Wind chill is a phrase often mentioned in weather forecasts, and even more often on walks on icy mountain tops (usually in the same breath as a few unprintable adjectives). The wind chill information given in forecasts is useful information for people who are going to spend some time out of doors, particularly in the winter, as it helps you to work out how cold you might feel. However, even the 'wind chill temperature' is not the whole story, as your ability to withstand such temperatures depends on such things as your age and physical fitness. If you are wet you will also cool down faster, which is why garments that wick away sweat are so useful. We will look at wind chill first, and then at the cooling effects of water.

How wind chill works
Air feels cool or cold when it is colder than your body temperature. If the air surrounding you is cooler than your body, heat energy will be transferred from you to the air around you. The colder the air, the faster the energy is transferred. If you are standing in still air, the warmth of your body heats up the air around you. This warm air will gradually rise and carry the warmth away from you, sucking in cooler air which heat from your body will warm up. This only happens relatively slowly, though, which is why calm air actually feels warmer than the air temperature measured at a point away from your body.

If there is a wind blowing, the wind blows this warmed layer of air away much faster, replacing it with cooler air. The faster the wind, the faster this warmed air is blown away, and the colder it feels. So if the air temperature is 5°C (40 °F) and the wind is 20 mph, it feels the same as it would do if the air temperature was -6°C (20 °F) on a still day.

Wind chill does not affect how cold something will get if it is left out long enough. Your body will never get colder than the air temperature. Again, this is academic as on a winter's day you would have been dead long before your body temperature got anything like that low. A windy day only feels colder than a still day because the wind is carrying away the air around us that our bodies have warmed up, and so we cool down faster. If something like a bottle full of water starts off warmer than the air temperature it will lose heat until it reaches the same temperature as the air. It will cool down faster on a windy day, but it will never get colder than the air temperature, no matter how fast the wind (unless the outside is wet – see below). If the temperature is 5°C (40 °F) and the wind is 20 mph, you might feel as if it is several degrees below freezing point, but the water in your bottle will not freeze.

Water and cooling
Most people know that we produce sweat when our bodies are too hot, and that the evaporation of sweat helps to cool us down. This cooling effect of evaporation also happens when the air is relatively cold. To go back to our bottle of water again – if the outside of the bottle is wet (from rain, for example), the evaporation of this water will cool the bottle, and may cool it to below the ambient air temperature. This effect will be less if the air is really cold, as the water on the outside will freeze. This helps to explain why it is important to stay as dry as possible if caught out in conditions that may lead to hypothermia.

This effect also explains why wicking undergarments and breathable waterproofs are so useful. No matter how carefully you dress for the conditions, you will find yourself overheating and sweating at some point, particularly if walking uphill. Before the advent of modern breathable materials you would have found yourself sweating as you went up hill wearing a waterproof jacket, and the layers of clothing next to your skin would become wet. Much of this would evaporate, only to condense again on the inside of your jacket, or it wouldn't evaporate at all because the air inside your jacket would become saturated (see below). This wasn't a problem while you were going uphill (although it may have felt uncomfortable), but when you stopped on the top to admire the view, your body would lose heat to the liquid water in your clothing, and you would cool down much faster than if you were dry.

Modern fabrics act in two ways to reduce this effect. Wicking undergarments (or 'thermal underwear') transfer the sweat rapidly away from the places where it is being produced, which allows it to evaporate faster while it is being produced. Breathable outer layers allow the evaporated water vapour to escape, so when you admire the view at the top you are not standing in garments full of your own sweat!

Evaporation, humidity and wind
The humidity of the air also affects how hot and sweaty you feel. Humidity is the amount of water vapour in the air compared to how much water vapour it can hold. This changes with temperature, with hotter air being able to hold more water vapour than colder air. Air at 100% humidity is saturated, and cannot hold any more water vapour, so any liquid water will not evaporate. (Strictly speaking, some of the liquid will evaporate, but water vapour in the air will condense at the same rate, so overall the net evaporation is zero.)

On a hot day with low humidity, sweat can evaporate quickly and you feel relatively cool. If the air temperature is the same but the humidity is high, you still sweat but it does not evaporate easily. You feel hot and sticky, and your body continues to make sweat (as it is still too hot), which makes you even wetter and sticker! This is why 'close' or 'muggy' weather can feel very uncomfortable.

Wind increases the cooling effect of sweating. On a still day, the air around your sweating body would soon become saturated with water vapour, making it very difficult for more sweat to evaporate. Just as a breeze will blow away air warmed by your body, it will also blow away the saturated air and allow more evaporation of sweat to take place.

How evaporation causes cooling
We need a bit of particle theory to explain this. All materials are made from particles (atoms or molecules, depending on the material). In solids, the particles are held in fixed arrangements, and can only vibrate. In liquids, the particles can move around but tend to stay in whatever container they are placed in, although some particles escape the surface of the liquid and become a gas (this is what evaporation is). In gases the particles whiz about energetically all over the place. In all cases, the more energy the substance has, the faster the particles vibrate (in solids) or move around (in liquids or gases).

The temperature of a liquid (as we are talking about water evaporating) is a measure of the average energy of the particles. Some particles will have more than the average energy and will be moving faster than the average; some will have less and will be moving slower than the average. It takes energy for a particle to escape from a liquid, and so the particles that evaporate are likely to be the ones in the liquid with the highest energies. If you remove the high energy particles from the liquid, the average energy of the ones that are left will be lower, so the temperature will be lower. This is why evaporation causes cooling in the liquid that is left behind.

The sweat your body produces will be at the same temperature as your skin. As the sweat evaporates, the water molecules with the highest energies escape, and the average energy of the ones that are left is lower. The sweat is now cooler than your body, and so it can absorb heat from your skin. More water molecules evaporate, cooling the sweat that is left and allowing it to absorb more heat, and so on until it has all evaporated.

You can feel this effect for yourself very easily. Alcohol (or liquids such as nail varnish remover) has a lower boiling point than water, and so it is easier for particles in it to evaporate. If you put a drop of alcohol on your skin and a drop of water, the alcohol will feel colder as more evaporation and so more cooling is taking place.