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14.2 Temperature Humidity Index (THI)

The formula for estimating THI is: 0.8*T + RH*(T-14.4) + 46.4

where T = ambient or dry-bulb temperature in °C and RH=relative humidity expressed as a proportion i.e. 75% humidity is expressed as 0.75.


Figure 14.1: THI chart, adapted from[7]

The temperature humidity index (THI) was developed as an indicator of heat load risk. The THI combines measurement of environmental temperature and humidity. THI does not incorporate impacts of solar radiation or wind speed and therefore is a cruder measure of heat load than more advanced indices that measure these inputs.

The THI remains useful, however, because it can be calculated easily from ambient or dry bulb temperature and relative humidity and both these measures are routinely collected on board export ships. 

A chart of THI is presented in Figure 14.1, with shading to indicate levels of varying levels of risk for EHL. The stress levels are based on observations for Bos taurus cattle and mild stress for Bos indicus cattle would not be expected to occur until the upper ranges of the severe stress category displayed for Bos taurus. If environmental conditions are in the severe stress range for several days in a row then there is a risk of severe losses from EHL.
7. Tropical dairy farming: feeding management for small holder dairy farms in the humid tropics; John Moran, 312 pp., Landlinks Press, 2005