Thermal Comfort observations
- ️Sun May 02 2010
Why are the indices useful?
We often use the air temperature as an indicator of how comfortable we will feel when involved in sports or other physical activities. However, the air temperature is only one factor in the assesment of thermal stress. In climates where other important factors, principally humidity, can vary widely from day to day, we need more than just the temperature for a more realistic assessment of comfort. However it is useful to be able to condense all the extra effects into a single number and use it in a similar way to the way we used the temperature. The Wet Bulb Globe Temperature (WBGT) and the Apparent Temperature are indices which attempt to do this.
Thermal Comfort observations are available from the observations index for each State
or go directly to Thermal Comfort observations in each State NSW & ACT, Vic, Qld, WA, SA, Tas, NT.
The Sports Medicine Australia Fact Sheet: Beat the Heat - playing and exercising safely in hot weather and their brochure: Drink up - Beat the heat explain how to use these observations to stay safe.
What causes thermal stress?
Human thermal comfort depends on environmental and personal factors. The four environmental factors are airflow (wind), air temperature, air humidity, and radiation from the sun and nearby hot surfaces. The personal factors are the clothing being worn and the person's level of physical activity. Thermal sensation is also significantly affected by acclimatisation/adaptation: people living in hot climates have been shown to be comfortable at higher temperatures than those living in cooler climates.
In hotter conditions the body must shed heat to maintain thermal equilibrium. The cooling effect of evaporation of sweat from the skin becomes an important factor. The efficiency of this cooling depends on the humidity of the air. 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. Fabrics with low vapour permeability (those that don't "breathe") will increase the humidity of air near the skin.
In colder conditions, the body must either reduce heat loss (eg by taking shelter from the wind) or increase heat production, for example, by greater physical activity. In these conditons evaporation and air humidity are relatively unimportant factors. The cooling of the exposed parts of the body by the wind now becomes the most important external factor affecting thermal balance.
The effect of radiation is important under all temperature conditions. Excess radiation always acts to increase the heat load on a person. This can be of assistance under cold conditions, but under hot conditions it's an extra heat load that must be shed.
Of the four environmental factors, wind and radiation are very much influenced by the immediate surroundings. For example, wind speed is reduced by the sheltering effect of belts of trees and solar radiation is affected by short term localised phenomena such as cloudiness. If these factors are to be used as inputs, they are best measured on location, as values can vary significantly over relatively short distances. The remaining two factors (temperature and humidity) are less spatially variable and can be used to give an indication of the general comfort level of a region.
In order to make comparisons between areas, 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 adjustments to the index value, either up or down, can be made to take them into account.
Most people use the temperature alone to provide some guide to the level of comfort. Generally 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 the humidity is usually quite low, but occasionally can become quite high, again reducing comfort to those people not acclimatised.
The Wet Bulb Globe Temperature (WBGT) and Apparent Temperature (AT) are just two methods 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 (light winds and fairly sunny conditions assumed). The AT can also be extended to take wind and solar radiation into account as well, though generally this is not done. In the AT values provided by the Bureau, wind is taken into account, but not solar radiation. Other indices such as the Physiologically Equivalent Temperature (PET) and the Predicted Mean Vote (PMV) can also be used.
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 its use widened. Because its use is recommended in the Standard, ISO 7243, it is often used in 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, (Tg), is measured by the black globe thermometer, which 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) measures 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 is measured by 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 = 0.7 × Tnwb + 0.2 × Tg + 0.1 × Ta
WBGT instruments are available commercially, but they are fairly expensive and require 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 screened to shade the instruments from the sun's radiation. 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 an 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 and 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. The indices apparent temperature (AT), PET and PMV are all based on such heat balance models. These are too complex to be used in practice by organisations such as sports clubs, but are used by meteorologists for tasks such as computing climatologies of thermal sensation. Free software to compute PMV and PET is available on the internet (see References below).
