Extreme heat is the deadliest weather in the U.S., on average, more than any other weather hazard.

It's a startling tally

Heat claimed an average of 273 lives in the country each year from 2015 through 2024, according to the U.S. Centers for Disease Control and Prevention and NOAA using the most recent data available at the time this article was published.

That's more than double the number of next deadliest weather phenomenon (flooding) and it easily overshadows the annual death tolls from tornadoes, hurricanes and lightning.

Surprising?

When you think of dangerous weather, what first comes to mind?

You've probably seen video of damaging tornadoes, hurricane winds and storm surge, flash flooding and lightning.

Hurricane Katrina claimed an estimated 1,392 lives in the U.S., according to the National Hurricane Center.

The April 2011 Super Outbreak of tornadoes killed 324 in the Southeast. One month later, the Joplin, Missouri, EF5 tornado was blamed for 158 deaths.

But other than those outliers, heat deaths typically outnumber those from other hazards each year, as the graph below shows.

Flooding remains the number two killer. Flash flooding can occur almost anywhere in the U.S., and given the continued problem of flood deaths in vehicles, this is not surprising.

Tornado warnings and Doppler radar have reduced tornado fatalities markedly since the early 20th century. Since 2010, lightning deaths have plummeted, to 20 or fewer each of the past five years.

(MORE: Over 1,300 killed in Europe heat wave)

It's a different kind of 'storm'

Heat tends to be most impactful and deadly in the form of a heat wave, any period of anomalous and uncomfortably hot and humid weather that lasts several days to week or more at a time.

A heat wave doesn't have to shatter temperature records to be dangerous.

How long it lasts is at least as important as how hot it gets.

One reason for this is how little it may cool down at night, particularly in larger cities.

Heat absorbed by a city's concrete and asphalt during the day is released more slowly at night, which can make urban areas up to 22 degrees hotter than surrounding rural areas, a phenomenon known as the urban heat island.

In a typical summer heat wave, overnight low temperatures may dip only into the upper 70s or low 80s in a larger city in the Northeast, Midwest or South.

In this way, there may be no break in the heat at night, perhaps for days.

Those living in the heart of a city may not have air conditioning or are fearful to open windows at night for safety reasons.

A fan may be of little help, as that may only blast and recirculate hot, humid air.

(MORE: Affordable ways to keep cool if you don't have AC)

What can happen to the body

The body no longer can cool itself because perspiration can't evaporate from skin into humid air. Continued sweating eventually leads to dehydration.

As heat stress builds, heat exhaustion could eventually lead to heat stroke, as the body's temperature climbs above 103 degrees, requiring immediate medical attention.

Everyone is susceptible to heat exhaustion or heat stroke, but the elderly, infants and children are particularly vulnerable.

In July 1995, a heat wave pushed high temperatures into the mid-90s to mid-100s in Chicago for five days. It was accompanied by oppressive humidity. Overnight lows at Chicago's Midway Airport failed to drop below 76 degrees.

At least 465 deaths in Chicago were attributed to the heat wave, according to NOAA's National Disaster Survey Report. In all, NOAA concluded more than 1,000 died from this Midwest heat wave. Most of the victims were elderly and confined to their homes and apartments without air conditioning. Hospitals weren't prepared to deal with the large volume of patients.

The report found that even though temperature forecasts for the area were accurate, local government and services failed to view the situation as a threat to public health. As a result, there was no communication to the public regarding the dire consequences for sensitive groups.

(MORE: Should I exercise today in this heat?)

Since 1995, local governments have taken measures to prepare for heat events.

Better lines of communication exist between local governments, public safety officials and the local National Weather Service. There is also much better communication about preparing for the heat and what measures can be taken when it arrives.

Cooling stations and shelters are now set up in cities when excessive heat becomes an issue.

The National Weather Service unveiled an experimental HeatRisk product that breaks down potential heat impacts into categories of severity over the next seven days.

How to stay safe

The first line of defense against extreme heat is to be aware of the forecast.

The National Weather Service issues extreme heat watches and warnings when the combination of heat and humidity is expected to pose a danger. When a warning is issued, it's time to take action.

Here are some important safety tips from the CDC:

• Stay in air-conditioned buildings as much as possible.
  
• Don't rely on a fan as your main cooling source when it's very hot outside.
  
• Drink more water than usual and don't wait until you are thirsty to drink.
  
• Don't use the stove or oven to cook – it will make you and your house hotter.
  
• Wear loose, lightweight, light-colored clothing.
  
• Take cool showers or baths to cool down.
  
• Don't engage in very strenuous activities and get plenty of rest.
  
• Check on a friend, neighbor or family member, especially the elderly – and have someone check on you.
  

Jonathan Erdman is a senior meteorologist at weather.com and has been covering national and international weather since 1996. Extreme and bizarre weather are his favorite topics. Reach out to him on Bluesky, X (formerly Twitter) and Facebook.​