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The Humidex

Real Estate Agent with Century 21 Lanthorn Real Estate LTD., Trenton, Ontario

I listen to the radio when I'm in my car. Usually every hour or so I catch the news, and being Canadian, one of the most important features is always the weather. The winter is a whole different ballgame for me, once the temperature drops below a certain point I can't really tell the difference anyway, so I'm resigned to suffering without caring just exactly how much I'm supposed to be suffering.

In the last few years it would seem that the "humidex" plays a larger role in the delivery of the summertime weather presentation. The presenter will say "It's 29 degrees outside, but with the humidex it feels like 36." I always thought this was silly, because I don't know what 36 feels like. Just yesterday it was 36, but they told me that with the humidex it felt like 43. How the heck am I supposed to know what anything feels like because they are always telling me it feels like something else.

I decided to get to the bottom of this phenomenon, and this is what I discovered;


Now, I hope this clears up a lot of the confusion as to how they arrive at the "humidex" readings. It's pretty straightforward, no? Well then maybe the Wikipedia explanation will be a little simpler;

The current formula for determining the humidex was developed by J.M. Masterton and F.A. Richardson of Canada's Atmospheric Environment Service in 1979. Humidex differs from the heat index used in the United States in being derived from dew point rather than relative humidity.

The record humidex in Canada occurred on July 25, 2007, when Carman, Manitoba hit 53.0.[1] This breaks the previous record of 52.1 set in 1953 in Windsor, Ontario (The residents of Windsor would not have known this at the time, since the humidex had yet to be invented).

When the temperature is 30 °C (86 °F) and the dew point is 15 °C (59 °F), the humidex is 34 (note that humidex is a dimensionless number, but that the number indicates an approximate temperature in °C). If the temperature remains 30 °C and the dew point rises to 25 °C (77 °F), the humidex rises to 42. The humidex tends to be higher than the U.S. heat index at equal temperature and relative humidity.

The humidex formula is as follows:

humidex = (air temperature in Celsius) + h
h = (0.5555)*(e - 10.0)
e = 6.11 * exp [5417.7530 * ((1/273.16) - (1/dewpoint in kelvins))]

I suppose I will never really know what those numbers mean. I will just know that it's hot. That's all they really had to tell me anyway.  


Tanya Nouwens
Immeubles Deakin Realty - Montreal West Island, QC
Montreal Real Estate Broker & Stager

Now that's funny, Malcolm.  I interpret it like this: If the current temperature is X, and with the humidex it is X + 5, my hair will frizz.  If it is X + 10, I will look like I put my finger in a socket. If it is X + 15, I will not go outside. Period.  Hope that helps : )

Aug 03, 2010 11:08 AM
Janice Ankrett
Burlington, ON
Staging Professional

Malcolm, you want to run that by me again? I'm still confused LOL. I don't care how high it is humidity is the pits!

Aug 03, 2010 11:57 AM
Brian Madigan
RE/MAX West Realty Inc., Brokerage (Toronto) - Toronto, ON
LL.B., Broker


Very informative.

People tell me I should be a weatherman, because I tell people really how hot it is.......


Aug 03, 2010 03:10 PM
Marcia Hawken
Naples Luxury Specialist

Malcom, Perhaps if I study those figures and charts it won't seem as hot in Naples this time of year.  Goood distraction!

Aug 03, 2010 10:56 PM
Larry Lawfer
YourStories Realty Group - Newton, MA
"I listen for a living." It's all about you.

Malcolm,  I know the word rolodex and being in Dallas, I guy named Tex, but humidex still escapes me.  It's cold I put on more clothes.  Thanks for the lesson.

Aug 04, 2010 04:40 AM
Malcolm Johnston
Century 21 Lanthorn Real Estate LTD., Trenton, Ontario - Trenton, ON
Trenton Real Estate

Tanya Nouwens, Montreal Real Estate Broker (JJ Jacobs Realty Inc/Ready, Set...Sold! Inc, Montreal Canada) I'm sure the frizzometer makes more sense than the humidex Tanya.

Janice Ankrett ASP (Janice Ankrett Home Staging) Janice, I don't quite understand the numbers myself, and I'm not going to stretch my brain by trying.

Brian Madigan LL.B. (Royal LePage Innovators Realty, Broker) Brian, you'd make a great weather man.

Marcia Hawken - Naples Luxury Specialist (Downing-Frye Realty, Inc.) I can think of a few better distractions in Naples Marcia.

Aug 04, 2010 04:44 AM
Malcolm Johnston
Century 21 Lanthorn Real Estate LTD., Trenton, Ontario - Trenton, ON
Trenton Real Estate

Larry Lawfer (EXIT Realty Metro Dallas) Ah, but in Dallas you'd have the heat index, not the humidex. Apparently humidity is something that Canadians worry about a bit too much.

Aug 04, 2010 04:47 AM
Bob Foster
Century 21 Lanthorn R. E. Ltd. Belleville, Ontario - Belleville, ON

Hi Malcolm,

I had an interesting converstation with someone at Environment Canada one day asking what they meant by POP (probability of precipitation).

If the POP is 50% does that mean -

- it will rain 50% of the time?

- it will rain on 50% of the area?

- on 100 days with a POP of 50% it will rain on approximately 50 days? (Follow up question - how much rain? Does one drop count?)

The poor guy who answered said he wasn't sure it meant any of those - it was just a "general indicator", which I'm sure it is.

Then I decided that it really isn't nice to harrass public employees, so I thanked him and said good-bye : )


Aug 04, 2010 10:43 AM
Malcolm Johnston
Century 21 Lanthorn Real Estate LTD., Trenton, Ontario - Trenton, ON
Trenton Real Estate

Bob Foster (Century 21 Lanthorn R. E. Ltd. Belleville, Ontario) Haha Bob, it's not nice to ask public servants complicated questions like that.

John B. Joseph (Groupe Sutton Centre-Ouest, Westmount) Bite your tongue John, October is never something that should be looked forward to in Ontario.

Aug 09, 2010 04:14 AM