How cold is it really outside? We often use the air temperature as an indicator of how comfortable we will feel when involved in sports or other physical activities. Simply knowing the temperature doesn't tell you enough about the conditions to enable you to prepare sensibly for all winter weather. Air temperature is only one factor in the assesment of thermal stress. Other factors including wind speed, relative humidity and sunshine play important roles in determining how cold you feel outside.

In climates where other important factors, 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 Apparent Temperature is an index which attempts to do this.

Wind chill temperature is how cold people feel when outside—the apparent temperature felt on the exposed human body due to the combination of air temperature and wind speed.

Wind chill quantifies the rate of heat loss from exposed skin caused by wind and cold. It accounts for loss of heat when warm air around a body is replaced with colder air. The factor is an indication of the effect of the combination of air temperature and wind speed on human comfort and safety.

As the air movement increases, it draws heat from the body, driving down skin temperature and eventually the internal body temperature. Therefore, the wind makes it feel much colder. The first wind chill formulae and tables were developed by Siple and Passel in the 1970s. In 2001 the formulae were revised to reflect more accurate theories and testing, specifically for the human body, or even more specifically for the human face.

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 air flow over the skin. Fabrics with low vapour permeability ( 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.

The Apparent Temperature (AT) is just one method of combining temperature and humidity into a single number. 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.

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. 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 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.

The Apparent Temperature (AT) - 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.

The US and Canadian formulae are best suited to extremely cold climates. Other formulae such as the Steadman Wind-Chill index (developed by Australian environmental scientist Robert Steadman) have been developed for temperate climates, but are less well known.

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.