5 Temperature Records Meteorologists Are Watching This Week

A major heat wave is spreading across a large part of the United States this week. Scientists call the cause a "heat dome," which traps hot air close to the ground and keeps temperatures very high. Forecasters say many temperature records could be broken from the South all the way up to New England. Here are five key records that meteorologists — scientists who study weather — are watching closely.

The first record to watch is how many days in a row a city stays at 100 degrees or above. Many places are expected to hit 100 degrees for two, three, or even four straight days. On some of those days, the heat index — which is how hot it actually feels when you add in humidity — could reach as high as 115 degrees. That kind of heat can be very dangerous for people who are outside for long periods of time.

Washington, D.C., is one city forecasters are keeping a very close eye on. The city was predicted to reach 100 degrees or more on Thursday, Friday, and Saturday, and possibly even on Wednesday. Washington has only had four days in a row at 100 degrees or higher two other times in its history. If it happens again this week, it would tie that rare record.

Philadelphia is also in the spotlight. The city's longest streak of days at 100 degrees or more is three days in a row, which happened in July 1966 and again in July 1993. This week, forecasters say Philadelphia could tie that record, and there is even a small chance it could break it. That would make this one of the hottest stretches the city has ever seen.

The second record involves New York City. The official weather station for the city is a thermometer at Belvedere Castle in Central Park. It has not hit 100 degrees since July 18, 2012 — more than a decade ago. This week, forecasters say the city could reach 101 degrees on both Thursday and Friday, which would be remarkable.

The third record is about daily high temperatures. A daily high record is the hottest temperature ever recorded on a specific date at a specific location. These records are the most common to break because there are 365 different dates in a year, giving the heat many chances to set a new mark. Experts expect more than 100 new daily high records to be set through Saturday, from Tennessee and Ohio all the way up to Maine.

Michael Mann, a professor at the University of Pennsylvania who studies the environment, explained why daily records fall so often. "It's really just about simple matters of probability," he said. Because there are so many dates in a year, there are simply more chances for a record to be broken on any given day. Monthly records, on the other hand, only have 12 chances per year to be broken.

The fourth record is about nighttime temperatures. Usually, temperatures drop after the sun goes down, giving people and animals a break from the heat. But this week, forecasters say the hot air mass sitting over the region will stop temperatures from cooling down at night. More than 200 "daily record high minimum" records — the hottest a location has ever been at night on a given date — could be broken, stretching from Texas to Maine.

This is especially dangerous for people's health. Our bodies need cooler nighttime temperatures to recover from the heat of the day. This week, overnight low temperatures are only expected to drop into the 70s or near 80 degrees, which gives people very little relief. People who do not have air-conditioning are at the greatest risk of getting sick from the heat.

The fifth and most extreme record is the all-time high temperature. This is the single hottest day ever recorded at a location since weather records began there. These records are very hard to break, especially in big cities where records have been kept for more than 100 years. Breaking one is a clear sign that a heat event is truly extreme.

Washington, D.C., has the best chance of setting a new all-time record this week. The current all-time high at Ronald Reagan National Airport is 105 degrees, set in July 2012. The latest forecast predicted a high of 103 degrees on Friday, which would fall short of the record but still be dangerously hot. Concord, New Hampshire, whose all-time record is 102 degrees, also has a real shot at falling, with a high of 102 expected on Thursday. New York City's all-time record of 106 degrees and Boston's record of 104 degrees are less likely to be broken, but both cities are still expecting temperatures well above normal.