A scorching heat wave toppled daily temperature records in 12 cities in Ontario and another dozen communities in Quebec Thursday.

The hottest spot in Canada was Pearson International Airport in Toronto, where the temperature hit 37.9 C, setting a record for July 21. With stifling humidity, it felt like 50 in Canada's biggest city.

Windsor, Ont., also set a record for this date when the temperature hit 37.6 C at the airport. With the humidex, it felt even hotter.

Jodi Stone brought her kids to a splash pool in Windsor. "It's unbelievably hot and humid, so we decided to get the kids out and get them cooled off because it's just too hot at home," she said.

Her four-year-old son James took a break from splashing through the water to list off the must-do's for staying cool, which include drinking juice and eating ice cream.

Ontario daily record highs: The lift bridge in Burlington hit 36 C, up from the previous record of 31.5 set in 2005. Cobourg hit 30.3 C, slightly above its previous high of 26.9 C set in 2005. Collingwood hit 33.9 C, higher than the 32.5 C it hit on this date in 1998. Elliott Lake airport hit 30.6, beating a previous high of 30.2 C in 2005. Goderich hit 32.5 C, well above the previous record of 28.7 C in 2004. Munro Hamilton International Airport hit 36.4 C. The previous record was 32.7 C in 2002. Mount Forest hit 32.6 C, compared with a previous high of 30.6 in 2005. North Bay Airport hit 32.1 C, up from 31.7 from 1955 Point Petre hit 28.8 C, slightly above its previous best of 27.2 C reached last year. Conditions at the Buttonville airport touched 37 C, above the 33.8 C record hit in 2005. Pearson airport broke its record, with the temperature at Canada's busiest airport hitting 37.9 C, higher than previous record set in 1955. Windsor airport hit 37.6 C, above the previous record of 35.4 C set in 1998. See the daily temperature records for July 21 set in Quebec. Source: Environment Canada

Kylie Clark was enjoying the heat at an Ottawa area beach. The temperature in Ottawa hit 36 C on Thursday but felt like 46 with the humidity.

"We complain about the winter, and then we complain about the heat," Clark said from Westboro beach. "I say pick a season and then don't complain about the other one."

'Heaven on earth'

Frank Olszynko, owner of Lois 'n Frima's Ice Cream, described the hot weather in the Ottawa area as "heaven on earth."

"It is hot, but we have to enjoy it," he said. "We can't complain when it's hot. We can complain only when it's cold."

Olszynko's workers kept cool by standing in buckets of cold water while they served up cones to eager customers.

But CBC reporter David Gerow said some Ottawa neighbourhoods looked like a "ghost town" as people stayed inside to avoid the afternoon sun.

At least 12 Ontario cities set daily temperature records for July 21.

CBC meteorologist Karen Matthews said that 12 cities in Quebec also broke daily temperature records.

"Montreal was the best record broken, hitting 35.2 C, breaking a daily record set back in 1955," Matthews said.

Montreal public health officials were reminding people that many are at a serious risk because of the heat. In May, the public health agency published a report stating that in 2010, 106 deaths in the city were likely caused by heat waves.

Agency officials said the elderly are the most vulnerable, but this year they're also reaching out to those suffering from mental health issues.

Cities set up cooling centres

The heat sent residents of several Ontario cities to emergency cooling centres, slowed commuter trains and prompted the Blue Jays to close the retractable roof of Toronto's Rogers Centre for their afternoon game against the Mariners (the first time it has been closed because of the heat).

Due to the extreme heat in Toronto, Air Canada Cargo imposed a temporary embargo on transporting animals via cargo for animal health and safety reasons. The embargo is reviewed regularly and has been extended for another 24 hours due to forecast temperatures, Air Canada spokeswoman Angela Mah told CBC News at 5 p.m. ET Thursday.

Thursday afternoon's thoroughbred racing card at Woodbine was cancelled because of the heat. The decision was made after the racetrack's management consulted with the Horsemen's Benevolent and Protective Association and the Ontario Racing Commission veterinarians.

Guide to the humidex The humidex is an index number without units that measures the combined effect of heat and humidity. The humidex number is equivalent to the dry temperature in degrees Celsius that it feels like to someone outside in the heat. What it feels like when the humidex is: 30 to 39: Some discomfort.

40 to 45: Great discomfort; avoid exertion.

Above 45: Dangerous.

Above 54: Heat stroke imminent.

"In the best interest of the horses and after having discussed it with the HBPA and the veterinarians, it's the right decision to cancel Thursday's thoroughbred card," said Steve Koch, WEG's vice-president of thoroughbred racing. Thoroughbred racing is expected to resume at Woodbine on Friday. at 2 p.m.

Canada Post mail carriers in heat wave regions were allowed to start their routes in the early-morning hours, so they're not out during the hottest part of the day, said spokesman John Caines. Carriers were also given sunscreen, hats and water.

"They're professionals as well, so they know how to conduct themselves in this kind of weather, but if they can get out early and get the mail before it gets too, too hot and get it all delivered, then it's better for everybody," he said.

Despite the soaring temperatures, the province's power supply was expected to meet demand. The Independent Electricity System Operator predicts peak demand will hit 25,591 megawatts today, which won't even be enough to crack the top 10 peak demand days.

The all-time high was Aug. 1, 2006, when Ontario needed 27,005 megawatts of electricity. IESO spokesman Terry Young said lower industrial demand is one of the reasons we won't set any power consumption records despite the soaring heat and humidity.

The heat also forced GO Transit commuter trains to travel slower than usual as a safety precaution because the metal tracks they ride on expand in the heat. The transit operator was warning that it could mean 10- to 15-minute delays.

The rising temperatures have been caused by a so-called heat dome — a hot, unmoving high-pressure area hovering over central Canada.

The dome is pushing the jet stream well to the north and keeping cooler or wetter weather out. The same phenomenon has also caused a heat wave in the U.S., the worst in more than a decade.