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How Did This July Become The Hottest Month On Record?

BOSTON (CBS) - The hottest month of the year is coming to a close, and in fact, it's not just the hottest month of the year. It's the hottest month of any year. At least as far back as records go in Boston, which is 1872. With nearly a year's worth of 90 degree days and a record amount of warm nights, it now stands up above all the others.

Average temperature for the month of July in Boston.(WBZ-TV)

How did we get to the record? It's interesting to look around southern New England. Everyone has had one of their hottest months on record, but not all of them achieved the top spot. Only Boston, Hartford, and Portland, Maine. The majority are somewhere between second and fifth place. There is certainly a measure of the urban heat island (UHI) that helps the city of Boston topple some records, and you can see it in the July data.

This month featured the highest number of 70F+ nights ever recorded in the city, beating the record set just last year. With all those warm nights, it also notched the warmest monthly average low of any month on record. Before last year, there had never been a month with an average monthly low temperature of 70F or higher. We did it last August (70F on the dot) and now have just broken that record with a significantly higher 71F average this July. On top of that, we set a record for the only year with two 80F+ lows back during the extreme heat of July 20th and 21st.

Graphic: WBZ-TV

Suburban cities do not see consistent overnight lows quite this warm, because they are able to radiate heat out into the atmosphere more efficiently at night. There are fewer surfaces that absorb solar radiation all day and continue to send it back out as longwave radiation after the sun sets. For example, Norwood had eight nights of 70F+ temps this month (though you can interpret them as similar because the eight in Norwood also ties the record for July).

There are two important things to note here. Firstly, Boston didn't develop last year. It has been a large, thriving, and paved city for over a hundred years, and it has always had an UHI effect. It is gradually growing over time, but if we are comparing Boston records to Boston records and rural to rural, it is more apples to apples. The other is that even though there is an UHI effect, it's still the actual temperature. Sometimes I have conversations with people who denounce city temperatures because of the enhanced warm night effect near their centers. Those temperatures are still real and are still the temperatures people must live, work, and use energy in. Global warming is seen in both increasing greenhouse gases and also land use changes as humans remake their surroundings. So the need to track and consider them is still quite important.

Another aspect to consider is the warming ocean temperatures, which increasingly help to keep overnight temperatures up (especially at the coast, and Boston's temperature sensor is nearly in the water out at the edge of Logan Airport). Sea-surface temperatures are several degrees warmer than average and that helps to keep readings up at night.

Sea-surface temperatures in July were well above average along the East Coast.

All that being said, the days were not cool this July and carried their weight. Boston had the third-highest monthly high temperature ever recorded in the city, coming just shy of the marks set in 1983 and 1952. The 12 days of 90F+ highs tie for the fourth most on record for the city. Down the road at Bradley Airport in Connecticut, a new record was set for most 90F days in a month with a whopping 19 this July. That's the most of any month on record, not just July.

Graphic: WBZ-TV

The gold-standard climate site here locally, Blue Hill, notched an impressive mark as well. While it was not the warmest on record, preliminary numbers show Blue Hill Observatory coming in with their 3rd hottest July on record.

There have only been seven years since 1885 that had an average monthly temperature of 75F or higher there, and those years are 2019, 2016 (twice, July AND August), 2013, 2010, 1994, and 1952. Safe to say there is a strong trend there with much less UHI to impact the numbers. July also adds another/ This telling graphic (below)...a whopping 36 'Top 10 Warmest' months on record this decade, compared to just one such cold mark (February 2015).

Ratio of Top 10 warmest months vs Top 10 coldest months on record at Blue Hill Observatory, 2010-2019. (Graphic: WBZ-TV)

Interestingly, I don't feel like most people viewed this month as a very hot one. Maybe there are a couple of explanations for that. One would be that we're just getting used to it. We've had eight 'Top 10 warmest' July/August months this decade and none in the 'Top 60' coldest. Last year, in particular, was a humid swamp of a grind with a record-setting number of 70F+ dewpoints. So could be some recency bias there.

Another may just be that there is low variability in summer. Or in other words, summer is either hot or hotter. The departure from average was a little over 5F this July. That is, in itself, very impressive in the warm season. But a departure like that in December wouldn't make a dent. When we had the warmest December on record in 2015, it was a whopping +10.6F! I am not sure if people can really tell the difference between a lot of 92F days and a lot of 87F days, but they definitely notice a very warm winter month.

And finally, there is just a ton more air conditioning in New England than there used to be. So perhaps a majority of people just don't care because of the easy access to cooling? I'm all ears if you have other ideas!

Ratio of Top 10 warmest months vs Top 10 coldest months on record at Blue Hill Observatory, 2010-2019. (Graphic: WBZ-TV)

In any case, a warm summer is usually a safe bet these days. We've already locked in a warmer than average summer across the area for the 10th straight year (there's nothing suggesting August will be remarkably cool enough to offset June/July). Calling for a cooler than average summer should come with heavy odds because it would be quite a leap/accomplishment. All part of the continuing trend of our warming climate across all seasons.

Find Eric Fisher on Twitter and Instagram.

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