Much of Canada and the U.S. is entrenched in extreme heat and humidity that are making people cranky and prompting them to crank up air conditioners and electricity demand.

Temperatures of 30 C and higher — with the humidity making it feel upward of 40 — are also good reason to take precautions to prevent heat exhaustion and other health threats.

Tuesday brought record-breaking temperatures to 11 locales in Ontario and six in Manitoba. The hot spot of the day seemed to be Weyburn, Sask., at 35.6 C, though Saskatchewan hadn't managed to beat any previous highs by 5 p.m. local time.

The humidex reached the mid- to high-40s in some cases in Saskatchewan and southern Manitoba, though a cold front sweeping across Saskatchewan Tuesday night is expected to bring thunderstorms, followed by cooler and gusty west winds.

The cold front will cross Manitoba Wednesday, but high humidex values are likely to redevelop over southeastern regions before the front moves through in the afternoon.

CBC meteorologist Johanna Wagstaffe said Tuesday will have been the most comfortable day of the week for the Greater Toronto Area. Humidex values were in the low- to mid-30s for most regions of Ontario. Windsor was the exception, where it hit 41.

Some areas of central Saskatchewan were hit by large hail Monday night. (CBC)

"High heat and humidity [are] back on [Wednesday] with the peak heat on Thursday," when it will be 37 C in Toronto with a humidex in the high 40s, she said.

The humidex advisory will likely be extended to more regions in northwestern Ontario for Wednesday as the uncomfortable conditions spread eastward, then on to regions in southern Ontario later.

The heat wave will continue on Friday and into the upcoming weekend for most of southern and eastern Ontario. Toronto and Middlesex-London have issued heat alerts.

Air conditioners and fans set on high to beat the heat helped set a new summer peak power consumption record in Alberta.

Tuesday will be one of the hottest days for Saskatchewan and Manitoba, with humidex values soaring into the mid-40s in both provinces. "Although weather will remain hot and humid, temperatures will come down to the high 20s starting tomorrow," Wagstaffe added.

On Monday, Atikokan, Ont., and Winnipeg both hit official heat-wave status — three consecutive days of 32 C or higher.

In Winnipeg, the forecast Tuesday is for a high of 37 C, with the humidity making it feel more like 48.

One Winnipeg homeless shelter has been handing out water bottles, lip balm and sunscreen to clients, but has run out of some items.

Many people are curtailing outside activities, including museum workers who cancelled a dinosaur fossil dig in Morden, Man.

A malfunction at a Manitoba Hydro station left about 5,000 Winnipeg residents without air conditioning Sunday night, but the problem has since been resolved.

In Saskatchewan on Monday, some central areas were hit by baseball-sized hail. Environment Canada is still trying to confirm whether tornadoes touched down, but winds everywhere were high — up to 130 km/h at Meadow Lake.

On Monday in Alberta, the recent hot weather has been cited as the driving factor behind a record summer peak in electricity, breaking a record set on Aug. 18, 2008.

"Our load is quite high, but the majority of units are operating at or near maximum capacity, therefore we have adequate supply to meet demand," says Matthew Davis, supervisor of market analytics for the Alberta Electric System Operator.

In northwestern Ontario, the hot, unsettled weather has combined lightning with tinder-dry conditions to spark more than 100 forest fires.

The sweltering conditions made their way into Canada from the U.S. Midwest, and forecasters say the extreme discomfort will soon spread to the East Coast.

The worst conditions Monday blanketed a broad band from Texas to Minnesota and the Dakotas, as 17 states issued heat watches, warnings or advisories. Some of the highest humidex values were reported in Newton, Iowa (52 C), Mitchell, S.D., (nearly 49 C) and Madison, Minn. (48 C).

Parking the car urged

In Canada, smog alerts are prompting transit authorities to encourage drivers to take public transit instead of their cars. In Laval, Que., for instance, the Société de transport de Laval (STL) announced it is reducing the single-ride fare on its buses on Tuesday from $2.75 to just $1.

It's estimated the special fare might prompt about 1,500 motorists to leave their cars at home on high-smog days, according to an STL news release.

The Canadian Red Cross is warning that people most at risk of suffering heat exhaustion and other adverse health effects from extreme heat are the elderly and young children, as well as those who:

• Work, exercise, or live outdoors.
• Have health problems, especially people taking certain medications.
• Have had heat-related illness in the past.
• Have heart disease or other conditions that cause poor circulation.
• Take medications to eliminate water from the body (diuretics).

The Red Cross also gives these tips for beating the heat:

• Avoid being outdoors in the hottest part of the day, between about 10 a.m. and 4 p.m. Try going out in the early morning or later evening when the sun is not as strong.
• Slow down activities that make you hot. Work and exercise in brief periods.
• Take frequent breaks.
• Dress in light, loose clothing, and wear a hat.
• Drink plenty of cool fluids, but avoid caffeine and alcohol.

With files from The Associated Press and The Canadian Press