If you go outside on a windy day, you may quickly find that the thermometer doesn't reflect how cold you really feel. This effect is what weather forecasters call the wind chill. Basically, the wind makes a cold day feel colder by wicking heat away from your skin. Although wind chill is well understood and easy to explain, it's not as easy to measure, and in fact there is no universally agreed-upon scale for describing wind chill.
You are nearly always losing heat to the surrounding air because the surrounding air is nearly always colder than you are. Heat is initially transferred from your skin to the surrounding air through conduction -- collisions between air molecules and molecules in your skin. Air is a poor conductor of heat, so this process is relatively slow and inefficient. The most efficient means of heat transfer in air is through convection, where hot air rises and cold air sinks to create a current.
The air immediately next to your skin becomes warmed through conduction. Because this process is slow, the rate at which you lose heat is slow unless the air near your skin moves away under the influence of a current like a convection current or a wind. The wind rapidly pulls warming air away from the surface of your skin and replaces it with cold air so that the rate at which you lose heat increases. In essence, the wind is wicking heat away from the surface of your skin.
The faster the wind blows, the worse the wind chill becomes -- and if you're not careful, the wind chill can have serious consequences for your safety and health. And the lower the temperature, the greater the effect the wind can have. If the temperature outside is negative 1.1 degrees Celsius (30 degrees Fahrenheit), for example, a 30-mile-per-hour wind will make you feel as if you're standing in air that's negative 9.4 degrees Celsius (15 degrees Fahrenheit) -- a wind chill reduction of about 8.3 degrees Celsius (14.94 degrees Fahrenheit). But at negative 26 degrees Celsius (negative 15 degrees Fahrenheit), the same wind will give you a wind chill reduction of 17.3 degrees Celsius (31 degrees Fahrenheit).
By increasing the rate at which your body loses heat, wind chill can cause you to succumb to hypothermia or frostbite more rapidly than you would otherwise. So if you're out on a day when there's a strong wind chill, it's important to wear enough clothing to make sure you stay warm.
Typically, weather forecasters report wind chill in terms of the difference between the temperature outside and the temperature you would need to get the same rate of heat loss that is caused by the wind. If the temperature outside is zero degrees Celsius (32 degrees Fahrenheit), for example, but the wind is causing your skin to lose heat at the same rate it would if the temperature were negative 10 degrees Celsius (14 degrees Fahrenheit), you have a wind chill of negative 10 Celsius (18 degrees Fahrenheit). Although there is no universally agreed-upon system, most weather forecasters use the one devised by the Joint Action Group on Temperature Indices in a 2001 experiment, when 12 volunteers walked in wind tunnels while wearing thermal sensors. The measured rates of heat loss were used to determine a mathematical relationship between wind speed, temperature and wind chill.