Air movement is an important factor in thermal comfort. Whilst stagnant air in artificially heated spaces often contributes to a feeling of stuffiness and promotes odour build-up, even the smallest air movement in cold environments is often considered draughty. In warm or humid conditions, however, air movement can increase heat loss without any change in air temperature.
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It achieves this in two ways:
- If the air temperature is less than skin temperature it will significantly increase convective losses. This occurs because more air is coming in contact with the warm skin as it blows past. With little or no air movement, the air close to the skin quickly heats up to the ambient skin temperature and air movement occurs only when it becomes more buoyant or when you fan yourself.
- In moderately humid environments (30-80% RH), it will also accelerate the evaporation of sweat by moving saturated air away from the skin and replacing it with unsaturated air. Below 30% RH there is unrestricted evaporation anyway so air velocity makes only minimal difference to the evaporation. Above 80-85% RH there is very little evaporative potential as the air blowing past is already close to saturation, making air movement relatively ineffective.
Unfortunately, many of our activities are also affected by air movement. For example, whilst it might be cooling to have a strong natural breeze blowing through the office, the extra work chasing all the papers blown out the window and onto the street below may preclude its use. The table below indicates general responses to different air speeds within a building.
| DESCRIPTION | m/s | km/h |
|---|---|---|
| Still | 0.0 | 0.0 |
| Not Noticeable | 0.1 | 0.4 |
| Barely Noticeable | 0.3 | 1.0 |
| Pleasant Breeze | 0.5 | 1.8 |
| Light Breeze | 0.7 | 2.5 |
| Hair and Papers Move | 1.0 | 4.0 |
| Noticeably Draughty | 1.4 | 5.0 |
| Unpleasant Breeze | 1.7 | 6.0 |
| Gusting Breeze | 2.0+ | 6.5+ |
Measuring Air Movement
Air movement is measured using anemometers. Air speeds inside a building tend to be relatively low, usually less than 1 m/s. Thus the anemometers used for this purpose have to be much more sensitive than those used in weather stations measuring wind. There are two main types of internal anemometer, thos based on a small propeller and those based on the cooling of a hot metal wire exposed to the air. Propeller based systems tend to be a little less sensitive at very low air speeds, however bearing technology and ultra-lightweight materials are increasingly proving this bias incorrect.
For more information on air movement outside buildings, see the wind speed topic in the climate section.