WBGT (Wet Bulb Globe Temperature) Index

The Wet Bulb Globe Temperature is being used as the heat stress index in Olympic Forecasts.

The formula is:

Outdoors: WBGT = 0.7 Twb + 0.2 Tg + 0.1 Tdb
Indoors: WBGT = 0.7 Twb + 0.3 Tg
where:
Twb = Wet bulb temperature (°C)
Tg = Black globe temperature (thermometer inside ~ 15cm diameter blackened copper sphere) (°C)
Tdb = Dry bulb temperature (°C)

Source: Environmental Physiology and Medicine

Approximation:

As the Bureau does not measure Tg, an approximation is used.

This approximation does not take into account variations in the intensity of solar radiation or of windspeed, and assumes a moderately high radiation level in light wind conditions. Use of this approximation may lead to incorrect estimates of thermal stress, particularly in cloudy and windy conditions. Under these conditions the approximation is likely to lead to an overestimate of the stress.

The following simplified formula is used:
WBGT = 0.567 Td + 0.393 e + 3.94

where:
Td = Dry bulb temperature (°C)
e = Water vapour pressure (hPa)

Source: American College of Sports Medicine, Prevention of thermal injuries during distance running - Position Stand. Med.J.Aust. 1984 Dec. 876