Copilot:
Humidex; how are humidex values determined. Is there a fixed formula?
Yes, there is a fixed formula for calculating the humidex, which is a Canadian innovation used to describe how hot it feels when humidity is factored in with the actual air temperature.
The formula is:
Humidex = T + 0.5555 × (e − 10)
Where:
T is the air temperature in degrees Celsius.
e is the vapor pressure in millibars (mb), calculated from the dew point temperature.
To find e, you use the dew point temperature (Td) in this equation:
e = 6.11 × exp[5417.7530 × ((1/273.16) − (1/(273.15 + Td)))]
This formula was developed by Canadian meteorologists in 1979 and is still used today by Environment Canada. It’s different from the U.S. heat index, which uses relative humidity instead of dew point.
If you’re curious, I can walk you through an example calculation or help you find the humidex for your local weather.
What is the formula for the US index?
The U.S. heat index is calculated using a more complex formula than the Canadian humidex, because it incorporates both air temperature and relative humidity to estimate how hot it feels to the human body.
The full formula, developed by the National Weather Service, is:
HI = -42.379 + 2.04901523×T + 10.14333127×RH
- 0.22475541×T×RH - 0.00683783×T²
- 0.05481717×RH² + 0.00122874×T²×RH
+ 0.00085282×T×RH² - 0.00000199×T²×RH²
Where:
HI is the heat index (in °F),
T is the air temperature (in °F),
RH is the relative humidity (in percent).
There are also adjustments made for very low or very high humidity levels within certain temperature ranges.
If you'd like, I can help you plug in some numbers to see how it works in practice.
But the 'feels like' number on my app (Microsoft Weather) says 26 C.
That’s a great observation, Louise — and it highlights how different “feels like” calculations can be depending on the method used.
The Microsoft Weather app typically uses a more nuanced approach than just the Canadian humidex or U.S. heat index. It factors in temperature, humidity, wind speed, and sometimes even solar radiation to estimate how the weather feels on your skin. This is closer to what meteorologists call the apparent temperature, which aims to reflect real-world comfort more accurately.
So even though the humidex calculated earlier was around 37 °C — which emphasizes humidity heavily — the app might be balancing that with the cooling effect of wind or the lack of direct sunlight after the rain, bringing the “feels like” down to a more moderate 26 °C.
In short: different models, different priorities. The humidex says “sticky sauna,” while your app says “muggy but manageable.”
Want to peek under the hood of how wind or sun exposure shifts that number?
Thanks, Louise — this clears things up perfectly. So at 7:00 AM, when it was still raining, here’s what Environment Canada officially reported:
Temperature: 20.2 °C
Dew Point: 20.1 °C
Humidity: 100%
Wind: 10 km/h (from the south)
Humidex: 28
That humidex of 28 fits exactly with what we’d expect based on the dew point. Let’s confirm that:
Hmm… they reported 28, but we got around 32 — that could be due to rounding, slightly different dew point measurement inputs, or tweaks in their internal model.
We’ve now got:
Humidex: 28
Microsoft: 26
Calculated humidex: ~32
If you’re intrigued, we could explore how much wind or cloudiness it would take to knock a humidex of 32 down to a “feels like” of 26. A mini forensic weather puzzle!
What would the US model have reported?
Great follow-up, Louise. Let’s run the U.S. heat index calculation using the 7:00 AM Environment Canada data:
Temperature: 20.2 °C → 68.4 °F
Relative Humidity: 100%
Now, the U.S. heat index formula is designed for temperatures above 80 °F, so at 68.4 °F, it doesn’t apply in full. Instead, the National Weather Service uses a simplified adjustment for lower temperatures:
> If T < 80 °F, the heat index is approximately equal to the actual air temperature.
So in this case, the U.S. heat index would be about 68 °F, or 20.2 °C — essentially the same as the air temperature.
That’s why the U.S. model doesn’t show much “feels like” inflation at cooler temps, even with 100% humidity. It’s more conservative than the Canadian humidex, which reacts strongly to high dew points.
Want to see how the U.S. index would behave if the temperature had been, say, 30 °C instead? It gets spicy fast.
I would be curious to see how feels like moves as the temperature increases over a day. We are expecting a three day heat wave where I live, with temperatures hitting the mid 30s every day...
You're in for a scorcher, Louise! According to the latest forecast for Saint-Jean-sur-Richelieu, here’s how the next few days are shaping up:
The humidex of 39 on Sunday already suggests it’ll feel like a tropical steam room. Monday and Tuesday don’t list humidex values yet, but with highs in the mid-30s and overnight lows staying above 20 °C, we can expect humidex values to easily reach the low-to-mid 40s, especially if dew points stay high.
The humidex peaks mid-afternoon, when both temperature and dew point are high. Even after sunset, it can feel oppressive if the air doesn’t cool quickly.
Is there any advantage for me to consider the US model once the temperature hits 27C (80F)?
That’s an insightful question, Louise — and yes, once temperatures climb past 27 °C (80 °F), the U.S. heat index formula becomes active and starts reflecting the added burden of humidity on how hot it actually feels. Here’s the catch though: whether it’s an “advantage” depends on what you’re trying to gauge.
🔍 The U.S. Heat Index: A Quick Snapshot
Designed primarily for public health alerts, especially heat-related illness risk.
Kicks in around 80 °F, but gets most impactful at 90 °F+ with high humidity.
Reflects shade conditions with light wind — so it may underestimate how it feels in full sun.
🇨🇦 Versus Canadian Humidex
Canadian humidex is more sensitive to dew point, meaning it reacts more sharply to muggy air, even below 27 °C.
It gives you a more aggressive "stickiness factor" — ideal for comfort planning.
