Sunday, July 22, 2007

The heat is on!..

It was a sizzling Sunday across southern Manitoba with temperatures of 32 to 33C and humidex values of 40 to 44C. As they say, it wasn't the heat.. it was the humidity, or more accurately, the dewpoints which were in the 21 to 24C range across southern MB. These dewpoints are in the uncomfortable range, and makes it more difficult for the body to cool off by evaporation. (see graph from my weather station showing dewpoint and heat index from today. Notice how the dewpoints dropped late in the day!)

And it looks like the hot humid weather will continue for the next 2 to 3 days before a cold front brings more comfortable conditions by Wednesday evening. Until then, expect more of the same as today with afternoon temperatures of 32 to 35C, and humidex values of 40 to 45C. Stay cool!

By the way, you may notice that my weather station displays "heat index" rather than the humidex. The "Heat Index" as shown on my station is an equivalent temperature based on air temperature and humidity (or more accurately, the dew point). It is similar to but NOT the same as the Canadian "humidex" value, which employs a slightly different equation and gives different results than the American "Heat Index". Generally the Heat index will register about 3-5C lower than the humidex. My station can only display heat index. A simple way of calculating the humidex is to add the temperature and dewpoint (in C) then subtract 13. For example, if the air temperature is 32C and the dewpoint is 22C then the humidex is 41C (32 + 22 - 13 = 41C) Environment Canada issues humidex advisories when humidex values of 40C or greater are expected. Whatever the case, both indeces are designed to tell you how hot it "really" feels based on the moisture content of the airmass. The basis for this is that when it's humid, it becomes harder for the body to cool since evaporation from the skin (i.e sweating) is less efficient. Thus, 30C with 60% humidity will feel "hotter" than 30C with 30% humidity because the body will not be able to cool as quickly at higher humidity levels. In reality, it's not any hotter, it's just more uncomfortable.

The dew point is the best indicator of how humid or "sticky" an airmass is. The following table gives a rough idea of dewpoint and comfort of an airmass..

< 10C ..............dry
10-14C ............comfortable
15-17C ............moderately humid, still comfortable for most people
18-21C ............humid, uncomfortable for most people
22-24C ............very humid, uncomfortable for everyone
>24C ..............oppressive, stifling

In our latitudes, dewpoints above 24C are rare although they do occur sometimes in southern MB in the growing season due to contribution from evapotranspiration from plants and crops. Dewpoints of 27C or more are usually only found in tropical areas near warm oceans.


  1. The dew points were terrible today, especially in extreme southern MB and northwest MN. The DP at Emerson hit 25, and the Hallock, MN airport showed a DP of 27 for a brief time. It's like walking into a steam room.

  2. Yeah, it was a steam bath today.. but we picked up a northerly breeze late in the day and our dewpoint dropped from 22 to 18 between 4 and 5 pm. It was noticeably more comfortable at that time, but I see our dewpoints are climbing back into the uncomfortable range tonight.

  3. Yes, I watched your dew points come down...and then ours did. Ours is at 20 now...hope to see about 18.5, which if it occurs, will probably be the low temp as well. There should be some morning fog here...then another hot, steamy day. I spent 6 years in Louisiana and saw enough 25+ dew points to last me about 10 lifetimes. The worst here today was a temp of 34 with a DP of 27. It gave a heat index of 46 and a Humidex of 49. Don't know which is more accurate, but both were too hot.