This past summer didn’t set U.S. records for just daytime temperatures but also for nighttime ones, posing a danger to the elderly and low-income people who depend on overnight cooling, a new study reveals.

At nearly one of four U.S. weather stations – 278 out of 1,218 – the average nighttime low temperatures for June, July and August were hotter than at any time since 1895, according to an analysis by the Natural Resources Defense Council, an environmental group. Record highs were set at stations in 37 states, including 40% of all those east of the Mississippi.

“Welcome to what might be termed the dark side of climate change,” said Dan Lashof, director of NRDC’s Climate Center. He said the summer’s temperatures reflect a longer-term trend. “The long, hot summer of 2010 follows the hottest decade on record and more record high temperatures can be expected in the future as heat-trapping pollution continues to build up in our atmosphere.”

The report, released Thursday, says nighttime temperatures are more sensitive to greenhouse gases than daytime ones because increases in cloud cover and atmospheric aerosols counteract some of the heat effect during the day.

“Hot, stagnant nights can prove even more harmful than daytime highs as vulnerable populations –particularly the elderly and low-income individuals without air conditioning – are unable to cool down and get relief from the stress of daytime heat that persists into the evening, ” said Kim Knowlton, a senior scientist in NRDC’s health program.

The report highlights the following states:

• In Maryland, 12 of the 16 stations in the Historical Climatology Network reported their hottest average nighttime low temperatures on record in summer 2010. All 16 Maryland stations reported average nighttime low temperatures among their five hottest on record in summer 2010.

• In Florida, nearly all – 21 of 22 – weather stations reported average nighttime low temperatures among their five hottest on record in summer 2010.

• The Midwest also experienced very warm nighttime temperatures. In Illinois and Indiana, 92% and 86% of the stations, respectively, reported average nighttime low temperatures among their five hottest on record in summer 2010.

• The Western United States was not as hot as the Eastern half of the country. Nonetheless, seven stations in Arizona reported average temperatures for this summer among their five hottest on record, and 11 stations in New Mexico reported average nighttime low temperatures among their five hottest.

The report says record-high temperatures are not the only weather extreme this year, citing also severe droughts and floods. It says Russia has seen hundreds of wildfires and thousands of deaths in Moscow during its worst heat wave on record, and in Pakistan, more than a thousand people have been killed and a million more displaced by floods. It says flooding this year has also killed more than a thousand people in China, and more than 50 in Iowa and Tennessee.