Here’s what you need to know:

When is a heat wave declared?

The designation happens when the temperature reaches at least 90 degrees for three consecutive days. Between 1876 and 2011, there were 263 heat waves in New York City, according to the National Weather Service.

Most of those heat waves lasted three days. One lasted 12 days.

What are the temperatures going to be this week?

Today’s high temperature will be near 90 degrees, the Weather Service predicted. Tomorrow it may hit a high of 91 degrees. Thursday may be a little cooler, at 85 degrees.

After that, watch out. The temperature really starts to increase: Friday through Sunday, the highs could reach 95 degrees or more.

Upper Manhattan and the Bronx are “particularly heat sensitive,” according to Jaime Madrigano, a policy researcher at the RAND Corporation who was featured in a video by the National Oceanic and Atmospheric Administration.