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. The Apparent Temperature (AT) (see below) does not have this ambiguity and consideration should be made as to whether this is a more appropriate guide for your activity.
The Apparent Temperature (AT) - Heat Index
The apparent temperature (AT), invented in the late 1970s, was designed to measure thermal sensation in indoor conditions. It was extended in the early 1980s to include the effect of sun and wind. Only the modification due to wind is taken into account on this site. The AT index used here is based on a mathematical model of an adult, walking outdoors, in the shade (Steadman 1994). The AT is defined as; the temperature, at the reference humidity level, producing the same amount of discomfort as that experienced under the current ambient temperature and humidity.
Basically the AT is an adjustment to the ambient temperature (T) based on the level of humidity. An absolute humidity with a dewpoint of 14°C is chosen as a reference (this reference is adjusted a little with temperature). If the humidity is higher than the reference then the AT will be higher than the T; and, if the humidity is lower than the reference, then AT will be lower than T. The amount of deviation is controlled by the assumptions of the Steadman human model. In practice the AT is more intuitive to use than the WBGT, as it is an adjustment to the actual air temperature based on the perceived effect of the extra elements such as humidity and wind. AT is valid over a wide range of temperature, and it includes the chilling effect of the wind at lower temperatures - see below.
The hot weather version of the AT (1984) is used by the National Weather Service in the United States. In the United States this simple version of the AT is known as the Heat Index.
The AT used here does not include the effect of the sun, but this can be factored in. Under Australian conditions the effect of full sun produces a maximum increase in the AT of about 8°C when the sun is at its highest elevation in the sky.
The Apparent Temperature (AT) - Wind Chill
The apparent temperature (AT), described in the previous section, can also be used as a measure of wind chill. There are a number of Wind Chill Indices in use around the world, generally for colder temperatures than usually experienced in Australia. Nevertheless, conditions in parts of Australia can be cold enough, under windy conditions, to cause significant chilling. Below is a conversion table with a temperature range suitable for Australian conditions.
When using the AT as a wind chill the Steadman model assumes an appropriately dressed adult for those conditions. However if clothing were to get wet, the cooling effect would be greater than that predicted by this model, and the chance of hypothermia would be greater than indicated by the AT. In wet, windy conditions, someone wearing inadequate clothing can become hypothermic in quite mild temperatures.
Using the Indices
The heat indices on the observation page are only a guide to help you make decisions relating to your activity. Your decision process would need to include a number of factors of which temperature and humidity only form part.
For more details, sporting clubs should also obtain a copy of the Sports Medicine Australia Heat Policy. It can be downloaded for free from www.sma.org.au. The SMA Fact Sheet Beat the Heat - playing and exercising safely in hot weather and their brochure Drink up - Beat the heat are also useful. Sports Medicine Australia (SA) also have a booklet containing a Decision Checklist for sport in South Australia. The actual values might not be appropriate in all states but the methodology might be a useful guide. The booklet is available on the web as Hot Weather Guidelines .
By using the conversion tables below, and your own measurements it is possible to determine the value of the heat index at your venue. Electronic thermometers containing relative humidity are available from electronic, and other suppliers, for less than $50. These instruments produce fairly accurate values of relative humidity. Care must be taken to correctly expose the temperature sensor. It should be about a metre above ground level, shaded and away from extraneous radiation sources (for example brick wall) so as to produce an accurate shade temperature. Compare your maximum temperature with a nearby Bureau of Meteorology weather station for reference. If your value is consistently high, for a reason you can't explain, try another location.
To estimate the average conditions of the index you can use the Bureau of Meteorology climate data. You will need to get averages of temperature and relative humidity. Choose your state and click on the location you are interested in. 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, in January we have a temperature of 29.4°C and relative humidity of 59%. From the tables this gives an apparent temperature of about 32°C and a WBGT of about 30°C.
See the section on Apparent Temperature for dealing with day/night variations.
