Jeremy Siegel: You’re listening to GBH’s Morning Edition. With 80-plus temperatures this week, many of us have turned on the air conditioner for the first time this year. And that means higher electricity bills and more energy use overall.

Paris Alston: Here now for his weekly deep dive into the science behind the weather and plants is GBH meteorologist Dave Epstein. Hey there Dave, good morning.

Dave Epstein: Good morning. Good to be here.

Siegel: So Dave, today we’re going to talk about something that’s called cooling degree days and also heating degree days, which help us measure the times when we end up using more energy to heat and cool our homes. What exactly is a cooling degree day?

Epstein: Yeah. So a cooling and a heating degree day are based on the number 65. So why 65? Because there’s this assumption that when the temperature outside is 65 degrees, we don’t need heating or cooling to be comfortable. So again — 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. And so 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. So if the temperature, if the average, was above 65, you have a cooling degree day. So let’s do a little math. High temperature’s 90, low temperature’s 66, divided by two, that’s 78. Seventy-eight minus 65 is 13 cooling degree days. And that is a way to measure how much energy you’re using.

Alston: So how do you use that to analyze the, for instance, Dave, the days that we have been seeing, those 90 degree temperatures we’ve been seeing?

Epstein: Yeah. So 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 compared to last year. So let’s just say that here’s an example: Like, let’s look at heating degree days. As I’ve gotten older, I put the heat on when it gets less cold in my house, right? So I used to when I was younger, I could handle down to 60-61 without any heat. Now it’s like 63, 64. So I can look at how much energy I’m using based on the temperature outside on a given year, because every year is different. So on the heating side, if the high temperature was 33, the lowest 25 divided by two, that’s 29, we’d have 36 degree days, 36 plus 29 is 65. So that’s how you get there as well. It’s just a way for energy companies to look at it. 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.

Siegel: How have we seen these days, the heating and cooling degree days, change over time?

Epstein: Yeah, it’s a great question. So obviously as the climate warms, we are seeing fewer heating degree days and more cooling degree days. So the energy use is changing. It doesn’t mean we’re using more energy, but the energy use is changing. And actually 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, 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.

Alston: So that makes me think about, Dave, how we’re seeing rising home energy costs with people trying to heat their homes, but then also our warming climate, or, even the fact that the UK doesn’t use AC, right? So how is our changing climate going to affect energy consumption in the years to come?

Epstein: Yeah, I mean, in the years to come, energy consumption will shift how we consume energy. So we may consume, you know, your energy bill overall from January through December 31st, may not change. But how it’s split up may change because we’re seeing more need for cooling for us to be comfortable in the summer, especially with the humidity and all of that, than we see in the winter. So we may see that you’re not turning your heat on until, you know, late October early November for some people, you may make it even deeper into November, but you may start cooling, for example, this week here in May with temperatures up near 90.

Siegel: That is GBH meteorologist Dave Epstein with his weekly deep dive into the science behind the weather and plants. Dave, thanks so much for your time.

Epstein: You’re welcome.

Alston: You're listening to GBH news.

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  thewakeup@wgbh.org, 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.”