The Heat Load Index, referred to as the ‘HLI’, is an index that brings together all the weather factors into one number to allow easy interpretation of the cooling capacity of the environment. The Heat Load Index has been developed in Australia from over 10 years worth of research into assessing heat events in Australian feedlot conditions for Australian cattle.
The basic weather information required to determine the capacity of the environment to cool a body consists of:
- radiative temperature – a combination of temperature and radiation called Black Globe Temperature (BGT)
- air movement (Wind Speed)
- a measure of the moisture in the air (Relative Humidity)
As you can see above, the HLI is calculated from measurements of BGT (°C), relative humidity (%) and wind speed (m/s) using a simple relationship. A tool is available on CHLT to calculate HLI from the available input parameters.
For the most part, the HLI on its own does not indicate the likelihood of heat stress in feedlot cattle. This is done by calculation of the Accumulated Heat Load Unit (or AHLU). The only time when heat stress may be indicated by the HLI alone is when there is a rapid change in the HLI over a short period of time.