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Boston hasn’t had a June heat wave like this in nearly a century

Fifth straight day of 90-degree heat ties monthly record set in 1925

A couple cooled off traveling down Cambridge Street in Boston on a scooter Monday.
A couple cooled off traveling down Cambridge Street in Boston on a scooter Monday.David L. Ryan/Globe Staff

Strong June sunshine helped to boost temperatures back at 90 degrees for a brief time Wednesday afternoon, leading to a fifth official day of our current heat wave. This will tie the record for the longest heat wave in June, which was set back in 1925. This is the seventh 90-degree day this year, many more than would be typical so early.

This particular heat wave has been accompanied by lots of humidity and temperatures at night that have failed to go below 70. Heat waves this early are not common and certainly not of this length, but not unprecedented. Lots of folks want to find reasons why we have had such hot weather so early, especially pointing to climate change, but it’s not that simple.

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There’s no doubt the climate is getting warmer and we are experiencing somewhat more 90-degree days and much warmer nights. In Boston we’ve gone from an annual average of 12 days of at least 90 degrees in the middle of the last century to 14 now. 1983 still holds first place for the most 90-degree days in Boston at 30.

Inland areas have warmed more. For example, Hartford, away from the effects of the ocean, has gone from experiencing an average of 15 90-degree days mid-century to 20 more recently. Last year there was a whopping 39 such days there. The modifying effect of the ocean on coastal communities like Boston has hidden some of the summer warming that’s been experienced in much of New England.

There are increasingly more days above average than below, in all months of the year. This trend has accelerated over the past decade.
There are increasingly more days above average than below, in all months of the year. This trend has accelerated over the past decade.NOAA

When we have any heat wave, whether in June, July, or August, we can’t directly attribute that type of short-term weather to a warming planet.

It’s not accurate to say this heat is happening because of man-made climate change, but we aren’t necessarily off the hook either. What we can do is look at it from the perspective of odds. It’s probable that even if humans had not contributed any greenhouse gas it would probably still have been hot this week. However the length of the heat wave and the strength of the heat wave are the variables that are impacted.

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Without man-made warming, perhaps our lows at night wouldn’t have been quite so warm or the heat wave would have lasted only four days instead of five. In 30 years, the same heat wave might last six days or be somewhat hotter.

As the climate continues to get warmer the chances of having a heat wave in June or even earlier in the year will continue to increase and when we do have a heat wave the number of days it lasts and just how hot it gets will also increase.

It’s been very humid this week and that is also part of our changing climate. Dew points, a measure of the amount of moisture in the air, are going up.

This doesn’t mean hot and humid days didn’t occur 100 or 200 years ago, they did. However, the number of hot and humid days is becoming more frequent. Since the temperature can’t go below the dew point, part of the reason for our warmer nights is also these higher dew points, or added moisture in the air.

Dew points, a measure of moisture have been increasing in Boston this century.
Dew points, a measure of moisture have been increasing in Boston this century.NOAA

Finally, an early heat wave like this doesn’t mean we will have a hot summer. Actually, it’s likely our weather will actually turn quite a bit cooler for the next week to 10 days.

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Thereafter, there will probably be more heat and humidity, but how much more is unknown.

Back in 1925, for example, after all that June heat, July and August were cooler than average. Summer months cooler than average are still relatively hot, but the extremes, including a 100-degree reading from earlier that year, didn’t repeat. Summers in recent decades are averaging warmer and the trend is likely to continue.

The current heat wave will come to an end Wednesday night, but over the long haul, we should be prepared for more — not less — of this type of weather in the coming decades.