Anyone
who has ever waited at a bus stop or taken a walk on a blustery winter
day knows that you feel colder when the wind blows.
We call the cooling sensation caused by the combined effect of temperature
and wind the 'wind chill'.
The
wind chill factor used by forecasters is designed to indicate the
dangers posed by different combinations of wind and temperature on
the bodies of humans and animals.
This
index was created as a public health tool to reduce hypothermia,
frostbite and other cold-related ailments.
For
practical purposes the wind chill index is supposed to tell people
how warmly to dress - a crucial decision for people who spend long
periods outdoors such as fishermen and farmers.
INSULATION
GETS BLOWN AWAY
On a calm day, our bodies insulate us from the outside temperature
by warming up a thin layer of air close to our skin, known as the
boundary layer.
When the wind blows, it takes this protective layer away - exposing
our skin to the outside air.
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| On
windy days our bodies lose more heat. |
It takes
energy for our bodies to warm up a new layer, and if each one keeps
getting blown away, our skin temperature will drop, and we'll feel
colder.
Wind also makes us feel colder by evaporating any moisture on our
skin - a process that draws more heat away from our bodies.
Studies show that when our skin is wet, it loses heat much faster
than when it is dry.
So
wind chill is the chilling effect of the wind in combination with
a low temperature.
Interestingly humans don't sense the temperature of the air directly.
When we feel it's cold, we're actually sensing the temperature of
our skin.
Theoretically,
the wind chill index is supposed to measure the rate at which the
body loses heat when exposed to cold and wind.
Until
recently the wind chill formula was based on experiments conducted
in Antarctica in 1945 in which scientists measured the effect of
wind and temperature on a can of water hanging from a pole.
Recently a new formula has begun to be used,
developed by representatives of seven government agencies, including
the U.S. National Weather Service and the Canadian Weather Service.
The
new wind chill index is an attempt to more accurately measure the
effect of combinations of low temperature and wind on humans or
animals.
First
published: December 2003
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