They warned us back in May. That's when Environment Canada suggested that the summer of 2011 would be warmer and drier than average across most of the country. And that's exactly what's happening — and not just in southern Ontario.

A large mass of hot, humid air has moved north out of the U.S. and has gripped Saskatchewan and Manitoba with sweltering temperatures. By Monday, July 18, Atikokan, Ont., and Winnipeg both hit official heat wave status: three consecutive days with highs of 32 C. With the humidity, it feels more like 40 C and higher.

Ontario and Quebec are anticipating very high temperatures by the end of the week.

Heat facts

Hottest day on record: July 5, 1937. Temperatures soared to 45 C in the towns of Midale and Yellowgrass, Sask.

The deadliest heat wave in history: July 5-17, 1936. Temperatures exceeding 44 C in Manitoba and Ontario and claimed the lives of 1,180 Canadians.

Highest humidex ever measured: 52.1 C in Windsor, Ont., on June 20, 1953.

Most of the time, high temperatures are a welcome change from chilly Canadian winters. Rising mercury usually means trips to the beach, ice cream and dips in the pool. But sizzling heat and high humidity can also be taxing on the body.

The human body normally cools itself by sweating, but in extreme heat and humidity that's not enough. Perspiration doesn't evaporate easily on hot, humid days, and that means the body must work harder to maintain a normal temperature.

People start to breathe rapidly, feel weak or faint, have headaches or feel confused. They can suffer heat stroke and heat exhaustion, or the hot temperatures can worsen existing health conditions. Over-exert yourself in the heat and you might find yourself in trouble.

Despite Canada's image of being a cold country, it can get quite hot. The hottest day on record was on July 5, 1937, in Midale and Yellowgrass, Sask., when temperatures peaked at 45 C.

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Hot days draw people to the beach for outdoor fun, but they can also be taxing on the body. (Darryl Dick/CP)

And in 1936, the deadliest heat wave in Canadian history hit Manitoba and Ontario. For almost two weeks in July, temperatures rose above 44 C and left 1,180 Canadians dead, mostly infants and the elderly. Four hundred of those drowned seeking refuge from the heat.

According to Environment Canada, the heat was so intense that steel rail lines and bridge girders twisted, sidewalks buckled, crops wilted, and fruit baked on trees. Similar tragedies have happened in recent times too. In 2003, a deadly heat wave that hit Europe claimed the lives of an estimated 11,000 to 15,000 people when the mercury hit 40 C and higher.

The current run of high temperatures is expected to last for a week.

Humidity and heat

But it's about more than just temperature — it's about how hot it feels, or the "humidex."

Guide to the humidex

If it feels like (expressed in degrees Celsius) Level of comfort
Less than 29 No discomfort
30-39 Some discomfort
40-45 Great discomfort; avoid exertion
Above 45 Dangerous
Above 54 Heat stroke imminent
Source: Meteorological Service of Canada

The humidex is an index, or calculated value, which describes how hot or humid weather feels to the average person.

This Canadian innovation — first used in 1965 — combines the air temperature and humidity into one number to reflect how the temperature is perceived.

When the air is moist and saturated, sweat can't evaporate easily and therefore, the body feels hot and sticky.

The more humid the air is, the more uncomfortable people feel. The highest humidex ever measured was 52.1 C in Windsor, Ont. on June 20, 1953. That number was calculated using historical data.

An extremely high humidex is a reading more than 40 C.

Extreme heat warnings are issued when the combination of heat, humidity and other weather conditions can be very dangerous.

Heat-related illnesses

Heat exhaustion

This happens when people exercise heavily or work in a hot and humid place, and body fluids are lost through heavy sweating. Blood flow to the skin increases, and blood flow to vital organs decreases, resulting in a form of mild shock. If the condition isn't treated, the body temperature will continue to rise, and the person may suffer heat stroke.

Heat stroke

When a person's temperature control system — sweating to cool down the body — doesn't work, heat exhaustion progresses to heat stroke. A person's body temperature can rise so high that it can cause brain damage and death if the body is not cooled quickly.

What exactly is too hot? Environment Canada issues extreme hot weather warnings when the air temperature is more than 30 C and the humidex is more than 40 C.

Municipalities also issue heat alerts, but the criteria vary. For example, in Toronto, the city's chief medical officer of health issues heat alerts and extreme heat alerts. These are ratings of how the weather affects human health, based on historical mortality data and meteorological data.

A heat alert is called when a hot air mass is forecast and the likelihood of deaths is more than 65 per cent. An extreme heat alert is issued when the heat has become more severe or is expected to last longer and the likelihood of deaths is more than 90 per cent.

How to cope with heat

Tips for exercising in the heat

  • Drink plenty of fluids.
  • Take frequent rests.
  • Reduce expectations from your workout.
  • If you feel dizzy, nauseous, or your vision becomes blurry, stop immediately and find a cool place.
  • If you really need a hard workout, considering taking it indoors.

To avoid heat-related illnesses, air-conditioned spaces, like shopping malls and libraries are good places to seek relief. If that's not possible, or if you must venture outside during extremely hot temperatures, here are some tips:

  • Stay hydrated. Drink lots of water and natural fruit juices even if you don't feel thirsty. Avoid alcoholic beverages, coffee and cola.
  • Avoid going out in the blazing sun or heat when possible. Seek shade as much as possible, or plan excursions for the early morning or evening.
  • Cover up. Wear a hat and loose-fitting, light clothing.
  • Reduce the heat. Keep the drapes drawn and blinds closed. Keep the lights off or turned down low.
  • Cool down with baths or showers periodically, or use cool, wet towels.
  • Avoid intense or moderately intense physical activity.
  • Be aware that fans alone may not be enough when the temperatures are high.