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.

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 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 Pacific Northwest, the heat and the drought are working in concert, exacerbating the problem and causing temperature records to fall day after day.

Which places in North America are currently experiencing a heat wave?

One of the hottest cities on the continent on Monday was Salem, Ore., about 45 miles southwest of Portland, where the high temperature reached 117 degrees Fahrenheit in the afternoon, a record for the city, the National Weather Service said.

At Portland International Airport, the high temperature was 112 degrees on Sunday and 115 degrees on Monday. The high on Monday was the hottest temperature recorded there since record-keeping began in 1940.

Canada broke a national heat record on Sunday when the temperature in a small town in British Columbia reached almost 116 degrees, breaking an 84-year-old record by nearly 3 degrees.

The Northeast, under a separate weather system from the Northwest, is also in the middle of a three-day heat wave that is expected to end on Thursday. Boston reached 97 degrees on Monday, tying its record for that date, the National Weather Service said.

Why is it hotter than normal in North America?

We have long known that the world has warmed by more than 1 degree Celsius (about 1.8 degrees Fahrenheit) since 1900, and that the pace of warming has accelerated in recent decades. The warmer baseline contributes to extreme-weather events and helps make periods of extreme heat more frequent, longer and more intense.

When will the heat wave end?

Forecasters say that temperatures will remain unseasonably hot into next week in the Pacific Northwest.

In Portland, temperatures have moderated somewhat but are expected to reach the mid-80s later in the week, said Clinton Rockey, a meteorologist with the National Weather Service in Portland. Temperatures will remain 10 to 20 degrees above average at least until next Tuesday.

How can I safeguard against possible power outages?

In Seattle, where until this month official weather stations in the past century had recorded only three days that exceeded 100 degrees, many more residents have recently bought air-conditioning units. While there have been no major power outages in the Pacific Northwest, the electrical grid could become overloaded with increased energy demands to cool and dehumidify buildings.

To help reduce demand, consider raising the thermostat by a few degrees and closing window shades and blinds. Avoid using large appliances like ovens, washing machines and dryers during the hottest part of the day, and turn off all lights and electronic devices not in use. With water heating accounting for about 18 percent of the energy consumed in your home, consider shorter or colder showers, the Energy Department suggests.

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