Remember when it used to cool down at night? Here’s why we rarely see cooler evenings anymore.

It used to be that summer evenings provided a much-needed break from the heat. But now our days are getting hotter and more humid.

Carina Sandoval, of Somerville, stood in front of her freezer Tuesday to get relief from the heat in her third-floor apartment, which had no air-conditioning until last week.Matthew J. Lee/Globe Staff

An ice-cold shower. Opening all the windows. Even getting out of bed to lie on the hardwood floor. For the past month, since that first 90-degree day in June, Carina Sandoval has tried everything she could think of to keep cool each night on the third floor of her Somerville apartment.

Nothing worked.

“Sometimes before I went to bed, I would go to the freezer and just stand with as much of my body in the freezer as I possibly could,” she said. “But regardless, I was going to be sweating, and basically just toughing it out.”

Sandoval, 23, is among many residents feeling the stifling heat firsthand — not just when the sun is up, but well into the night. Gone are the days, it seems, when folks could seek relief from a sweltering summer afternoon by depending on cooler temperatures to take over after sundown, or by simply opening the windows overnight to allow a refreshing breeze to flow through.

“We are seeing an increase of temperatures across the United States and the Northeast, with overnight temperatures warming faster than daytime temperatures,” said Jared Rennie, research meteorologist with the National Oceanic and Atmospheric Administration (NOAA).

Since 1936, Boston has seen its average nighttime low temperature from June to August (meteorological summer) jump 3 degrees — from roughly 62 degrees to 65 — more than double the rate of increase in daytime highs, according to NOAA data. The average summertime high temperature in the city has risen 1.2 degrees.

During the summer, the average overnight temperature (low) for Boston has increased 3 degrees since 1936.ACIS

Expanding beyond summer and looking at annual trends for Boston overnight lows, 2023 clocked the second-warmest average of overnight temperatures on record, at 47 degrees — falling just 0.2 degrees behind 2021′s record of 47.2 degrees. (Records date back to the late 1800s.)

The average yearly overnight temperature in Boston has increased 2.7 degrees since the 1930s.boston globe

Across the entire Northeast, 2023 marked the warmest nights on record, with the region this year currently on pace for a top-five warmest on record. The graph below details the yearly average minimum temperatures dating back to 1895 across the Northeast — where a staggering increase can be seen in recent years. There has not been a below-average year for nighttime lows across the Northeast since 1994 and that year was only 0.1 degree below normal.

Since 1994, the northeast has faced above average overnight temperatures, a streak likely to reach 30 years at the end of this year.NRCC

Nearly everywhere in New England this year is on pace for a top-five average warmest nighttime temperature placement in the record books — a change impossible to ignore for many people across the region. As we start the second half of July, there have already been 10 nights with an overnight temperature of 70 degrees or above in Boston — two more than the average (with half of the month remaining) — and Wednesday night’s lows should stay above 70 as well.

The current record for July nights with overnight temperatures above 70 is 17, set in 2019. With the added humidity, some nights have felt much closer to 80 degrees, providing no relief whatsoever.

The humidity, or moisture in the air, has also been at some of the highest levels on record, making it more difficult for the nights to cool off.

So far this summer, nearly all of New England is facing a top-5 summer of warmest overnight temperatures.NRCC

What’s causing these extra warm nights?

A one-two punch has led to the steep rise in nighttime temperatures, especially during the summer — worsening climate change and a growing abundance of moisture in the air.

“Because of increased greenhouse gases, we see increased temperatures. And because of increasing temperatures, there is the ability for the atmosphere to hold more moisture, and as such, makes it harder for the actual temperature to cool off overnight,” Rennie said.

For every 1-degree increase in temperature, the atmosphere can hold 4 percent more water vapor. With daily highs averaging nearly 6 degrees above normal in some areas so far this summer, plenty of heat has built up moisture levels within the air, which in turn acts like a blanket and slows down the release of heat back through the upper atmosphere — and as a result, traps the warmth close to the ground overnight.

