The Wind Chill Effect
Your body always works hard to try to maintain a body temperature close to 37oC. However, air movement will carry heat away from your body so that you will feel colder in a windy day than what the thermometer measures. This effect, termed "wind chill", will be more significant with increasing wind speed.
In some countries like U.S. and Canada, something called the "wind chill temperature" is reported along with the actual temperature measured. It was created based on scientific research involving human volunteers and computer modeling, as well as medical understanding of how the body loses heat when exposed to cold. The table below shows the wind chill temperatures for various air temperatures and wind speeds based on the formula used in both U.S. and Canada. For example, if the air temperature is 4oC and the wind is blowing at 30 km/h, your face will feel as cold as -1oC in a calm day without any wind.
One point to note, however, is that the wind chill temperature does not apply to inanimate objects. An inanimate object will not cool below the actual air temperature in wind. For example, no matter how hard the wind blows, a pile of water on the ground will never freeze if the air temperature is kept above the freezing point.
"Cool Met Stuff" - Wind Chill Effect (in Cantonese only)