There have a been a few hot spells in the Northeast so far this summer, but the core of blistering summer heat has been locked over the West so far this year. Until now that is.

The Fourth of July kicked off a period of sweltering heat from the Midwest to the Northeast that will last through the rest of the week. Kansas and the Gulf Coast saw heat advisories earlier this week. On Wednesday, the National Weather Service posted them for New York City.

For New York City, a heat advisory is issued when the combination of heat and humidity, or the heat index, is expected to make it feel like 95°F or hotter for two consecutive days, or if that value is 100°F (38°C) or more for any length of time. Before Wednesday, the temperature had reached 90°F only twice this year in New York City, and both of those days were at the end of May.

In Philadelphia, an excessive heat warning is in effect each afternoon for the rest of the week, indicating that the heat index will be around 100°F during the afternoon hours. While the city had hit 90°F a few more times than New York this year, its first occurrence of 95°F did not happen until Wednesday.

Until this week, the Southwest and the Southern Plains had been the seat of the summer heat in the U.S., with temperatures frequently 10-15°F above normal. In many cases, that meant temperatures higher than 100°F.

The heat index value takes into account the effects of both the heat and humidity on the body. For example, if the air temperature is 96°F and the relative humidity is 38 percent, the heat index is 100°F.

While that humidity level sounds low, it is important to remember that relative humidity is relative to temperature, as warmer air enables more water to evaporate into the atmosphere. As a result, the air can still feel humid even when the relative humidity is low.

To figure how much actual moisture is in the atmosphere, meteorologists normally use the dew point temperature, which is the temperature at which condensation occurs. More physical water vapor in the atmosphere means a higher dew point.

The dew point takes on added importance in the East because it’s generally more humid than the West, particularly when it comes to potential heat impacts on health. The higher dew point makes it harder for the body to cool itself, as sweat does not evaporate as easily. Evaporation is a cooling process, which is why you feel chilly after getting out of a pool. But if the humidity is high, the evaporation process, and thus the cooling process, is slowed.

Climate change is driving up both temperatures and evaporation rates. That means summers are getting more and more muggy.

Much of the Northeast, from Washington, D.C., to Boston, is under an air quality alert. This usually happens when there’s no wind to disperse air pollution, leading to stagnant air. Stagnation depends on the wind speed and the temperature at different levels of the atmosphere, and while it can occur during the winter, especially in deep valleys, it is more common in summer.

Stagnation adds insult to injury regarding heat, as the lack of a breeze also makes it feel hotter, and the number of stagnant summer days has increased in most areas east of the Great Plains since 1973.

The Northeast will get a brief respite from the heat this weekend. Showers and thunderstorms late Friday will help cool things down into the 80s. But there are signs the 90s will return to the Northeast again next week, making any relief short-lived.