麻豆传媒

EVER wondered whether Santa Claus wears anything under his red suit? He
certainly ought to. In the far north, temperatures can plummet to around
鈭�30 掳C, and it鈥檚 likely to be a mite breezy on top of his sleigh. What
with wind chill, he could end up with serious frostbite in minutes.

Yet Santa鈥檚 lucky. He only has to brave the elements once a year. For those
who live and work in cold climates, knowing how cold they鈥檒l feel each time they
venture outside can be a matter of life or death.

So scientists dreamed up the wind chill index鈥攖he scale that tries to
express just how cold you will feel at a given temperature and wind speed. But
it鈥檚 a controversial topic. Some experts think it鈥檚 baloney. The Canadian and US
wind chill index, for example, which is used around the world, is based on
limited data collected over 60 years ago, and scientists are still divided over
the best way to indicate the potential risks.

To try to improve matters, earlier this year 12 volunteers spent hours
walking on a treadmill inside a refrigerated wind tunnel in Toronto. They were
part of a study designed to come up with a more accurate wind chill index so
those who live at the planet鈥檚 frozen extremes will know when to wrap up warm.
It could be the first step towards combining the effects of temperature and wind
speed with humidity and even sunlight, creating a new, highly accurate universal
standard for all weathers鈥攂oth cold and hot.

If you鈥檝e ever huddled out of the wind to try to stay warm, you鈥檒l
intuitively understand wind chill. In reality we are usually surrounded by an
envelope of air several centimetres thick that is warmed by our body heat. But
when the wind whips off that warm envelope, it exposes our skin to the colder
air and we shiver with cold. Yet it鈥檚 surprisingly difficult to measure and
express the combined effect of cold and wind on people鈥檚 bodies.

The first person to tackle the problem was the American geographer Paul
Siple. In 1940 he began a series of experiments in the Antarctic to measure how
quickly water froze in small plastic bottles under various conditions. He came
up with a formula to express rates of cooling that was used in countries
including the US and Canada until this year.

Using Siple鈥檚 index, you can express wind chill in one of two
ways鈥攅ither as the amount of heat lost in watts per square metre of
surface area, or more usefully as an equivalent temperature鈥斺�滻t feels like
it鈥檚 minus 20 out today.鈥�

But there鈥檚 a pretty obvious problem with Siple鈥檚 index. Small plastic
bottles are nothing like people鈥攚e have different sizes, shapes and
thermal properties. So his index noticeably overstates how cold you鈥檒l feel at
low temperatures and very high winds.

To correct the errors, in 1999 Randall Osczevski of the Canadian Defense and
Civil Institute of Environmental Medicine in Toronto and Maurice Bluestein, an
engineering professor at Purdue University, Indiana, began work on the 鈥渇acial
cooling model鈥� for determining wind chill. The idea is to relate the temperature
of the face to the temperature inside the body, and the way this depends on wind
and cold. 鈥淭he thermal comfort of the face is an important part of how you
feel,鈥� Osczevski says.

To test their model, the two researchers started with dummies and computer
models of the human head, then this year shut volunteers in a refrigerated wind
tunnel to measure just how quickly their faces cooled under different
conditions. One of the volunteers, meteorologist Pierre Tourigny, spent hours
walking on the treadmill with temperature sensors on and in his body. The one in
his rectum didn鈥檛 bother him as much as you might think. 鈥淭he one on the inside
of my cheek was worse. It forced me to keep my mouth shut,鈥� he says.

The results apply more directly to people than the old numbers based on water
bottles, Osczevski and Bluestein believe. Their new scale鈥攃alled the North
American Wind Chill Index鈥攁lso corrects another problem with Siple鈥檚
index. Wind speed is typically measured at a height of 10 metres. But your face
is usually about 1.5 metres off the ground where the wind is weaker. So the new
index adjusts the reported wind speed by cutting it by a third.

Many meteorologists claim the new index is a better measure of what people
feel, especially under extreme conditions. Under the old system, a temperature
of 鈭�25掳C and winds of 50 kilometres per hour would have translated
into a frigid wind chill of 鈭�51掳C. The new scale predicts a balmier
鈭�42掳C, closer to the temperature you鈥檇 really feel.

Next, the researchers plan to add the effects of relative humidity and
sunshine into the equation, at least in a simplified form. 鈥淲e can get really
complicated,鈥� Osczevski says. 鈥淲e could calculate the angle of the Sun every
second of the day, every day of the year. But weather people don鈥檛 want to run a
model every time they calculate the wind chill. They want a simple formula they
can plug numbers into.鈥�

Even with these improvements, their index won鈥檛 be the last word. It鈥檚 likely
to be replaced in two or three years鈥� time by a much more ambitious index called
the Universal Thermal Climate Index (UTCI). This is a sort of grand unified
theory of apparent temperature being developed by the International Society of
Biometeorology.

The ISB wants an index that takes into account temperature, wind speed,
humidity and sunlight, and that will even work in the heat as well as the
cold鈥攕o that hot, humid days would also be assigned their own apparent
temperatures. And unlike the North American wind chill index, the UTCI will be
determined using a computer model of the whole body that predicts how wind and
temperature affect the core body temperature. This will make the UTCI especially
useful in survival situations where people risk hypothermia.

Osczevski isn鈥檛 convinced there鈥檚 an advantage to a whole body model. 鈥淚
think the sensation of wind chill is an acute sensation. It occurs within the
first minute of going outdoors. It is not the long-term cooling of the whole
body,鈥� he says.

Nevertheless, in a meeting in June, the ISB decided to press on with the
whole body model and most countries are expected to adopt the index once it鈥檚
ready in about 2004. (Even though Britain already uses its own whole body model
to calculate wind chill, it could adopt the more sophisticated international
model once it鈥檚 finished.)

Yet some critics question whether a wind chill index will ever have any
practical value. Edwin Kessler is a former director of the National Severe
Storms Laboratory in Norman, Oklahoma. He thinks wind chill is often just a tool
for TV weather forecasters who want to report a dramatically low number. 鈥淚 feel
that sometimes technology is a little overused. We got along just fine without
wind chill for centuries,鈥� he says. 鈥淧eople know what to do. If it鈥檚 a 30 miles
per hour wind and 30 below zero鈥攆or Pete鈥檚 sake, you wear your parka.鈥�
Santa, are you listening?