Granite Geek: It's Not Your Imagination - Nights Are Getting Warmer, Too

Jul 10, 2018

Credit NWS

During last week's oppressive heat, you may have found it particularly hard to cool down at night. Over the past several decades, our nights are getting, on average, warmer. 

Granite Geek David Brooks of the Concord Monitor spoke about why with NHPR's Peter Biello.

(This transcript has been edited lightly for clarity.)

You write in your column this week that over the years temperatures have been rising during both the day and night. But the boosts in the nighttime temperatures are more pronounced and that has to do with something called the atmospheric boundary layer. What's that?

So the atmospheric boundary layer is the layer of the atmosphere that is most affected by the land, basically radiant heat and other such things that directly affect the temperature and humidity within the atmospheric boundary layer. And this layer is not static. It gets taller and shorter over the course of the day. When the sun's out, heats up the air, air expands when it's warm, the atmospheric boundary layer gets taller, a couple kilometers high depending on where you are and the circumstances. At night when the temperatures go down the atmospheric boundary layer shrinks and it gets just perhaps a few hundred metres.

You also write in your column this week that there is a relationship between greenhouse gases and the atmospheric boundary layer and the reason why the temperature has been increasing on average at night. What's that relationship?

So, greenhouse gases, the reason they're heating us up is they're not adding more heat, they're preventing heat from escaping, like in a greenhouse. The sun comes in, turns into heat but the glass doesn't let it out again, same thing with greenhouse gases.

And that happens both day and night?

That happens both day and night, it's fairly static between day and night. What's happening is you're getting the same amount of heating effect or non-cooling effect, shall we say, from the greenhouse gas both day and night, but at night the atmospheric boundary layer is compressed. There's less volume. And if you have the same amount of heating and less volume, what happens is each portion of that volume gets more energy added to it, so the temperature goes up more. So the temperature goes up more at night because there's a smaller volume of air being affected.

And you spoke with the New Hampshire State Climatologist Mary Stampone about this effect. She's done some research on this. What did she discover in her research?

She's a UNH professor as well as the state climatologist and she was nice enough to crunch some data for me to make sure that the data actually shows that this is happening here. And she looked basically at Concord, which is the official weather station for the National Weather Service at the airport, and going back to about 1940, which is when really the complete data started being collected. Actually what she did is she looked at days in which temperatures went over a certain point, or didn't go below a certain point. And she found that the daytime, that increased by about one day a year over the course of this 77 year period, but that's not statistically significant.

But whereas the nighttime, the nights when the temperatures did not go below 60 degrees, 60 degrees or lower, nights that didn't really cool off. She found the number of those nights increased significantly by about nine nights a year over the course of this period. And that's really what I was looking at. Not that it gets hotter but that it never really cools off.  

So what's the significance for people like you and me living in New Hampshire, or anywhere for that matter, if the nights aren't cooling off enough?

Cooling off affect at nighttime is important for the environment and public health as well. To a certain extent people can accept really, really hot weather. You know, our bodies are able to cope with it for a while as long as we get to release some of that heat at some point during that 24 hour period. If our ability to release it at night gets reduced because the nights aren't getting cool, that has a cumulative effect on our health. And all the environment, everything out there has evolved knowing that in the nighttime you can get rid of so much energy. And if you can't do that anymore it's going to have long term effects. 

So on Mt. Washington, for example, this recent heat wave tied the all time record for the highest ever nighttime low. In other words, overnight it never got below 61 degrees on Mt. Washington. As I say, that's only ever happened once before. Usually the temperature gets down into the 50s at night, even on a warm night usually it gets down at least into the 50s on Mt. Washington. So that's a perfect example of it's not cooling off as much. And this is continuing to happen. It's not like we've reached this bad point, now we're going to stay here. The way circumstances are, greenhouse gases and changes in land use and all that, it's going to continue to get worse. As I say in the column, in 2028 we're going to look back on 2018 and say "Gosh, it was so cool that summer. I wish it was like that again."

That's David Brooks. He's a reporter for The Concord Monitor who's geeky interests keep him up at night whether it's hot or cold at GraniteGeek.org.