History: The earliest wind chill index
was based on the research of Antarctic explorers Siple and Passel who
first measured the combined impacts of varying wind speed and
freezing temperatures in 1945. They did this by measuring heat loss
from water as it froze in a plastic container suspended from a tall
pole.
For over a year, there have been discussions between the National
Weather Service and the Meteorological Services of Canada about
updating the Wind Chill Temperature Index. During the Fall of 2000, a
special group – consisting of the National Weather Service, the
Meteorological Services of Canada and the International Society of
Biometeorology – evaluated the existing wind chill formula and made
changes to improve it. The group's goal was to internationally
upgrade and standardize the Wind Chill Temperature Index.
The new Wind Chill Temperature Index, by Randall Osczevski of
DCIEM and Maurice Bluestein of Purdue University in Indiana, makes
use of advances in science, technology and computer modelling to
provide a more accurate, understandable and useful formula for
estimating the dangers arising from winter winds and freezing
temperatures. In addition, clinical trials were conducted and the
results of those trials has been used to verify and improve the
accuracy of the new formula.
Twelve volunteers (six men and six women) participated in the
clinical trials. These consisted in four walks, at 4.8 km/h, on a
treadmill in a refrigerated wind tunnel at the Defence and Civil
Institute of Environmental Medicine in Toronto, Canada: one walk at
each of -10°, 0° and +10°C, plus a "wet trial" at +10°
during which participants received, every 15 seconds, a light
one-second splash of water in their faces. During each 90-minute
walk, the volunteers were walking while facing a wind of 2 metres per
second (m/s) for 30 minutes, followed by 30 minutes at 5 m/s, and 30
minutes at 8 m/s (or about 4, 10 and 16 mph, respectively). Sensors
were fixed to participants' forehead, cheeks, chin and nose, as well
as to the inside of one cheek, to measure skin temperature and heat
loss. The results from these trials were used to determine the
various thresholds for frostbite, as seen on the new wind chill
chart.
The new wind chill equation is now in use in both Canada and the
United States. Therefore, there is now a consistent wind chill
formula across North America.
The Wind Chill Temperature Index has been implemented in Canada
and the United States, resulting in a consistent index provided to
help the public protect itself against the dangers of frostbite and
hypothermia.
Specifically, the new Wind Chill Temperature Index:
- uses calculated wind speed at an average height of five feet
(typical height of a human face) based on readings from the
national standard height of 33 feet (typical height of an
anemometer);
- is based on the latest heat transfer theory, i.e., heat loss
from the body to its surroundings, during cold and breezy/windy
days;
- is based on a human face model because this is the part of the
human body most often exposed to the elements;
- uses a standard factor for skin tissue and assumes a no
sunlight scenario.
Frostbite Threshold: For the first time, the wind chill
temperatures include specific threshold values that provide specific
warning of time-to-frostbite at given levels of wind chill. For
example, a temperature of 5 degrees Fahrenheit and a wind speed of 30
mph equal a wind chill of -19, which will produce frostbite in 30
minutes. The chart also shows how frostbite will occur sooner if the
temperature is lower or the wind speed higher. Since it is the
responsibility of the National Weather Service to help protect lives,
this is an important service to the American people.
Clinical Testing Process: The new wind chill temperature
index was tested on human volunteers at the wind tunnel and climatic
chamber of the Defense and Civil Institute of Environmental Medicine
in Toronto, Canada. In these tests, the faces of six men and six
women were exposed to various temperatures and winds. By means of
sensors attached to various parts of their bodies, researchers
measured how fast the temperatures of the exposed skin dropped. A
time-dependent model was used to determine the time of frostbite and
the model incorporated the results from the tests.