Discomfort Index¶
The Discomfort Index (DI), also called Thom's Discomfort Index or the Temperature-Humidity Index, estimates human heat discomfort from air temperature and relative humidity alone. It was introduced by Thom (1959) — originally from dry-bulb and wet-bulb temperatures — and is implemented here in the Celsius and relative-humidity form of Giles et al. (1990). It remains widely used as a simple heat-stress indicator.
More information:
Thom, E.C. (1959). The Discomfort Index. Weatherwise, 12(2), 57-61. https://doi.org/10.1080/00431672.1959.9926960 (origin of the index)
Giles, B.D., Balafoutis, C. & Maheras, P. (1990). Too hot for comfort: the heatwaves in Greece in 1987 and 1988. International Journal of Biometeorology, 34(2), 98-104. https://doi.org/10.1007/BF01093455 (the temperature / relative-humidity formulation used here)
Epstein, Y. & Moran, D.S. (2006). Thermal comfort and the heat stress indices. Industrial Health, 44(3), 388-398. https://doi.org/10.2486/indhealth.44.388 (review and discomfort categories)
How to use¶
You need 2 m air temperature in Kelvin and relative humidity in percent. It returns the Discomfort Index in Kelvin.
Relative humidity can be obtained from temperature and dew point with
calculate_relative_humidity_percent.
Interpret the output¶
The index uses the relative-humidity form DI = T - 0.55 (1 - 0.01 RH)(T - 14.5), with T in °C and RH in %. At 100% relative humidity the index equals the air temperature; in warm conditions (above 14.5 °C) drier air pulls it below the air temperature, reflecting the reduced heat stress of low humidity (below 14.5 °C the sign reverses, but the index is intended for warm-season use). This is Thom's index on the Celsius scale; the same index also appears in a Fahrenheit relative-humidity form (with 58 °F in place of 14.5 °C) and in Thom's original dry-bulb/wet-bulb form.
The commonly used thermal-sensation categories (Giles et al. 1990; Epstein & Moran 2006) are:
| DI (°C) | Thermal sensation |
|---|---|
| < 21 | No discomfort |
| 21 - 24 | Less than 50% of the population feels discomfort |
| 24 - 27 | More than 50% of the population feels discomfort |
| 27 - 29 | Most of the population feels discomfort |
| 29 - 32 | Everyone feels severe stress |
| ≥ 32 | State of medical emergency |
The output is not clamped to this range: out-of-range inputs return the raw
index and the caller is responsible for any masking. Convert the returned Kelvin
value back to °C (e.g. with kelvin_to_celsius) to look it up against the table.