Day after day of unrelenting heat can have a cumulative, and deadly, effect.
Lost in the glare of a pair of high temperature records this week -- including one in the rare triple digits -- were three other records, which most Twin Cities residents may have slept through. Perhaps fitfully.
Those were the low temperatures Monday, Wednesday and Thursday, which were the warmest on record for those dates and a measure of the current heat wave's potential threat to human health.
While most people can cope with a day or two of extreme heat, it's the long siege without cooling that can lead to heat stroke and even fatalities, said William Roberts, professor of family medicine at the University of Minnesota.
In July 1995, Chicago suffered through a five-day stretch with some of its highest temperatures on record, including overnight lows in the 80s. That heat wave killed 750 people, many of them residents of buildings without air conditioning and who had no access to it.
"Cumulative heat like we're having does increase the risks of heat stroke for people who are active, but also people who are inactive," said Roberts. "You don't cool off enough to let the organs rest, especially if you allow yourself to dehydrate, or you're not real well-nourished. Things fall apart in a hurry."
In Chicago in 1995, he said, the difference between life and death among the elderly was in some cases 20 to 30 minutes in air conditioning.
Roberts also noted that Korey Stringer, an all-pro offensive tackle for the Minnesota Vikings, died of heat stroke after the second day of training camp in July 2001, not on his first. Likewise, the military has documented that heat complications in soldiers are more common on the second and third day of exposure than the first, Roberts said.
There have been no reports of heat-related fatalities in the Twin Cities since the heat wave began last Friday. One key factor, Roberts said, is the prevalence of air conditioning. Also, in strategies that emerged from the Chicago debacle, cities and other agencies publicize "cooling centers" -- air-conditioned public places where people can go to get a break from the heat.
In 1936, deaths mounted over time during the worst heat wave on record in the Twin Cities, noted assistant Minnesota state climatologist Pete Boulay. In a nine-day period that July, the Twin Cities experienced eight days with highs of 100 or more, including the all-time record 108, as well as the all-time warmest overnight low of 86; that reading was part of a streak of seven straight overnight lows in the 80s. But deaths peaked on Day 9, with 22 in Minneapolis and 49 in St. Paul.
National Oceanic and Atmospheric Administration spokesman Chris Vaccaro said improved forecasting has helped tremendously.
"We know these heat waves are coming, and local communities can make adjustments." Vaccaro said.
As in most heat waves, the suburbs have been somewhat less hot than the core cities this week. University of Minnesota professors Peter Snyder and Tracy Twine have been tracking Twin Cities temperature patterns at more than 150 metro sites as part of a project for the university's Institute on the Environment. They have found nighttime temperatures at Lake Elmo running about 7 degrees cooler this week than those at Minneapolis-St. Paul International Airport, where the Twin Cities' official temperatures and rainfall are recorded.
Urban cores tend to stay relatively warm at night, Snyder said, because the common materials in that environment -- asphalt, concrete and brick -- absorb more heat than vegetation and water and continue to release it after sundown.
Twin Cities dew points were in the 70s -- a key discomfort threshold -- for almost all of Tuesday and Wednesday before dropping back into the 60s Thursday afternoon. Even so, they were still higher than many readings between Minnesota and the Gulf Coast late in the afternoon.
The Twin Cities are has experienced 88 hours with dew points of 70 or higher this season. The average for a season is 168. Thursday's high temperature of 96 degrees was also the 15th of 90 or higher this season. The average is 13.
Friday's predicted high of 93 for the Twin Cities is the last high in the 90s in the forecast through Thursday. That means the current heat wave will end as memorable, if not historic.
"It could have been worse, I guess," Boulay said.
Bill McAuliffe • 612-673-7646