Humidex

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.^[1] 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^[2]^[3]

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.^[4]

This value was beaten on 25 July 2007 when Carman, Manitoba, hit 52.6.^[5]^[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:^[7]^[8] ${\text{Humidex}}={T}_{\text{air}}+0.5555(e-10.0)$

where $e$ is the vapour pressure in millibars given by:

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

also note:

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

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

Humidex for range 15–43 °C
		Temperature (°C)
		15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40	41	42	43
Dew point (°C)
	10	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40	41	42	43	44
	11	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40	41	42	43	44	45
	12	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40	41	42	43	44	45
	13	18	19	20	21	22	23	24	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40	41	42	43	44	45	46
	14	18	19	20	21	22	23	24	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40	41	42	43	44	45	46
	15	19	20	21	22	23	24	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40	41	42	43	44	45	46	47
	16		21	22	23	24	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40	41	42	43	44	45	46	47	48
	17			22	23	24	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40	41	42	43	44	45	46	47	48
	18				24	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40	41	42	43	44	45	46	47	48	49
	19					26	27	28	29	30	31	32	33	34	35	36	37	38	39	40	41	42	43	44	45	46	47	48	49	50
	20						28	29	30	31	32	33	34	35	36	37	38	39	40	41	42	43	44	45	46	47	48	49	50	51
	21							29	30	31	32	33	34	35	36	37	38	39	40	41	42	43	44	45	46	47	48	49	50	51
	22								31	32	33	34	35	36	37	38	39	40	41	42	43	44	45	46	47	48	49	50	51	52
	23									33	34	35	36	37	38	39	40	41	42	43	44	45	46	47	48	49	50	51	52	53
	24										35	36	37	38	39	40	41	42	43	44	45	46	47	48	49	50	51	52	53	54
	25											37	38	39	40	41	42	43	44	45	46	47	48	49	50	51	52	53	54	55
	26												39	40	41	42	43	44	45	46	47	48	49	50	51	52	53	54	55	56
	27													42	43	44	45	46	47	48	49	50	51	52	53	54	55	56	57	58
	28														44	45	46	47	48	49	50	51	52	53	54	55	56	57	58	59

References

^ "Spring and Summer Hazards". Environment and Climate Changes. Government of Canada. Retrieved 22 September 2016.
^ Meteorological Service of Canada. "Humidex". Spring and Summer Weather Hazards. Environment Canada. Retrieved 20 June 2016.
^ Hong, Jackie (17 August 2015). "7 things you probably didn't know about the Humidex". The Star. Retrieved 23 September 2016.
^ "Spring and Summer Weather Hazards: Heat and Humidity". Environment and Climate Change Canada. Archived from the original on 14 July 2018. Retrieved 5 July 2018.
^ Cbc.ca Archived 16 January 2009 at the Wayback Machine
^ "Canada's Top Ten Weather Stories for 2007". Environment and Climate Change Canada. Retrieved 15 January 2017.
^ "Calculation of the 1981 to 2010 Climate Normals for Canada". Archived from the original on 27 June 2013. Retrieved 4 October 2014.
^ Canada, Environment and Climate Change (31 October 2011). "Glossary - Climate - Environment and Climate Change Canada". climate.weather.gc.ca. Retrieved 27 December 2019.

[1] "Spring and Summer Hazards". Environment and Climate Changes. Government of Canada. Retrieved 22 September 2016.

[SMC-2] Meteorological Service of Canada. "Humidex". Spring and Summer Weather Hazards. Environment Canada. Retrieved 20 June 2016.

[3] Hong, Jackie (17 August 2015). "7 things you probably didn't know about the Humidex". The Star. Retrieved 23 September 2016.

[4] "Spring and Summer Weather Hazards: Heat and Humidity". Environment and Climate Change Canada. Archived from the original on 14 July 2018. Retrieved 5 July 2018.

[5] Cbc.ca Archived 16 January 2009 at the Wayback Machine

[6] "Canada's Top Ten Weather Stories for 2007". Environment and Climate Change Canada. Retrieved 15 January 2017.

[7] "Calculation of the 1981 to 2010 Climate Normals for Canada". Archived from the original on 27 June 2013. Retrieved 4 October 2014.

[8] Canada, Environment and Climate Change (31 October 2011). "Glossary - Climate - Environment and Climate Change Canada". climate.weather.gc.ca. Retrieved 27 December 2019.

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v t e Meteorological data and variables
General	Adiabatic processes Advection Buoyancy Lapse rate Lightning Surface solar radiation Surface weather analysis Visibility Vorticity Wind Wind shear
Condensation	Cloud Cloud condensation nuclei (CCN) Fog Convective condensation level (CCL) Lifting condensation level (LCL) Precipitable water Precipitation Water vapor
Convection	Convective available potential energy (CAPE) Convective inhibition (CIN) Convective instability Convective momentum transport Conditional symmetric instability Convective temperature (T_c) Equilibrium level (EL) Free convective layer (FCL) Helicity K Index Level of free convection (LFC) Lifted index (LI) Maximum parcel level (MPL) Bulk Richardson number (BRN) Significant tornado parameter (STP)
Temperature	Dew point (T_d) Dew point depression Dry-bulb temperature Equivalent temperature (T_e) Forest fire weather index Haines Index Heat index Humidex Humidity Relative humidity (RH) Mixing ratio Potential temperature (θ) Equivalent potential temperature (θ_e) Sea surface temperature (SST) Temperature anomaly Thermodynamic temperature Vapor pressure Virtual temperature Wet-bulb temperature Wet-bulb globe temperature Wet-bulb potential temperature Wind chill
Pressure	Atmospheric pressure Baroclinity Barotropicity Pressure gradient Pressure-gradient force (PGF)
Velocity	Maximum potential intensity

Humidex

History

Computation formula

Table

See also

References