Brochures

Humidity

• Relative humidity
• Humidex
• Guide to summer comfort
• Hair and humidity
• See the humidex table
• Humidity in the forecast - brochure
• Frequently asked questions

It's a well-known fact that the body retains more heat when the weather is hot and humid than it does during a drier but equally warm day. Why? As a response to excessive heat in the surrounding environment, our bodies perspire to make us feel cooler. Evaporation works best when the air is dry. In moist, saturated air, perspiration cannot evaporate as readily. The combination of excess heat and moisture will cause us to feel hot and sticky. As a rule of thumb, the higher the humidity, the greater the discomfort.

Relative humidity

Relative humidity is the amount of moisture that the air contains compared to how much it could hold at a given temperature. A figure of 100 per cent would mean that the air has become saturated. At this point, mist, fog, dew and precipitation are likely.

Relative humidity is normally at its maximum when the temperature is at its lowest point of the day, usually at dawn. Even though the absolute humidity may remain the same throughout the day, the changing temperature causes the ratio to fluctuate.

Humidex

The humidex is a Canadian innovation, first used in 1965. It was devised by Canadian meteorologists to describe how hot, humid weather feels to the average person. The humidex combines the temperature and humidity into one number to reflect the perceived temperature. Because it takes into account the two most important factors that affect summer comfort, it can be a better measure of how stifling the air feels than either temperature or humidity alone.

The humidex is widely used in Canada. However, extremely high readings are rare except in the southern regions of Ontario, Manitoba and Quebec. Generally, the humidex decreases as latitude increases. Of all Canadian cities, Windsor, Ontario has had the highest recorded humidex measurement: 52.1 Celsius on June 20, 1953. The hot, humid air masses which cause such uncomfortable weather usually originate in the Gulf of Mexico or the Caribbean.

Guide to summer comfort

Range of humidex: Degree of comfort

• Less than 29 Celsius: No discomfort
• 30 to 39 Celsius: Some discomfort
• 40 to 45 Celsius: Great discomfort; avoid exertion
• Above 45 Celsius: Dangerous
• Above 54 Celsius: Heat stroke imminent

An extremely high humidex reading can be defined as one that is over 40 Celsius. In such conditions, all unnecessary activity should be curtailed. If the reading is in the mid to high 30s, then certain types of outdoor exercise should be toned down or modified, depending on the age and health of the individual, physical shape, the type of clothes worn, and other weather conditions.

If working outdoors is an absolute necessity, drink plenty of liquids and take frequent rest breaks. In hot, humid conditions, there is a considerable risk of heat stroke and sun stroke.

During the dog days of summer, remember that Fido too feels the heat. When the humidex is high, take special care to ensure that your pet is well-protected from the heat and has plenty of water to drink.

Despite its limitations, the humidex remains a useful, and hence popular, means of determining how hot it actually feels outside.

Hair and humidity

Did you know that hair length increases with humidity? The range between dry and saturated air can account for a difference in hair length of about three per cent.

In moist air, people with naturally curly hair experience the frizzies as their hair increases in length. Under the same conditions, people with long, straight hair find it going limp.

Hair is such a reliable indicator of good or bad weather, in fact, that it is the primary element of the hair hygrometer, an instrument that was used for years to measure humidity. Invented in 1783, it was used until more sophisticated technology was developed in the 1960s. For its time, it was a very accurate humidity-measuring device, although not in widespread meteorological use today.

This document is published by authority of the Minister of the Environment. Copyright Minister of Supply and Services Canada 1995. Catalogue Number En57-24/23-1995E. ISSN 0715-0040. ISBN 0-662-22990-8.

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