Meteorologist Dave Epstein is our go-to person for pressing weather questions on everything from winter blizzards to summer droughts. He’s also a horticulturist, meaning he’s an expert in anything that grows leaves and flowers. GBH’s Morning Edition asked our audience for weather and gardening questions, and Epstein graciously answered them on the air.

Have a gardening or weather question for meteorologist Dave Epstein? Tweet him @GrowingWisdom, email us at, or text 617-300-2008.

What are heating degree days, or cooling degree days?

As temperatures reached 80 degrees this week, some people in the Boston area might have reached for something they don’t usually see this time of year: The ‘on’ button on their air conditioners.

And as they think about what that means for their energy bills, they may come across an unfamiliar term: Cooling degree days, or their cold-weather counterpart, heating degree days.

The terms are a measure of how many degrees people may heat or cool their homes to reach a comfortable environment of about 65 degrees, Epstein said.

“It’s a way for energy companies especially or even homeowners to kind of look at how much energy they’re using on a yearly basis,” Epstein said. “And it’s also why, if you have oil or propane and they come to your house right at the right time, it’s because of that. They’re actually looking at the cooling and heating degree days.”

It’s a simple calculation, Epstein said: Average the day’s high and low temperatures, and subtract that number from 65.

“What you do is: you look at the high and the low, you average it. And depending on whether it’s above or below 65, you have a cooling or heating degree day,” he said. “So if the temperature, if the average, was above 65, you have a cooling degree day. … That is a way to measure how much energy you’re using.”

Why 65? It’s assumed to be a temperature at which most people are comfortable, he said.

“There’s this assumption that when the temperature outside is 65 degrees, we don’t need heating or cooling to be comfortable,” Epstein said. “Somebody else could say wait a second, that’s too cool for me or too warm for me. But in general, 65 is the number that we look at.”

As our climate changes, the number of heating degree days and cooling degree days may shift — perhaps people will turn their heat on later in the fall and start using air conditioning earlier in the spring, Epstein said. But how that affects energy bills remains to be seen.

“I had some students once in one of my classes look at: is there more energy being used as places like Florida to cool or more energy in northern tier to heat?” Epstein said. “And actually you’re using more energy to heat than there is to cool. So overall, there’s probably a net saving because we’re using less heating in the winter.”