WASHINGTON — The killer lurking in the shadows of the current heat wave may be hot nights.
“Everybody kind of gets fixated on how hot it gets: ‘Did we break 100?’ ” observed Illinois state climatologist Jim Angel. “But the nighttime temperatures can be just as important.”
For people without air conditioning, a nighttime respite from the worst of the heat gives the body a vital chance to recover from the stresses of the day.
But while the current heat wave has recorded 12 all-time daily highs so far this month, it also has registered 98 all-time overnight highs, the National Oceanic and Atmospheric Administration reported at a briefing Thursday.
And that’s just all-time highs.
When it comes to a record high for a particular date, 1,279 locations have tied or broken daytime records this month while 3,128 nighttime highs have been tied or broken.
For example, Eppley Airfield in Omaha on Wednesday had an overnight low of 82 degrees. That was a full 5 degrees warmer than the previous warmest overnight on that date, set in 2002.
Litchfield, Minn., also posted an overnight low of 82, besting a warm nighttime record for that date of 74 degrees set in 1964. The overnight low of 82 at Lambert Field in St. Louis edged out the 1998 mark by 1 degree.
When temperatures overnight do not cool to levels that provide relief, it increases the stress on people without air conditioning, on livestock and on crops, said Deke Arndt, chief of the Climate Modeling Branch at NOAA’s National Climatic Data Center.
In general, both daytime and nighttime temperature increases “are consistent with what we would expect in a greenhouse-warmed world,” he added.
High overnight readings also increase energy consumption as air-conditioning units run deeper into the night and start earlier in the morning, he said.
NOAA reported that last month was the seventh-warmest June on record, and so far 2011 is the 11th-warmest year on record worldwide.
The long-range U.S. forecast for August calls for warmer than normal conditions across the southern tier of states from Arizona to North Carolina, with the hottest conditions concentrated in Texas and Louisiana, where it is also expected to be drier than usual.
No rain
