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Dew Point: The Measure Of Moisture In The Air

Humidex Table

During the current warm spell,  the conversation often refers to the humidity.   Humidity is essentially determined by comparing the temperature and the dew point.   Since temperature varies through the day, so does the (relative) humidity… typically highest (90-100 percent) in the early morning when the temperature is coolest; being closest to the dew point temperature (and when the dew forms).  In the afternoon, while the relative humidity lowers as the temperature rises, it actually feels more uncomfortable.

While the relative humidity can change considerably during the day, the dew point temperature remains fairly constant, and is a better measure of the total amount of moisture in the air, and the level of discomfort.

The Environment Canada weather page displays the temperature, dew point and humidex:

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The Humidex is a calculation that uses the temperature and dew point measurements to determine the level of discomfort or danger.

Humidex Table

Humidex Table

Humidex values in the 30’s are uncomfortable, while humidex values in the 40’s can be dangerous.  This week, high temperatures will be reaching 32C in a number of Maritime communities.  The dew point was in the mid teens on the weekend, so humidex values were around 35C.  Today, dew point temperatures are approaching 20, and humidex values will approach 40C.  However, as the week goes on, dew point temperatures could climb into the low 20’s.  The outcome will be very uncomfortable humidex values of around 40C, and Environment Canada would issue a Heat Warning.  Higher dew point temperatures are also associated with warm nights that stay above 20C, as well as thicker and more widespread fog along the Atlantic and Fundy coasts.

The computer models agree that warm days and warm nights will continue into the weekend.

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Experts in occupational health and safety recommend additional rest periods and plenty of water during periods of high humidex.

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While we welcome this fine and warm summer weather, we should remember to protect ourselves from the sun and the heat…..



About Jim Abraham

Jim has spent about 40 years in the weather business. He has been an operational forecaster from Halifax to Whitehorse. Jim started the Canadian Hurricane Centre, and has flown into a couple of these storms. As a senior executive within Environment Canada, Jim has managed weather research, weather services, and weather/water/climate observing programs. Retired from Environment Canada, Jim is the Atlantic Director for the Canadian Climate Forum, the president of the Halifax chapter of the Canadian Meteorological & Oceanographic Society, a partner in Climaction Services, and a part-time meteorologist on CBC radio. He is still participating in national and international activities related to weather preparedness. Having witnessed unprecedented advances in the science and technology of meteorology. Jim hopes that this blog will also be educational; enabling users to better understand weather-related phenomena, better interpret available information, and ultimately better able to make decisions to protect themselves, their family and their property. Jim welcomes any questions and suggestions.


The views and opinions expressed in this content are those of the author and do not necessarily reflect those of haligonia.ca.


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