(CBS News) The heat has been oppressive in Chicago and across much of the country. Temperatures climbed into the 90s and 100s yet again today, and the humidity is making it feel a lot worse.
Heat warnings and watches are up all over the heartland. We've seen heat waves before, but never like this.
It's topped 90 degrees in New York for four days. Chicago sweat through three straight days of 100 degrees. And in Washington, it's been 95 degrees for nine days, the longest stretch in 140 years. Across the country, there have been at least 13 heat-related deaths.
"Eighteen significant weather sites in the United States with long periods of record have set their all-time maximum temperatures," meteorologist and weather historian Chris Burt told CBS News.
What's behind this historic heat is a warm air mass that began over the Rockies, crept East and retreated back to the Midwest.
Overheated lakes killed fish in Wisconsin. One of the few places to find relief in St. Louis was a sliver of shadow under the Gateway Arch. In Iowa -- corn crops are in jeopardy. Farmer Andy Hall said that's pushed prices up 18 percent this month.
"You can tell the corn's been stressed by the way the leaves are curled up. It looks like pineapples," Hall explained.
In the control room of New York's main electricity supplier, Con Ed, they're keeping a close eye on the grid that supplies power to 9 million people.
"The build up of time and heat in the ground can have an impact on equipment," power company Con Ed spokesman Mike Clendennin said.
It's a mere 91 degrees in New York's Times Square. Temperatures will hit 100 tomorrow. Meteorologists predict the heat wave across the Midwest and the East should break by Monday.