After weeks of dry heat, farmers in central Illinois say their corn crops could use a good shower or two, June 19, 2007. (Seth Perlman/Associated Press)

Young residents fetch water in recycled plastic cans as they walk on the dry bed of the Sabarmati River in Ahmadabad, India. At least 34 people died during a heat wave in the region in May 2006.
(Ajit Solanki/Associated Press)

Most of the time, warm temperatures are a welcome change from chilly Canadian winters. A rising mercury is associated with trips to the beach, ice cream and dips in the pool. But, sizzling heat can be taxing on the body. The body normally cools itself by sweating, but in extreme heat and humidity, it's not enough. Perspiration doesn't evaporate easily and 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.

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 45C.

QUICK FACTS

• Hottest day on record: July 5, 1937. Temperatures soared to 45C in Midale and Yellowgrass, Sask.
• The deadliest heat wave in history: July 5-17, 1936. Temperatures exceeding 44C in Manitoba and Ontario claimed 1,180 Canadians (mostly the elderly and infants) during the longest, deadliest heat wave on record.
• Highest humidex ever measured: 52.1C in Windsor, Ont. on June 20, 1953. (calculated using historical data).
• Spring 2006 is the warmest Canada has experienced since nationwide records were kept in 1948, at 3.0 degrees above normal, based on preliminary data. The previous record was 1998 at 2.9 degrees above normal.

And in 1936, the deadliest heat wave in Canadian history hit Manitoba and Ontario. For almost two weeks in July, temperatures more than 44C 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 40C and higher.

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

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. And the further south you are, the higher the humidex is. The highest humidex ever measured was 52.1C in Windsor, Ont. on June 20, 1953, calculated using historical data.

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

Here's a guide to humidex comfort levels, according to the Meteorological service of Canada:

• Less than 29C: no discomfort
• 30 to 39C: some discomfort
• 40 to 45C: great discomfort; avoid exertion
• Above 45C: dangerous
• Above 54C: heat stroke imminent

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

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

Municipalities also issue heat alerts, but the criteria varies. 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.

As of July 5, 2006, Toronto has issued 38 heat warnings and 29 extreme heat warnings since 2001.

So, how can a person cope with the heat?

To avoid heat-related illnesses, air-conditioned spaces, like shopping malls and libraries, are good places to seek relief. But, if that's not possible, or 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.