Humidex

Key takeaways

Humidex belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Humidex to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Humidex from memory before moving on to harder problems.

Reference excerpt

The humidex (short for humidity index) is an index number used by Canadian meteorologists to describe how hot the weather feels to the average person, by combining the effect of heat and humidity. The term humidex was coined in 1965. The humidex is a nominally dimensionless quantity (though generally recognized by the public as equivalent to the degree Celsius) based on the dew point. Range of humidex: Scale of comfort

20 to 29: Little to no discomfort 30 to 39: Some discomfort 40 to 45: Great discomfort; avoid exertion Above 45: Dangerous; heat stroke quite possible

History 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 the dew point rather than the relative humidity, though both dew point and relative humidity (when used in conjunction with air temperature) are directly related to atmospheric moisture. For a long time, the record humidex in Canada was set by Windsor, Ontario, which hit 52.1 on 20 June 1953, as reported by Environment Canada. This value was beaten on 25 July 2007 when Carman, Manitoba, hit 52.6.

Computation formula When the temperature is 30 °C (86 °F) and the dew point is 15 °C (59 °F), the humidex is 34. If the temperature remains 30 °C (86 °F) and the dew point rises to 25 °C (77 °F), the humidex rises to 42. The humidex is higher than the U.S. heat index at equal temperature and relative humidity. The humidex formula is as follows:

Humidex = T air + 0.5555 ( e − 10.0 ) {\displaystyle {\text{Humidex}}={T}_{\text{air}}+0.5555(e-10.0)}

where e {\displaystyle e} is the vapour pressure in millibars given by:

e = 6.11 exp ⁡ [ 5417.7530 ( 1 273.16 − 1 T dewpoint ) ] {\displaystyle e=6.11\exp \left[5417.7530\left({\frac {1}{273.16}}-{\frac {1}{T_{\text{dewpoint}}}}\right)\right]}

also note:

T air {\displaystyle {T}_{\text{air}}} is the air temperature in degrees Celsius

T dewpoint {\displaystyle {T}_{\text{dewpoint}}} is the dewpoint in kelvins, which can be calculated from dewpoint in degrees Celsius using { T } K = { T } ∘ C + 273.15 {\displaystyle \{T\}_{\text{K}}=\{T\}_{^{\circ }{\text{C}}}+273.15}

5417.7530 is a rounded constant based on the molecular weight of water, latent heat of evaporation, and the universal gas constant. The humidity adjustment approximately amounts to one Fahrenheit degree for every millibar by which the partial pressure of water in the atmosphere exceeds 10 millibars (10 hPa). At the time the humidex was originally developed in 1965, Canada was still on the Fahrenheit scale, and thus the humidex was originally based on that. The 1979 reformulation added the 0.5555 factor (from the relation 1 °F = ⁠5/9⁠ °C), to address metrication in Canada as the country switched to the Celsius scale.

Table

See also Heat index (with temperature chart in both °C and °F) AccuWeather RealFeel temperature Thermal stress on humans Wind chill

References

Illustrations

Worked examples

Example 1 — a first encounter with Humidex

Start with the simplest possible case. Write down what Humidex claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Humidex before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Humidex ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Humidex

In research: Humidex appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Humidex in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Humidex is common in secondary-school and first-year university syllabi. It links to neighbouring topics 1979 introductions, Atmospheric thermodynamics, Climate of Canada, so understanding it makes those chapters shorter.
In everyday life: Look for Humidex outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Humidex in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Humidex means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Humidex out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Humidex in simple terms?

The humidex (short for humidity index) is an index number used by Canadian meteorologists to describe how hot the weather feels to the average person, by combining the effect of heat and humidity. The term humidex was coined in 1965.

Why does Humidex matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Humidex?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Humidex.