Heat waves can form when a high-pressure system moves into a region and stalls.
The system can force warm air downward, acting like a cap as it keeps the cool air near the surface from rising, according to AccuWeather.
What qualifies as a heat wave can vary greatly from region to region.
In the northeast or Great Lakes region, a heat wave is generally considered to include three straight days of temperatures in the 90s or higher -- though folks in other parts of the country are used to different kinds of weather.
Should you find yourself in a heat wave, try to limit your time outside to the mornings and the evenings, when the air will be coolest. You'll also want to drink plenty of water throughout the day.
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