As the sun heats the planet, some energy is reabsorbed by greenhouse gases, including water vapor. This will warm the Earth. The more greenhouse gas is present, the more possible warming.Nasa

“Climate change is a contributing factor in the trend of increasing overnight temperatures,” said Samantha Borisoff, a climatologist with the Northeast Regional Climate Center at Cornell University. “Warmer air can contain more moisture and increase cloud cover. At night, some of the heat radiated from the Earth’s surface is reemitted back towards the surface. In cities, this warming is exacerbated by paved surfaces and buildings, and other factors, such as unusually warm ocean temperatures, can also play a role.”

A warming ocean is directly linked to a warming atmosphere. And when combined, they can make evening temperatures feel even more uncomfortable — due to the absorbing heat trapped in the atmosphere and its slow release. In the summer, with high sun angles and long durations of sunlight, there’s simply not enough time to release the heat before the next sunrise, usually resulting in a warmer starting temperature the following morning.

Sea surface temperatures immediately east and south of New England have been mostly averaging 1 to 3 degrees above average — an average noticeably warmer than much of the Atlantic Ocean.

Sea surface temperatures are well above average east and south of New England, helping keep temperatures at night elevated.Climate Reanalyzer

How the evening heat affects us

Experts said the elevated nighttime temperatures have serious effects on our health.

“This impacts folks that don’t have or cannot afford air conditioning,” Rennie said. “The lack of cooler overnight temperatures prevents their bodies from recovering from the heat, especially during heat waves.”

For Samela St. Pierre, of Framingham, the intense heat this summer makes her fatigued. “I’m forced to navigate what I want to do around the heat, but it’s literally so exhausting sometimes,” St. Pierre, 33, said.

“Falling asleep with temperatures hot at night is a challenge,” she added. “I take cold showers usually multiple times a day, in the morning and definitely at night, to lower my body temperature before going to bed.”

St. Pierre also washes her face with cold water “dozens of times a day,” especially since she works from home. She said the key for her is also drinking “lots of water.”

For Sandoval, the heat also directly affected the quality of her sleep, with her usual 11 p.m. bedtime pushed back to 1 or 2 a.m. as she tossed and turned. Last week, a roommate got a new air conditioning unit and gave Sandoval her old one. The difference, she said, has been dramatic.

“I had a moment on my first night, where I was like, ‘Oh my God, I can’t believe I was living like this!’ " she said. “I had almost forgotten what it was like to have a good quality night of sleep.”

James Aikey, 14, of Marshfield, leaped from the Beach Street bridge in the Green Harbor neighborhood of Marshfield. His mother said of the heat, “This is the only place to be right now. At least there’s a breeze.”Craig F. Walker/Globe Staff

The humidity is what’s most taxing on the body, according to Brigitte Griffin, who’s lived without air-conditioning in her Framingham apartment for seven years. She uses fans and sometimes wet towels draped over them for some relief at night.

“The heat and humidity affect my sleep. I wake up more often,” said Griffin, 60, who added that occasionally she heads downstairs to the basement to get cool. “Years ago it didn’t seem as hot.”

Constant exposure to heat will diminish hydration and keep the body working overtime to try and cool itself, according to health experts. “With humid air present, the difference between skin and ambient air is much less, so the more humid it is, the harder it gets for the body to cool, resulting in less heat transfer from the skin,” said Dr. Paul Biddinger, chief preparedness and continuity officer at Mass General Brigham in Boston.

Growing up without air conditioning, Sandoval said that for years she never really saw the point, and much preferred to have a nice breeze blowing through an open window than the icy blast of an AC vent. This summer, though, is the first time she’s felt differently.

“I feel like during the summer, it used to be a lot more rare to see 90-plus degree days. But it’s just felt super consistent,” she said. “This summer has actually officially forced me to change. I owe my life to AC now.”

Marianne Mizera of the Globe staff contributed to this report.

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