Two years ago I wrote a piece about the health effects of higher nighttime temperatures. The National Weather Service’s regional office just reported that Portland, Maine, broke its record for most number of nights where the low temperature was 70 degrees or higher – 9 of them, and it’s still August. The previous record of 8 such nights was in 2018. Concord hasn’t been that hot, says NWS as if Aug. 13: “It’s had 3 nights so far where the low temperature was 70 degrees or greater. This would place them in 29th place for the year if there are no more nights where the low temperature stays at or above 70.”

So I thought I’d rerun the 2018 piece here, with some edits/cuts:

Perhaps you noticed that it got really hot last week. What I noticed is that it never got cool.

That sounds like a distinction without a difference, but it’s important, both for what it does to our health and as a reflection of how the changing climate alters things in complex ways.

Computer models have long predicted that increasing greenhouse gases in the atmosphere will raise nighttime temperatures more than it raises daytime temperatures. That seems weird to me, but Mary Stampone, a UNH professor of geography who is the New Hampshire state climatologist, says the data supports it.

“Twenty years ago, when they were starting to pick out the different trends, different variables other than just global average, one of the projections is that the colder temperature are going to increase at a faster rate than the warmer ones … and nighttime temperatures increase at a faster rate than daytime. We’re seeing that happen,” she said.

Stampone crunched some numbers from the National Weather Service’s official weather station at Concord airport, dating back to 1940, when full data started being recorded. I sometimes crunch numbers for my stories; having a Ph.D. crunch them is much more reassuring.

Stampone found that the number of really hot days per year – the number of 24-hour periods that had a maximum temperature above 90 degrees – didn’t change during this period. The annual average increased by about one day per year, which is not statistically significant.

On the other hand, the number of not-cool nights per year – those where the minimum temperature never fell to 60 degrees or less – did change. On average, the number of nights that don’t get below 60 degrees over the course of the year has increased by 9 over the period of record.

Stampone also looked at maximum and minimum temperatures each day over the 77-year period during the three months that meteorologists consider to be summer: June, July and August.

“There is no trend in maximum daily temperature from 1940 to 2017, but minimum temperatures have increased by 2.5 degrees Fahrenheit,” she said. “That is a pretty stark difference.”

What causes this difference? It seems to be at least partly based on behavior of the atmospheric boundary layer, the air that is most changed by reflected heat from the ground.

The boundary layer shrinks at night as temperatures fall, going from kilometers high during the day to a few hundred meters at night.

Greenhouse gases are warming the planet by preventing the escape of heat, which doesn’t change much from daytime to nighttime.

Because the volume of atmospheric gases is much smaller at night, this greenhouse gas effect has more effect per volume than it does during the day. It’s sort of like the way a given amount of stovetop heat will raise the water temperature more when the pot is half-full than when it’s completely full.

It is possible to find some positives to this night-versus-day discrepancy. For example, it alleviates some of the strain that increased heat puts on the electric grid, because at night the power load goes down so there’s more capacity to run extra air conditioning.

But there are more negatives. People can handle a surprising amount of heat as long as some is released overnight, so reducing our nighttime cooling can have serious health consequences. It hurts the environment for the same reason.

And here’s something else to remember: We’re continuing to change. It’s not like we’ve gone to a new pattern of warm and cool and will stay there – if current trends in pollution and land use continue, in a decade we’ll look back on the summer of 2018 and wish we could be that comfortable again. Keep that in mind next time your bedroom is too hot to sleep in.