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This is my column, which ran Tuesday July 10 in the Monitor. By coincidence (or, rather, because multiple reporters get story ideas from obvious phenomena like a wicked bad heat wave) a couple of major outlets ran similar analysis later in the week, including Inside Climate News and the New York Times. But I’m the only one who focused on New Hampshire! … Also, a reader pointed me to Penn State research that says jet contrails (no, not “chemtrails”) affect nighttime temperature more than daytime, which is kind of weird.. 

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.

There were plenty of anecdotes about this effect during the recent heat wave: Mount Washington Observatory tied its warmest-ever nighttime, bottoming out at 61 degrees on July 2, while Burlington, Vt., broke its warmest-night record that same night when the temperature never fell below 80. Most eye-popping of all, the city of Quriyat in Oman broke the world record for “highest low temperature” – it never got below 109 degrees on June 28. Yes, 109.

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.

In case you’re wondering, Concord airport didn’t get down to 60 degrees for a single night during the heat wave. The minimum overnight temperature was at least 61 from June 27 through last Friday.

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.

This is also part of the reason colder areas, especially polar regions, are being hurt more by the changing climate, although many other factors are probably more important, such as patterns of global movement of the atmosphere and ocean currents.

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.

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