The National Weather Service issued another excessive-heat warning on Tuesday for much of Washington State and Oregon that will remain in effect for the next few days. There are also heat advisories in the Northeast, from Philadelphia to Boston. Here is what you need to know about these heat waves:
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.