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'Heat dome' bringing scorching hot temperatures to London and rest of Southwestern Ontario beginning on Thursday

Dale Carruthers

By Dale Carruthers, The London Free Press

Theresa Barritt of Halifax and her London granddaughter, Madeline Doyle, eight months, beat the heat Monday at the Gibbons Park splash pad. There’s more heat and humidity heading London’s way. (MORRIS LAMONT, The London Free Press)

Theresa Barritt of Halifax and her London granddaughter, Madeline Doyle, eight months, beat the heat Monday at the Gibbons Park splash pad. There’s more heat and humidity heading London’s way. (MORRIS LAMONT, The London Free Press)

The combination of a “heat dome” hovering over the London region and fast-growing corn crops pumping moisture into the summer air will pack a one-two weather punch later this week.

Forecasters say a heat dome moving east across the U.S. will hit parts of Canada, including Southwestern Ontario, causing daytime highs that will feel above 40 C with the humidex.

The high-pressure system that takes rising warm air and pushes it down again — heating it in the process — will result in three straight days of hot and sticky weather without any nighttime relief beginning Thursday.

“It’s like having a lid on you,” Environment Canada senior climatologist Dave Phillips said Monday. “There’s no relief, there’s very little ventilation.”

Adding to the misery, corn crops are in rapid-growth phase across Southwestern Ontario, the nation’s corn belt, releasing moisture into the already humid air and pushing the humidex up in the process.

One hectare of corn sends more than 19 millimetres of moisture into the atmosphere, a process known as transpiration or crop sweat, said Peter Johnson, a Real Agriculture agronomist.

Corn plants release less water during dry growing seasons, but this year’s crop has had enough to drink, Johnson said. “Most of the crop in the region looks excellent, so it’s going full steam,” he said.

Environment Canada expects the heat to arrive Thursday, when a high of 32 C is expected, before rising to 33 C Friday and Saturday.

But what will make the three-day stretch especially brutal, said Phillips, are the nighttime lows that won’t dip below 20 C.

“It will correspond to what we call the dog days of summer for London,” said Phillips.

Already suffering through a hot, dry summer, London is coming off three straight months of above-average temperatures. The city has already recorded eight days above 30 C, compared to just five in all of last year.

The Middlesex-London Health Unit has issued two heat warnings — both lasting two days — already this year, up from one the previous summer. The alerts are put out when forecasters call for daytime highs above 31 C and nighttime lows that don’t fall below 20 C for two straight days, or when the humidex breaks 40 C for two consecutive days.

Known as the humidex, the heat index — the summer equivalent of winter’s wind chill — measures what the temperature really feels like after humidity is added. 

About this heat dome

  • High-pressure system moving east
  • Arriving in Southwestern Ontario Thursday
  • Rising warm air pushed down, making it hotter
  • Brings clear skies and little chance of rain

dcarruthers@postmedia.com

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