Meteorology, Occupational Hygiene & Environment Canada Humidex
Canadian Humidex Vapor Pressure & Thermal Discomfort Formula
Calculate Environment and Climate Change Canada (MSC) Actual Water Vapor Pressure e (hPa) from Dew Point via Clausius-Clapeyron, Humidex Increment h = 0.5555*(e - 10), Total Humidex Score, and MSC Public Heat-Advisory Tier.
Compute Environment Canada’s Clausius-Clapeyron Humidex directly from Dew Point!
Unlike polynomial relative-humidity heat tables, the Meteorological Service of Canada (MSC) Humidex (Masterton & Richardson) derives perceived mugginess directly from actual atmospheric water-vapor pressure e (hPa) using the Clausius-Clapeyron relation at Dew Point T_dew (°C): e = 6.11 * EXP(5417.753 * ((1 / 273.16) - (1 / (T_dew + 273.15))))! Because human sweat evaporation stalls once ambient vapor pressure exceeds the 10 hPa comfort baseline, Humidex adds h = 0.5555 * (e - 10) onto dry-bulb air temperature T_air (°C): Humidex = T_air + 0.5555 * (e - 10)! Remembering that physical T_dew can never exceed T_air prevents corrupted weather-station sensor imports from generating bogus heat alerts!
Verify on an Oppressive Great Lakes Afternoon—DryBulb_Air_Temp = 32.0°C (B2) and Dew_Point_Temp = 24.0°C (B3): Clausius-Clapeyron Vapor Pressure reaches e = 30.30 hPa (E2), generating a Humidity Increment h = 0.5555 * (30.3006 - 10) = +11.28 (E3) and a Total Humidex Score of 43.28 (E4)—triggering MSC’s "Great Discomfort: Avoid Exertion (40-44.99)" advisory (E5)!
| Canadian MSC Humidex Metric (T_air = 32°C, T_dew = 24°C) | Formula | Verified Output |
|---|---|---|
| Vapor Pressure e (hPa) | 6.11 * EXP(5417.753 * (1/273.16 - 1/(Td+273.15))) | 30.30 hPa (12.30 @ Td=10°C) |
| Humidity Increment h | 0.5555 * (e - 10) | +11.28 (+1.28 @ Td=10°C) |
| MSC Humidex Index | T_air_C + 0.5555 * (e - 10) | 43.28 (33.28 @ Td=10°C) |
| MSC Discomfort Advisory | 40–44.99 Great Discomfort; >=45 Dangerous | Great Discomfort: Avoid Exertion (40-44.99) |
Environment Canada Clausius-Clapeyron Humidex Formula
Guard against unphysical Dew_Point > Air_Temp or temperatures outside [-40°C, 60°C]!
=LET(ta_c, B2, td_c, B3, IF(OR(ta_c < -40, ta_c > 60, td_c < -50, td_c > ta_c), "Invalid Temp / Dew Point", LET(vp_e, 6.11 * EXP(5417.753 * ((1 / 273.16) - (1 / (td_c + 273.15)))), ROUND(ta_c + 0.5555 * (vp_e - 10), 2))))Dry Continental Air-Mass Check (Set Dew Point = 10.0°C at Same 32.0°C Air Temp)
Lower Dew_Point_Temp B3 from 24.0°C to 10.0°C while keeping T_air = 32.0°C: Vapor Pressure E2 drops from 30.30 hPa to 12.30 hPa, slashing Humidex E4 by 10 full points to 33.28 ("Some Discomfort (30-39.99)" in E5)!
=IF(ISTEXT(E4), E4, IF(E4 >= 45, "Dangerous: Heat Stroke Risk (>=45)", IF(E4 >= 40, "Great Discomfort: Avoid Exertion (40-44.99)", IF(E4 >= 30, "Some Discomfort (30-39.99)", "Little to No Discomfort (<30)"))))Try the example
Use the original values or change them to test your understanding.