Heat Waves: Weather's #1 Killer

heat wave sun horizon
Heat is the #1 weather killer in the U.S. Steve Krongard/The Image Bank/Getty Images

If you had to guess which weather event is the most hazardous of all, which would you pick? Tornadoes? Hurricanes? Lightning? Believe it or not, heat waves — prolonged periods of abnormally hot and humid weather that last for three or more days up to several weeks, typically during the summer season — kill more people in the United States on average per year than any other single weather disaster.

How Hot Is a Heat Wave?

Also called excessive heat or extreme heat events, heat waves are characterized by higher than normal temperatures, but just how high depends on where you live. That's because what's normal temperature-wise for one location will be different than what's normal for another. In other words, there is no one set of criteria for how hot temperatures must be before being called a heat wave. Instead, you'll find that what qualifies as a heat wave in one region differs from that in another. For example, the National Weather Service in Milwaukee, WI issues heat wave warnings whenever the ​heat index (an estimate of how hot it feels from the heat and humidity combined) reaches 105°F or higher during the day and 75°F or higher at night for at least 48 hours. On the other hand, temperatures in the 90s would be warm enough for the makings of a heat wave for places like Seattle, WA.

Despite the local nature of this criteria, whenever you hear the term "heat wave" mentioned it typically refers to events where temperatures, even in the shade, reach 90°F or more.

High Pressure Brings the Heat

Heat waves form when high pressure in the upper atmosphere (also known as a "ridge") strengthens and remains over a region for several days to weeks.

This most commonly happens during the summer season (from May to November in the Northern Hemisphere) when the jet stream "follows" the sun.

Under high pressure, air subsides (sinks) toward the earth's surface. This sinking air acts as a dome or cap that allows heat to build up at the surface rather than allowing it to rise. Since it can't lift, there's little or no convection, clouds, or chance of rain -- only warm and dry weather conditions.

The Hazards of Too Much Heat

Uncomfortably high temperatures and humidity aren't the only hazards associated with heat waves. Watch for these as well:

  • Heat Illness: Extreme heat makes it difficult for our bodies to maintain a safe internal temperature of 98°F. This is why prolonged exposure to heat and humidity poses a significant health risk and can lead to life-threatening heat-related illness like heat exhaustion, heat cramps, and heat stroke.
  • Poor Air Quality: Sinking air helps trap pollutants and ozone near the surface, making the air we breathe unhealthy.
  • Fire Weather: The dry, windy conditions associated with a dome of high pressure readily contribute to an increased risk of the start and spread of wildfires.
  • DroughtDrought and heat go hand in hand. Heat waves are often either the reason why drought conditions begin or why they worsen if they're already in place. 

    Expect More Heat Waves in Our Warming World

    Scientists warn that it is very likely heat waves will occur more often, and when they do occur, will last longer as a result of Earth's present global warming. Why? A rise in global average temperatures means you're starting from a warmer baseline. This boosts the odds that temperatures during the warm season will be that much higher.

    Now that you know what to expect from heat waves, here's how to keep yourself safe when they arrive.

    Edited by Tiffany Means

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    Your Citation
    Oblack, Rachelle. "Heat Waves: Weather's #1 Killer." ThoughtCo, Aug. 16, 2016, thoughtco.com/heat-waves-weathers-number-one-killer-3443910. Oblack, Rachelle. (2016, August 16). Heat Waves: Weather's #1 Killer. Retrieved from https://www.thoughtco.com/heat-waves-weathers-number-one-killer-3443910 Oblack, Rachelle. "Heat Waves: Weather's #1 Killer." ThoughtCo. https://www.thoughtco.com/heat-waves-weathers-number-one-killer-3443910 (accessed January 18, 2018).