If you’re thinking Maine summers, known for their warm days and cool nights, aren’t like they used to be, you’re right.
They’re not just getting hotter — they’re also getting stickier.
The dew point, a measure of how much moisture is in the air, has been gradually rising across the state. As it climbs, our hot days feel even hotter and those conditions intensify risks of heat-related illnesses like heat exhaustion and heatstroke.
The past few years have seen some of the highest average summer dew points on record in Maine, according to Weather Underground data. Measured in degrees Fahrenheit, dew point represents the temperature at which the air becomes fully saturated with moisture. High temperatures paired with dew points above 65 degrees can feel particularly oppressive.
“The reason you measure dew points in degrees, the dew point temperature, is because if you cool the air to the (air temperature), it would start to form dew or fog,” said Andrew Pershing, director of climate science at Climate Central, an independent climate research group, and formerly of the Gulf of Maine Research Institute. “It’s a very stable indicator of basically how moist the air is.”
This week’s heat and humidity make those effects tangible. Wednesday and Thursday had high temperatures in the mid-80s and lows at night of upper-60s.
Meanwhile, dew points hovered between 62-66 degrees, creating muggy conditions and providing little relief at night.
Friday will feature much of the same, according to the National Weather Service forecast. Friday will likely bring a high of 86 degrees and lows around 66 degrees, and chances of rain are about 50% with possible scattered thunderstorms.
The weather service does not list a dew point forecast; however, the latest predictions in Gray suggest current conditions will likely persist, keeping an elevated heat index and prompting a heat advisory for much of southern and western Maine.
It’s not just this week. From Portland to Augusta to Bangor, average July temperatures and dew points have risen over the past several decades, culminating in a true hot mess just over the past few years.
Portland’s average July temperature and dew point have risen gradually since the late 1940s, according to Weather Underground data. Temperatures have risen about 4 degrees, with the average climbing from 67 degrees in 1948 to over 72 degrees in the past decade.
The average dew point increased from about 58 degrees in 1948 to over 62 degrees since 2020.
Bangor and Augusta show similar patterns, according to Weather Underground. Bangor’s average July temperature rose from 66 degrees in the 1950s to about 71 degrees in recent years, and the dew point went from about 56 degrees to over 61 degrees. The July averages in Augusta trend even higher, with temperatures most recently reaching over 73 degrees and dew points over 63 degrees.
A few summers over the years stand as an exception to the rule, as Maine had cooler stretches in the late 1970s and early 1990s.
However, the long-term pattern is clear: Maine’s increasingly warm summer season air is holding more water vapor than some 75 years ago.
“Climatologically speaking, the air mass is retaining more moisture content,” said Sarah Jamison, senior service hydrologist with the National Weather Service in Gray. “Not just at the surface with the dew points, but throughout the air column. … We’re living in a wetter world than we used to.”
DIRECT CONNECTIONS
As the atmosphere heats up, it can hold more water vapor, Jamison said. Think of a sponge — increasing temperatures causes expansion which, in turn, allows more moisture to accumulate, she said.
“Just by warming the air column even 1 degree Celsius, you’re able to hold up to 7% more moisture,” Jamison said. “Over time, that translates to higher extremes in things like dew points and rainfall intensity.”
Pershing said the gradual trend in air and dew point temperature increases are directly tied to climate change.
“For the big picture, a warm air mass can hold more water, so if you have a source of water — which, for New England, you’re going to tap into the Gulf of Mexico or more locally from the Gulf of Maine — and you have warmer air, you’re going to be able to have more moisture in the air,” he said.
That kind of change also intensifies our experience of heat. Small increases in dew point or air temperature usually mean a significant rise in heat index values — how hot it actually feels. The more humid the atmosphere is, the less we experience the cooling effect of sweating. That can exacerbate negative effects like heat exhaustion and even severe effects like heatstroke.
“It’s amazing what 1 degree difference can do to the human body,” Jamison said.
Pershing added that “hot” is also a matter of perspective depending on where you live, and, for Mainers, it doesn’t take extreme temperatures to have immense heat-related stress.
“We’re really seeing conditions now, especially with the added moisture, where air conditioning is becoming something that’s very important for keeping people safe … (and) the infrastructure is not there.”
A decidedly humid day sees dew points reach above 60 degrees and an “oppressively” humid day, above 70 degrees. When dew points remain high after sunset, it becomes harder for the environment to cool down and provide relief at night, Jamison said. Dew points temperatures in the upper 60s at night will often, if not always, result in starting the next day with warmer temperatures keeping “the heat trend going.”
Pershing said nighttime temperatures are already showing that trend. For example, nighttime temperatures in Portland have risen by about 3.8 degrees since 1970, he said. “That moist air … it basically sets the lower limit on how cool you can get at night.”
RIPPLE EFFECTS
High humidity doesn’t just affect hot summer days, Jamison said. It also has ripple effects in the winter, when excess moisture can cause mixed precipitation, which leads to a less consistent snowpack, Jamison said.
Pershing added that increasing moisture also raises risks of extreme rainfall much like Maine saw in the December 2023 floods.
“In the warm season, it can mean hotter, hazier days,” Jamison said. “But a few extra degrees of dew point (in the winter) can turn what might have been an all-snow event into a mix or rain-snow event. That’s part of the feedback loop we’re seeing.”
While Maine doesn’t experience as many extreme heat days as other parts of the country, that doesn’t mean it’s immune.
Heightened humidity comes with peripheral issues, specifically with ticks and Lyme disease. “Part of the story with ticks is humidity. … The trend towards just more moisture availability is one of the factors that explains why we’re seeing more Lyme disease ticks throughout the country,” Pershing said.
Most of all, it makes summer days more concerning, especially for people with underlying health conditions, pregnant individuals, the young, and elderly and outdoor workers.
“These are very dangerous levels for people to be outside,” Pershing said.
