Thom discomfort index

Sector Health, human comfort
Description Thom discomfort index is a physiological thermal stress indicator for people based on dry-bulb and wet-bulb temperature (Thom 1957, Epstein and Moran 2006).
End User General public, Bologna
Calculation method

TDI = 0.5TW+0.5Ta, where Tw is wet-bulb temperature and Ta is air temperature (Eq. 4, Epstein and Moran 2006). Tw can be calculated from moisture information according to Stull (2011):

Tw = T atan[0.151977(RH% + 8.313659)1/2] + atan(T + RH%) – atan(RH% – 1.676331) +
0.00391838(RH%)3/2 atan(0.023101RH%) – 4.686035

The highest hourly TDI value for each day is summed into the number of days per year with TDI above 24 and 28, respectively. 

ID Title Period Statistical processing Unit Threshold Comment
thomindex Thom discomfor index yearly Number of days exceeding the defined thresholds (24 and 28 Thom index), using daily maximum air temperature days    
Provenance Computed by the HARMONIE model.
Validation The simulations made by HARMONIE-AROME in Urban SIS has been validated against observations in Urban SIS deliverable 5.1, where an overview is given in Table 4.  
Calculation caveats

Spatial representation: S1
Other caveats: O4
Could be compared to: 
Could be used with: 


Assessing thermal stress is important for worker health and safety, industrial productivity, and athletic performance. This heat stress index incorporates relevant meteorological variables related to physiological thermal stress into a single value.

Many thermal stress indices have been developed since the early 1900s (see review and comparison by Epstein and Moran 2006). For example, the wet bulb globe temperature index (WBGT) developed by the U.S. Navy in 1957 has been used widely and adopted as an ISO standard (ISO 7243). The Thom index is selected for this work because it is highly correlated with the WBGT index and has the advantage of being technically easier to implement.

Experience user See other metrics provided to compare underlying assumptiobs

 Epstein Y, DS Moran 2006: Thermal comfort and the heat stress indices. Industrial health 44.3: 388-398.

Stull R 2011: Wet-bulb temperature from relative humitidy and air temperature.  J Appl. Meteorology and climatoloy, 50, 2267-2269.

Thom EC 1959: The discomfort index. Weatherwise 12.2: 57-61.