating hot and dry conditions.

The high daily and nightly temperature and high humidity of heat waves are very dangerous for the elderly, infants and children, those with other existing health problems, or other vulnerable populations. Low income Americans can also be at risk when unable to access air conditioning and other cooling services. Heatwaves occurring in the off-seasons of spring and fall can be particularly damaging because people are often unprepared and therefore more likely to suffer increased exposure to the health risks associated with heat waves.

Heatwaves also cause damage to agriculture, property, and infrastructure. Heat-stressed animals are in danger of sickness or death. Although all food production can suffer from extreme heat, some plants which require cool nighttime conditions or specific temperatures during their growing season are particularly vulnerable. The energy grid during heat events experiences increased electricity demand for cooling, but heat can also reduce the ability of power lines to carry energy. This combination can lead to dangerous and deadly power outages. For other types of infrastructure and property, the increased risk of drought and wildfire associated with extreme heat conditions causes significant harm.

Heat Risk Mitigation Options for Homeowners

Protecting your property and the health of your family from extreme heat is becoming an increasingly important consideration. You may want to consider getting air conditioning in your home as a preventative measure, even if you are outside an area that is typically subject to high heat. Below are some suggestions for mitigation tactics you can employ against dangerous heat conditions.

Create a preparedness plan.

  • Establish a plan for your family in the case of extreme heat, especially if you lack access to air conditioning
  • Identify and help vulnerable individuals within your community in the case of high temperatures including: the elderly, infants and children, those with health conditions, outdoor workers, and low income residents
  • Advocate for and set up cooling centers in community buildings for those who lack cooling access and implement other community planning measures in the case of a serious heat wave

Reduce the urban heat island effect by installing heat resistant home upgrades. 

  • Install green or cool roofs made of highly reflective or light-colored materials and cool pavement options which provide more solar reflectance; these options can help reduce temperatures and energy demand drastically
  • Increase canopy cover and vegetation around your home to reduce heat exposure and help prevent drought-induced soil dryness

Pursue energy efficiency. 

  • Energy efficiency reduces demand on the electricity grid during heat waves which can decrease the risk of a black out.


Extreme heat is a very serious weather phenomenon that is getting significantly worse due to climate change. Over the next several decades and beyond, it will be important to advocate in your community for infrastructure changes, energy efficiency, and care for the most vulnerable to mitigate the increasingly dangerous effects of extreme heat.

See the ClimateCheck property report for more information on your personal heat situation.

Read More About Heat Risk and Climate Change