Heat Waves Are the Most Deadly Weather Events

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 anywhere from three days to several weeks— 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 "normal" temperatures differ based on the region. 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, sustained temperatures in the 90s would be warm enough to qualify as a heat wave in places like Seattle, WA.

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 or 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 global warming. Why? A rise in global average temperatures means you're starting from a warmer baseline. This usually means that temperatures during the warm season will be that much higher.

Edited by Tiffany Means