A primer on thermal comfort

Most people conceptualise thermal comfort in terms of air temperature. This is is the figure conventionally given by weather stations and observed from a home thermometer.

However, the situation is far more complex as can be observed in the following circumstances:

- Why it suddenly feels warmer when the sun comes out from behind the clouds
- Why it feels colder riding a bicycle on a cold winter day than standing still
- Given the same temperature, why a humid day feels more uncomfortable
- Why touching a piece of metal sitting in a room feels cold.

The answer ****** in the multiple ways that heat energy can be transferred. Heat energy can occur by radiation, convection, conduction and evaporation.

* Conducted heat - occurs when two objects are in physical contact. Energy flows down a temperature gradient at a rate which is proportional to the thermal conductivity of the object. Metals conduct well. Air conducts poorly.
* Convective heat - occurs when heat is transferred by a moving gas or liquid medium. It is the reason treating a burn under cold running water removes heat faster than plunging into a bucket of water. The flow of heat is proportional both to the temperature gradient, the rate of flow and the physical characteristics of the gas/liquid. Convective flow can be reduced by reducing the flow of the medium. Wearing lots of thin layered clothes traps a warm layer of still air against the body which insulates us against the surrounding environment. Pink batts work the same way for a house as does the air layer in double glazed windows.
- Radiant heat - is heat transferred by electromagnetic waves. It is emitted by any object whose temperature is above absolute zero (-273 deg celcius). It is hard to understand but even ice actually emits heat energy. Transfer is not dependent on a medium, it can even occur through a vacuuum. Solar energy includes radiant heat. However, all objects radiate this heat energy regardless of the behaviour and temperature of nearby objects. What is important is the net transfer of energy between two objects and the distance by which they are separated. Standing closer to a fireplace feels warmer than standing away from it.
- Latent heat of (evaporation) - is heat transferred when a material changes state e.g. from solid to liquid, liquid to gas. Heat energy is required to do this and is extracted from nearby objects. When sweat evaporates it removes heat energy from the skin and you feel cooler. This effect is accentuated by air movement. It is reduced if the atmosphere is already saturated with moisture i.e. humidity.

Now applying this to human comfort:

Constantly our body gains or loses heat by radiation, convection. evaporation and conduction in different proportions. The net direction of heat flow depends on the surrounding conditions. When there is no net heat loss or gain then we feel 'comfortable'.

Sometimes the heat flows are in opposite directions for different mechanisms. For instance in a cold room we lose heat by convection and feel cold. We can compensate for this effect in several ways: a) Heat up the air in the room with a ducted gas heater and reduce or reverse the convective losses c) Put on a sweater and reduce both convective and evaporative losses b) Turn on a bar radiator to provide a source of radiant heat which compensates for our convective loss.

There are other examples. If the air temperature rises too high we gain heat by convection. We can compensate for this by encouraging sweat to evaporate with a ceiling fan. Evaporative losses compensate for the convective gains and we feel cooler. This effect is inhibited in humid climates and why evaporative air conditioners don't work as well. If we jump into a pool or put on a ice pack then heat is conducted rapidly away from the body.

We can address excessive radiant heat directly. Solar radiation is a potent heat source. Radiant heat can be reflected away by light coloured objects or shiny metals. Test cricketers demonstrate this example. Sisalation and white roofs for houses achieve the same effect.

As can be seen there are several factors that act in concert to ultimately influence the feeling of 'thermal comfort'. The parameters to consider therefore are air temperature, radiant heat, air movement and humidity. Hopefully this primer provides people some insight on how to improve their own comfort and the dwellings in which they live in.

See also this summary by the Bureau of Meteorology:

http://www.bom.gov.au/info/thermal_stress/

* oops