Alert: High Heat and Humidity = High Heat Index Values Monday

Monday will be the third day in a row for many areas in Massachusetts, making it an official heat wave.

Three or more consecutive days of temperatures of at least 90 degrees is what forecasters use to determine a “heat wave” in New England. Some years will have multiple heat waves, while others won’t have any. The last time the Boston area had an official heat wave was July 14-20, 2013.

High heat indexes are expected Monday as temperatures are expected to climb into the 90s across many areas, according to the National Weather Service.

What is the heat index?

“It’s not the heat, it’s the humidity”. That’s a partly valid phrase you may have heard in the summer, but it’s actually both. The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature. -- National Weather Service

Along with the heat comes high humidity and that combination will produce heat indexes in the mid to upper 90s through late afternoon Monday.

Following a hot and uncomfortable weekend, Monday will be another day of heat and humidity, making it the third day of high temperatures at 90 degrees or higher for many areas in the state, NWS officials say. Tuesday may also reach 90 or higher in many spots.

Officials say area residents should take precautions if they are going to be outdoors for extended periods of time today. Click here to see Massachusetts Emergency Management Agency’s extreme heat tips.

Image: Courtesy National Weather Service