More Vermont 90s, And Interesting Hot Green Mountain State Trends

Burlington managed to eek out another day above 90 degrees on Monday, making it the 15th day this year already in which it has gotten to the 90 degree mark.

A so-called cold front had passed through earlier in the day, but the slightly cooler air lagged far back in Ontario, so it wasn't really until evening that we began to feel some refreshing breezes from the northwest.

Ben Frechette, a meteorologist at WPTZ-TV, loves looking at data, as I do, shared some more stats yesterday.

I've been saying all along that the average number of days that get to 90 degrees in Burlington each year is five.

That's true if you look at the data starting in the 1880s and going through until now.  However, many climatologists look at the average going back the past 30 years or so.  If you do that, the average number of 90 days in Burlington is eight.

Frechette shared the chart you see in this post.  (Click on it to make it bigger and easier to see).

The chart shows the yearly number of 90 degree days in Burlington dating back to 1883. The darker line you see in the chart is the 30 year rolling average for the entire data set.

As you can, there is a definite trend toward increasing numbers. Such a long term trend suggests to me that climate change is playing a role, but it's probably not the only thing going on.

There are probably some cycles thrown in amid the general climate change warming. You can clearly see a spike in the number of 90 degree days in the 1940s.

Part of that might be because there was less forest in Vermont back then than there is now.  If you have more forests, you tend to have cooler temperatures in the summer because of all the transpiration and water evaporation from the trees.

But there was about the same or even even less forest cover in the decades before the 1940s, and no corresponding higher number of 90 degree days.  For whatever reason, though, the 1940s were generally warmer and drier than previous decades.

Vermont had as little as 20 percent forest cover in the late 1800s due to farmers clearing the land. By the 1980s, forest cover had reached about 75 percent.  It has stayed relatively steady since.

Moreover, rainfall has increased in Vermont during the past few decades, at a rate of about 1.5 inches per decade since 1960 or so.

Scientists think climate change is largely behind the increased rainfall. Remember, all that water from the trees and those rain storms might make it feel more humid in the summer, but the moisture also works to keep the temperatures from spiking.

That's because in wet times, more of the sun's energy goes toward evaporating water than heating the air. Plus, excess moisture can at times create extra clouds, blocking the sun, though that's not always the case.

You'd think the increasing forests and the heavier rainfall would work against an increasing trend of summertime 90 degree weather. But you see the rising trend in Frechette's chart.

One big caveat:  The immediate Burlington area has seen more suburbanization and sprawl over the past four decades than most other parts of Vermont.  The National Weather Service in South Burlington makes sure to calibrate things to ensure accurate temperature readings.

Still, I don't see as much of a warming trend in areas of Vermont that haven't changed as much as the Champlain Valley has. This part of the Champlain Valley has probably lost of bit of forest cover since 1970 or so.

I admit I have not deeply analyzed data from places like Montpelier or St Johnsbury, but I suspect the rising temperature trend lines in those communities might be a bit more tempered.

Maybe Frechette can look at that, although the dude is already incredibly busy with his weather duties at WPTZ.