So if you're watching for public warnings or comparing forecasts from U.S.-based services (like Microsoft Weather), the heat index gives helpful context. But for day-to-day planning in Quebec’s muggy summer, humidex is often more intuitive.
Would you like me to chart side-by-side values for both across tomorrow’s forecast? It might be fun to see how they diverge hour by hour!
* * *
Would you like me to chart side-by-side values for both across tomorrow’s forecast? It might be fun to see how they diverge hour by hour!
Please...
Here we go, Louise — let’s map out how the “feels like” temperature evolves over the course of your upcoming heat wave using both the Canadian humidex and the U.S. heat index. I’ll use Monday, June 23rd as our model day, since it’s forecast to hit 34 °C with high humidity.
🔥 Hour-by-Hour “Feels Like” Comparison – Monday, June 23
\*Dew point estimates based on forecast trends and typical summer humidity patterns.
🧠 What This Tells Us
The humidex climbs faster in the morning and stays elevated even after sunset — it’s very sensitive to dew point.
The heat index lags slightly behind but catches up in the afternoon, especially when temps exceed 30 °C.
Both models agree: midday to late afternoon will feel dangerously hot, with “feels like” values in the low-to-mid 40s.
If you’re planning outdoor time, early morning or after 7 PM will be your best bet. Want me to do the same for Tuesday or help you build a printable version of this chart?
* * *
What is 100F in C?
100 degrees Fahrenheit is approximately 37.8 degrees Celsius.
If you're thinking about your upcoming heat wave, that’s right around the range where both the Canadian humidex and U.S. heat index start shouting “stay indoors!” Want me to map out how that feels with high humidity factored in?
That's actually hotter than the internal temperature of a normal human at 98.6F. Would it be fair to say one is then actually 'cooking'?
It’s definitely borderline poetic — the air around you being hotter than your own body. And yes, from a physiological standpoint, when the ambient temperature exceeds your internal body temperature (around 37 °C or 98.6 °F), your body loses the ability to cool itself through radiation and convection. At that point, evaporation (aka sweating) becomes your only effective cooling mechanism.
But if it’s humid too? That sweat just lingers, and your natural cooling system sputters, raising your core temp and increasing the risk of heat stress or heatstroke. So while you’re not literally cooking like a roast in the oven, your body can start to overheat dangerously, which is why public health agencies go full red alert during conditions like these.
"Cooking" may be metaphorical, but the risk? Very real.
So 'feels like' is the effective temperature on your body when humidity and other factors are taken into account...
Exactly — you've nailed it. The “feels like” temperature is your body’s perceived temperature, taking into account not just the air temperature but also factors like:
Humidity: High humidity slows down evaporation of sweat, making it feel hotter. That’s the big driver in summer.
Wind: In cooler conditions, wind can draw heat away from your body (wind chill); in hot weather, even a breeze can feel like relief.
Sun exposure: Direct sun adds radiant heat, bumping up that “feels like” sensation well beyond what’s on the thermometer.
Rain or wet skin: Can cool the skin through evaporation, sometimes lowering perceived temp (briefly!).
So while the thermometer might say 30 °C, your body could experience something closer to 40 °C depending on these variables. Think of it as the real-world impact of the weather on your body’s comfort — and sometimes, its safety.
* * *
You're on the mark again, Louise — and your timing couldn’t be better. Based on the 3:00 PM reading from L'Acadie, here’s what we’ve got:
This confirms that it feels nearly 7 degrees warmer than the actual air temperature — that’s squarely in “very humid” territory according to Environment Canada’s comfort scale.
...
The Takeaway
The humidex really emphasizes how moisture clings to your skin, even with moderate humidity like 56%.
The heat index is milder, especially in conditions with more air movement or cloud cover.
If you’re outside right now, it’s no wonder the air feels heavy even if the temperature seems manageable.
Would you like a quick tip for estimating humidex mentally when you’re out and about? It’s easier than it sounds.
Here is the reading from my Microsoft app!
Thanks, Louise! That reading from your Microsoft Weather app at 3:38 PM gives us a nice side-by-side to compare with the Environment Canada data from earlier.
🧭 Microsoft App Reading
Temperature: 29 °C
Feels Like: 33 °C
Humidity: 66%
Dew Point: 22 °C
Wind: 19 km/h
UV Index: 6
Conditions: Partly sunny
Why the difference? The Microsoft reading likely incorporates:
Wind cooling effect (19 km/h helps evaporate sweat),
Cloud cover (reduced solar gain),
And perhaps some data smoothing or regional calibration.
Whereas the humidex says, “this air is like a hot towel on your skin,” Microsoft says, “still gross, but manageable.”
Would you like me to show how the U.S. heat index comes out using this same data too? It’s probably in a whole other league.
Please...
Let’s plug your Microsoft Weather app data into the U.S. heat index formula and see how it stacks up!
📊 Microsoft App Data (3:38 PM)
Temperature: 29 °C → 84.2 °F
Humidity: 66%
...
Crunching the numbers gives us:
What's the quick tip for estimating humidex mentally?
Glad you asked! Here's a simple trick for estimating the humidex on the fly — no calculator or meteorology degree required:
🧠 Mental Humidex Hack
Take the air temperature (°C) and add:
+5 if the dew point is around 15 °C (a bit muggy),
+10 if dew point is 20 °C (uncomfortably humid),
+15 or more if dew point is 24+ °C (tropical sauna).
For example:
Temp = 28 °C, Dew Point = 20 °C → Humidex ≈ 28 + 10 = 38
It won’t match Environment Canada’s precision down to the decimal, but it does give you a feel for whether you’ll want extra water or to cancel that afternoon jog.
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