About the WBGT Index

Content

• Thermal Stress Factors
• The Wet Bulb Globe Temperature (WBGT)
• Using the Index
• Conversion Table
• About the approximation to the WBGT used by the Bureau of Meteorology
• Reference

Thermal Stress Factors

In hotter conditions, where the temperature is significantly above 25°C, the body must try to shed heat to maintain thermal equilibrium. The evaporative cooling of sweat from the skin becomes an important cooling factor. The efficiency of this cooling depends environmentally on the humidity. A high humidity reduces the effectiveness of evaporative cooling significantly. The amount of clothing will also affect this cooling efficiency due to its restriction of the air flow over the skin and increasing the air humidity near the skin.

In colder conditions, where the temperature is significantly below 25°C, the body must reduce heat loss or increase the heat produced, by greater activity, or physiological responses such as shivering. Evaporation and air humidity is no longer a factor. The cooling of the exposed parts of the body by the wind now becomes the most important external factor affecting thermal balance.

Under all temperature conditions the effect of radiation can be extremely important. Radiation always acts to increase the heat load on a person. Under hot conditions, when a person is trying to cool, radiation makes things worse. In cool conditions when the person is trying to stay warm, radiation helps.

Of the four mentioned environmental factors, wind and radiation depend very much on the immediate surroundings and short term localised phenomena such as cloudiness. If these are to be used as inputs, they must be measured on location. Values just a few kilometres away could be quite different.

The remaining two factors temperature and humidity are less variable and can be used to give a general idea of the comfort level. Most people use the temperature alone to provide some guide to the level of comfort. In most cases this is quite reasonable because humidity doesn't often vary a lot, particularly in the tropics. However people moving from a less humid to more humid environment will immediately notice the effect of the greater humidity. In many sub-tropical regions of Australia humidity is usually quite low, but occasionally can become quite high, again reducing comfort to those people not acclimatised.

It is convenient to combine the effect of temperature and humidity into one index. This does not mean we can ignore the other environmental and non-environmental factors, but the user can adjust the index slightly to take them into account.

The WBGT (Wet Bulb Globe Temperature) is just one method of combining temperature and humidity into a single number. In fact the real WBGT is also affected by wind and radiation, but the WBGT provided by the Bureau is only an approximation, which ignores variations of wind and radiation. Other simple temperature/humidity indices such as the Steadman Apparent Temperature (used in the United States) and the Humidex index (used in Canada) also exist.

The Wet Bulb Globe Temperature (WBGT)

The WBGT was developed in the late 1950s for the US Marine Corps Recruit Depot on Parris Island in South Carolina. Humidity in this region can be quite high and Marines have to undergo vigorous training exercise in military clothing, under full sun. There is a significant risk of heat injury if precautions are not taken. The WBGT was later used by researchers as an easily measured general heat-stress index. In time it's use widened. It's use is often incorporated within Occupational Health and Safety guidelines for working in hot environments. It has been advocated for use in sports requiring continuous exertion, such as the marathon. It is also used for horses in equestrian events. The WBGT is measured by a simple three-temperature element device similar to the picture on the right.

Photo: Courtesy of Richard de Dear, Macquarie University.

The first temperature is the black globe temperature (Tg). It usually consists of a 150 mm (6 inch) black globe with a thermometer located at the centre. The black globe temperature represents the integrated effects of radiation and wind.

The second thermometer (not easily viewed on the picture) is the natural wet-bulb temperature (Tnwb). It consists of a thermometer with its bulb covered with a wettened cotton wick supplied with distilled water from a reservoir. Evaporation from the wettened bulb cools the thermometer. The natural wet-bulb thermometer, like the black globe thermometer is not shielded from wind or radiation. This thermometer represents the integrated effect of humidity, wind and radiation.

The final temperature element is the (shade) air temperature (Ta). It consists of a thermometer shielded from radiation - generally by being placed in a weather screen. It is the standard temperature normally quoted in weather observations and forecasts.

The three elements Tg, Tnwb, and Ta are combined by into a weighted average to produce the WBGT.

WBGT instruments are available commercially, but they are fairly expensive, requiring regular maintenance if they are to produce accurate values. The Bureau of Meteorology does not have WBGT instruments at any of its observation sites. Air temperature and humidity are measured using standard meteorological instruments and enclosures. Values for the black globe temperature and natural wet-bulb temperature cannot be accurately determined from a standard meteorological site.

Instead the Bureau uses an approximation to the WBGT. This approximation uses standard meteorologically measured temperature and humidity to calculate and estimation of the WBGT under moderately sunny light wind conditions. Real variations of sunshine and wind are not taken into account. The formula is likely to overestimate the WBGT in cloudy or windy conditions, or when the sun is low or below the horizon. Under clear full sun conditions in low humidity conditions the approximation underestimates the WBGT slightly. The formula for the approximation is shown at the end of this document.

Although the WBGT is still widely used for heat stress measurements, the basis for its use as a model for human response to heat has been questioned. The effects of the four environmental factors on the WBGT do not necessarily match those of humans under all conditions. In modern heat stress research, sophisticated mathematical models of human response are more often used. These are too complex to be used in practice for general guidance.

Because the Bureau of Meteorology uses an approximation to the WBGT, the user should clearly understand the limitations of this approximation as compared to a real measured WBGT. The fact that the values we produce assume a constant 'moderately sunny' day with 'light winds' may be overlooked, and it might be assumed that we have measured the true WBGT. This can cause confusion.

Using the WBGT Index

By using the conversion table below, and your own measurements it is possible to determine the value of the WBGT heat index at your venue. Electronic thermometers containing relative humidity are available from electronic, and other suppliers, for less that $50. These instruments produce fairly accurate values of relative humidity. Care must be taken to expose the temperature sensor away from extraneous radiation sources so as to produce an accurate shade temperature. Compare your maximum temperature with a trusted nearby reference weather station. If your value is consistently high, for a reason you can't explain, try another location.

To estimate the average conditions of the WBGT index you can use the Bureau of Meteorology climate data. Click on the Climate Averages link. You will need to get averages of temperature and relative humidity.

On the lower part of the Climate Averages page, choose your state, lookup the nearest observation site to your location, click on the station number.

For afternoon conditions you should use the Mean Daily Maximum Temperature and Mean 3pm Relative Humidity. Do not use this relative humidity for other times because relative humidity changes quite a bit during the day.

For example if we choose BRISBANE REGIONAL OFFICE, in January we have a temperature of 29.4°C and relative humidity of 59%. From the tables this gives a WBGT of about 30°C.

Conversion Table

Wet Bulb Globe Temperature Approximation

About the approximation to the WBGT used by the Bureau of Meteorology

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 approximation will also overestimate night-time and early morning conditions when the sun is low or below the horizon.

The simplified formula is:

The vapour pressure can be calculated from the temperature and relative humidity using the equation:

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

Reference

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