Ever wondered why you feel much hotter or colder than the temperature gauge indicates?

Australian Bureau of Meteorology, Thermal Comfort observations, January 2018:

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 assessment 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….

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 conditions 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.

An example of how this works on the ground:

To check thermal stress in your area on any given day go to Thermal Comfort observations index for each State or go directly to Thermal Comfort observations in each State NSW & ACT, Vic, Qld, WA, SA, Tas, NT.