**Document ID**: 10_31_01_2

**Date Received**: 2001-10-31 **Date Revised**: 2002-01-16 **Date
Accepted**: 2002-01-18

## **Curriculum Topic Benchmarks: **M3.3.5, M8.3.1, M8.3.3, S11.3.1, S11.3.4, S11.3.5

## **Grade Level: **\[6-8\] Middle School

**Subject Keywords: **wind chill, heat transfer, hypothermia,
computation, exponents, cold weather, weather, winter, energy transfer,
frostbite

**Rating: **Moderate

**What is Wind Chill?**

**By:** Peter Kiefer, Michigan Technology University, Houghton, MI 49931

email: pakiefer@mtu.edu

**From**: The PUMAS Collection http://pumas.jpl.nasa.gov

©2001, California Institute of Technology. ALL RIGHTS RESERVED. Based on
U.S. Gov\'t sponsored research.

Why is it that we feel colder when the wind is blowing than when the
wind is calm? What does it mean when the local weatherman gives the
daily high along with the wind chill? How is the wind chill determined?

Everyone has experienced wind chill on one level or another. It is the
reason why on a cold winter day we feel warmer when the wind is blocked
by an object like a building than when exposed to the wind. It is on
those winter days that the wind chill is most dangerous. The possibility
of frostbite increases as the temperature drops or the wind speed
increases. Thus it can be an average winter day, but if the wind speed
is high enough, it can become very hazardous to be outside. This
dangerous situation is present throughout the winters of the northern
Midwest, where I am from. It's common to receive wind chill warnings
where exposed flesh can freeze in less than a minute.

Our bodies go through life trying to maintain one constant temperature.
While it varies slightly from person to person, the average human body
temperature is 98.6F. Our bodies try to maintain this temperature
regardless of the temperature of our environment. This is evident on a
hot summer day when we sweat, or on a cold winter day when we shiver.

As a result of our bodies maintaining a constant temperature in a cold
environment, a thin layer of warm air is trapped near our skin. This
layer acts as a 'buffer' to the environmental temperature around us. If
this layer were to be disturbed or destroyed, our body would have to
work harder to maintain our body temperature since the 'buffer' air
directly next to the skin is now colder. Since the 'buffer' air is
colder, this makes us feel colder. It is for this reason that wind can
make us feel colder. The stronger the wind, the more this 'buffer' layer
is disturbed or even destroyed, making us feel even colder. The reverse
is also true in that if we trap the 'buffer' air, we can make ourselves
feel warmer (like a winter jacket).

The wind chill describes the sense of feeling colder due to wind. Paul
Siple and Charles Passel did the first experiments in Antarctica in
1939. They experimented on the time it took water to freeze when exposed
to the elements. They found out that the time for the water to freeze
depended only the initial temperature (human body temperature), the
environmental temperature and the wind speed. From their data they
derived an empirical relationship, but it wasn't a useful form for
everyday use, so the National Weather Service uses a revised formula,
given by:

WC = 35.74 + (0.6215 T) -- (35.75 ( ^0.16^)) + (0.4275 T ( ^0.16^))

Where WC is the wind chill index in Fahrenheit, is the wind speed in
miles per hour and T is the temperature in Fahrenheit. This is easy to
compute since the quantities are easily determined. It is from this
equation that modern wind chill charts are constructed. The wind chill
index gives the perceived temperature equivalent for the combination of
cold air and wind. The following graph demonstrates the change in the
perceived temperature for a range of wind speeds.

Wind chill is related to another dangerous wintertime occurrence,
hypothermia. Both of these situations can become dangerous even in fair
weather. Hypothermia can occur by two different means, either by wind or
by the evaporative cooling of water. Both of these cool the body heated
'buffer' layer air near the skin. For activities associated with
hypothermia see related PUMAS examples: Preventing Hypothermia and
Hypothermia in the Little House.

Wind chill is dangerous because it can actually cause tissue to freeze
in a process called frostbite. The severity of the wind chill is
determined by the amount of time it takes for frostbite to occur on
exposed flesh (this is why it is very important to cover your face and
hands on days with low wind chills). Frostbite can occur in a matter of
minutes, which makes wind chill an essential tool for forecasters to
make people aware of the severity of the conditions outside.

Wind chill is only used to describe the perceived temperature for
warm-blooded animals. An inanimate object exposed to the elements can
only become as cold as the ambient temperature regardless of wind speed.
If the object is at a different temperature than the ambient temperature
the wind will aid in the cooling (or warming) of the object to the
ambient temperature.

**Activities:**

Heat up two identical pie pans of sand (or any other similar object)
such that there is a sufficient difference in temperature between the
sand and the surrounding air. Place a thermometer in each pan of sand,
and put one pan to the side and place the other pan by a fan so that the
air is being moved across the sand's surface (but not too close so as to
make a mess). Contrast the differences in the temperature as the two
pans of sand cool at different rates.

Calculate the wind chill that you might encounter in the Antarctic
winter (average temperature of -71F and wind speeds average more than 35
miles per hour). Compare this to the wind chill of an average winter day
in your area. The wind chill can be calculated by using the wind chill
equation. The temperature and the wind speed are the only values needed
for the calculation. The order of operations is very important.

Example Calculation: = 30mph, T = -15F

WC = 35.74 + (0.6215 T) -- (35.75 ( ^0.16^)) + (0.4275 T ( ^0.16^))

WC = 35.74 + (0.6215 (-15)) -- (35.75 (30 ^0.16^)) + (0.4275 (-15) (30
^0.16^))

WC = 35.74 + (0.6215 (-15)) -- (35.75 (1.723)) + (0.4275 (-15) (1.723))

WC = 35.74 -- 9.323 -- 61.605 -- 11.050 = -46.24F

Gives a wind chill of --46F.

Use the local forecast of high and low temperatures along with predicted
wind speeds to estimate wind chills for the current day.

Create a wind chill chart for a range of temperatures and wind speeds.