What Causes a Heat Wave?

In summer 2021, Oklahoma City, Oklahoma, didn’t record a single three-digit temperature. In summer 2022, it has recorded 19 days when the temperature hit 100°F or higher—and summer’s not even over yet.

Areas around the world are experiencing record heat, which often leads to discussion of heat waves. But what exactly does that mean, and what causes them?

According to the National Weather Service, a heat wave is a period in which higher-than-normal temperatures are recorded for at least two days. (This definition isn’t universal. The World Meteorological Society, for example, defines a heat wave as lasting five days at temperatures nine degrees higher than normal.) The extreme heat is caused by high pressure systems, which push warmer, compressed air down toward the ground, raising the temperature. These systems also decrease wind and cloud cover, reducing protection against the heat. High humidity often adds to the unpleasant experience.

A heat wave indicates when these high pressure systems are active. Elevated humidity raises the heat index, which assesses both temperature and humidity. The latter makes it harder to cool off because it slows the evaporation of sweat from the body.

A heat warning or heat advisory is a public health notice cautioning people that excessive heat could have adverse health effects, ranging from dehydration to heat stroke. An advisory typically urges people to drink plenty of water and avoid outdoor activities, at least during the hottest part of the day.

While most concerns over heat waves are rightfully focused on human safety, it’s important to remember that excessive heat can have a negative impact on infrastructure, too. Pavement can grow warped and power grids can be overwhelmed as people rush to use air conditioning.

If you need to cope with a heat wave, drink plenty of non-alcoholic fluids, wear light clothing, and seek a cooling center in your community if your residence doesn’t have AC.

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