Conversion Tables
Steadman Apparent Temperature
Legend: Red values, apparent temperature above air temperature; blue values, apparent temperature below air temperature
Steadman Apparent Temperature as a Wind Chill
Legend: Colours added to visually delineate increasingly colder values.
Wet Bulb Globe Temperature Approximation
About the approximation to the WBGT used by the Bureau of Meteorology
The approximation used by the Bureau of Meteorology 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 approximation will also overestimate night-time and early morning conditions when the sun is low or below the horizon.
The simplified formula is:
WBGT = 0.567 × Ta + 0.393 × e + 3.94
where:
Ta |
= Dry bulb temperature (°C) |
e |
= Water vapour pressure (hPa) [humidity] |
The vapour pressure can be calculated from the temperature and relative humidity using the equation:
e = rh / 100 × 6.105 × exp ( 17.27 × Ta / ( 237.7 + Ta ) )
where:
rh |
= Relative Humidity [%] |
Source: American College of Sports Medicine, Prevention of thermal injuries during distance running - Position Stand. Med.J.Aust. 1984 Dec. 876
About the formula for the apparent temperature
The formula for the AT used by the Bureau of Meteorology is an approximations of the value provided by a mathematical model of heat balance in the human body. It can include the effects of temperature, humidity, wind-speed and radiation. Two forms are given, one including radiation and one without. On this site we use the non-radiation version.
Version including the effects of temperature, humidity, and wind:
AT = Ta + 0.33×e − 0.70×ws − 4.00
Version including the effects of temperature, humidity, wind, and radiation:
AT = Ta + 0.348×e − 0.70×ws + 0.70×Q/(ws + 10) − 4.25
where:
Ta |
= Dry bulb temperature (°C) |
e |
= Water vapour pressure (hPa) [humidity] |
ws |
= Wind speed (m/s) at an elevation of 10 meters |
Q |
= Net radiation absorbed per unit area of body surface (w/m2) |
The vapour pressure can be calculated from the temperature and relative humidity using the equation:
e = rh / 100 × 6.105 × exp ( 17.27 × Ta / ( 237.7 + Ta ) )
where:
rh |
= Relative Humidity [%] |
Source: Norms of apparent temperature in Australia, Aust. Met. Mag., 1994, Vol 43, 1-16
References
- Robert G. Steadman. 1994:
Norms of apparent temperature in Australia.
Aust. Met. Mag., Vol 43, 1-16.
- Robert G. Steadman. 1984:
A Universal Scale of Apparent Temperature.
Journal of Applied Meteorology: Vol. 23, No. 12, pp. 1674.
- American College of Sports Medicine,
Prevention of thermal injuries during distance running - Position Stand.
Med.J.Aust. 1984 Dec. 876.
- RAYMAN software to compute PMV and PET http://www.mif.uni-freiburg.de/rayman/intro.htm
- Beat the Heat - playing safely in hot weather (SMA policy and guidelines to minimising the risk of heat illness and injury in hot weather conditions)
http://sma.org.au/resources/policies/hot-weather/
- Drink up - Beat the heat (SMA brochure)
- ISO (1989). ISO 7243. Hot environments - Estimation of the heat stress on working man based in the WBGT-index (wet bulb globe temperature). Geneva, Switzerland, International Organisation for Standards.
- SMA Heat Policy http://sma.org.au/resources/policies/hot-weather/
- de Dear R.J. and G.Schiller Brager (1998) Developing an adaptive model of thermal comfort and preference, ASHRAE Transactions., V.104(1a), pp.145-167. Reprinted in Schiller Brager, G. (ed.) (1998) Field Studies of Thermal Comfort and Adaptation - ASHRAE Technical Data Bulletin, V.14(1). pp. 27-49.
- SMA Hot Weather Guidelines http://www.smasa.asn.au/resources/hotweather.htm
Last updated: 5/2/2010