Forecasting

Wind Chill Temperatures

in South Africa

by

Stephan Steyn

Dept. of Soil, Crop and Climate Sciences

University of the Free State

Air Temperature

and Human Comfort

• The human body’s perception of

temperature changes with varying atmospheric conditions

• The reason for these changes is related to how we exchange heat energy with our environment

• There is a constant exchange of heat –especially at the surface of the skin – between the body and the environment

• To maintain a constant temperature, the heat produced and absorbed by the body must be equal to the heat it loses to its surroundings

Sensible Temperature

• On a cold day, a thin layer of warm

air molecules forms close to the skin

• This air layer protects the skin from the surrounding cooler air and from the rapid transfer of heat

• Thus, in cold weather, when the air is calm, the temperature we perceive is often higher than a thermometer might indicate

• Once the wind starts to blow, the insulating layer of warm air is swept away

• Heat is rapidly removed from the skin by the constant bombardment of cold air

Sensible Temperature

• In cold weather, the peripheral blood vessels of

the body constrict to counteract rapid heat loss

to the environment

• In hot weather, the blood vessels enlarge,

allowing a greater loss of heat energy to the

surroundings

• As we perspire sweat evaporates and the skin

cools because it supplies

the large latent heat of vaporization

(about 2.43x106 J/kg)

All other factors being the same…

The faster the wind blows

the greater the heat loss

the colder we feel

Frostbite• High winds, in below-freezing air, can remove

heat from exposed skin so quickly that the skin may actually freeze and discolour

• The freezing of skin usually occurs first on the body extremities (fingers, toes, nose, ears) because they are the greatest distance from the source of body heat

Wind Chill Temperature Index

(WCTI)

• An attempt to measure the effect of combinations of low temperature and wind on humans or animals

• Created as a public health tool to reduce hypothermia, frostbite and other cold-related ailments

• When forecasters say the "wind chill is -10 C" they are NOT saying that the chilled object is cooled to -10 C

WCTI: General Limitations

The WCTI index is at best a rough estimate, since

It doesn’t take into account the following factors:

• The fit and type of clothing

we wear

• The amount of sunshine

reaching the body

• The actual amount of exposed skin

• Whether the skin is wet or dry

In addition, the wind chill index can only be used

for low temperatures and winds stronger than

5 km/h

History of the WCTI

• During the 1940s, Paul Siple and Charles Passelconducted experiments in Antarctica

• They measured the time needed to freeze water in a plastic cylinder that was exposed to the elements

• They found that the time depended on how warm the water was, the outside temperature and the wind speed

• Produced an empirical formula:

where: V = wind speed in km/h

T = air temperature in C

33)4.598.1)(0183.02637.02135.0( TVVWVTI

Problems with the old WCTI

• Human skin freezes at a different rate than

water (different parts of the body freeze at

different rates)

• The official wind measurements used in the old

formula are taken 10 m above ground, where

wind blows much faster than it does at the

surface

• The index-values become

unreliable at high wind speeds

(cut-off at 40 km/h)

The old WCTI

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

0 10 20 30 40 50 60 70 80 90 100

Wind speed (km/h)

WC

T-i

nd

ex

(ºC

)

10 ºC

5 ºC

0 ºC

-5 ºC

-10 ºC

The new WCTI

• In 2001, the U.S. National Weather Service and the Canadian Weather Service replaced the formulas with new ones

• The new formula use "modern heat-transfer theory" instead of empirical equations based on the 1945 experiments

• Wind speeds used in the new formula are from winds 1.5 m above the ground

(http://www.weather.gov/om/windchill)

(http://www.weather.gov/om/windchill)

The new WCTI• New WCTI formula:

where: V = wind speed in km/h

T = air temperature in C

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

0 10 20 30 40 50 60 70 80 90 100

Wind speed (km/h)

WC

T-i

nd

ex

(ºC

)

10 ºC

5 ºC

0 ºC

-5 ºC

-10 ºC

)(3965.0)(37.116215.012.13 16.016.0 VTVTWVTI

Applying the WCTI to Observed

Data

Station 10m Wind

(km/h)

Screen

Temp

(ºC)

WCTI

old

(ºC)

WCTI

new

(ºC)

Bloemfontein 18.5 5 -9.5 1.3

Cradock 27.8 7 -8.2 2.9

De Aar 46.3 7 -7.7 1.7

Ermelo 37.0 6 -10.1 0.9

Kimberley 37.0 5 -11.6 -0.4

Calculations based on 12:00Z data for 2 August 2006

(data supplied by SAWS)

Applying the WCTI to Model Data

(model data courtesy of the UP modelling group)

CCAM prognosis for 06:00Z on 2 August 2006

Temperature (ºC)Wind speed (km/h)Temperature difference (ºC)Temperature difference (ºC)Temperature difference (ºC)

Concluding Remarks

• The new WCTI can be applied with ease to

observed data and numerical model fields

• Knowledge of the general limitations is crucial

for interpretation of the WCTI

• Standardisation of the WCTI among the

meteorological community is necessary in order

to provide an accurate and consistent measure

to ensure public safety