Boston hasn’t seen a six-day heat wave since 2016. This summer, we’ve had two.

Some looked to take a break from the oppressive heat on Massachusetts Avenue in Boston.Jonathan Wiggs/Globe Staff

After a relatively comfortable June, punishing heat has overtaken the Boston area in July and early August, breaking temperature records and resulting in the warmest three-week period ever recorded in Boston.

It’s also meant heat waves ― measured as three days in a row with temperatures above 90 degrees — that seem to drag on. Prolonged heat waves are relatively uncommon in Boston, and since 1990 the city has only experienced a six-day heat wave five times, most recently in 2016. But so far this year, it’s already happened twice.

July’s punishing heat wave lasted seven days, from July 19 to July 26. And the heat wave that’s expected to end on Tuesday has stretched into its sixth day as temperatures crossed 90 degrees in Boston around midday. During each of the heat waves, temperatures climbed into the upper 90s, and even hit 100 on July 24.

In addition to the sheer length of time Bostonians have spent sweating this summer, the average temperatures each day have climbed far above normal. During the most recent 21 days, high temperatures soared past the low 80s, which are normal highs for this time of year. According to meteorologist Dave Epstein, the average temperature for this period in Boston was the warmest 21 days ever in 151 years of record keeping.

This type of heat is unusual, but it’s becoming more common. Data analyzed by UMass Amherst show that since 1872, the average temperatures for the period between July 25 and Aug. 7 have crept up steadily and hit a record of 80.4 degrees in 2022.

For example, in 1872, the average temperature between July 25 and Aug. 7 was 69.9 degrees, according to the data. In 1988, it was 78.1 degrees, and in 2018 it had jumped to 80.3 degrees.

As the climate warms, scientists expect this type of extreme heat to become the norm. By the end of the century, scientists say the Northeast could see 13 to 56 days above 90 degrees each summer, meaning “heat waves” could become just an average summer day.