What to Know About Heat Waves
The summer of 2025 is shaping up to be a hot one. Almost all of the United States can expect a summer with above-normal temperatures, according to the National Oceanic and Atmospheric Administration's seasonal outlook, with the Northeast and much of the West due for an especially scorching time.
Here's what to know about heat waves and their causes.
What causes a heat wave?
In most parts of the country, temperatures must be above the historical average in an area for two or more days before the label 'heat wave' is applied to a hot spell, according to the National Weather Service. But the definition can vary by region; in the Northeast, it is defined as three straight days in the 90s or above.
Heat waves begin when high pressure in the atmosphere moves in and pushes warm air toward the ground. That air warms up further as it is compressed, and we begin to feel a lot hotter.
The high-pressure system pressing down on the ground expands vertically, forcing other weather systems to change course. It even minimizes wind and cloud cover, making the air more stifling. This is also why a heat wave parks itself over an area for several days or longer.
What is a heat dome?
As the ground warms, it loses moisture, which makes it easier to heat even more. And in the drought-ridden West, there is often plenty of heat for the high-pressure system to trap.
As that trapped heat continues to warm, the system acts like a lid on a pot — earning the name 'heat dome.' In the summer of 2021, there was plenty of heat for the high-pressure system to trap, leading to triple-digit temperatures in late June and early July that killed hundreds of people in Oregon, Washington and British Columbia.
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