forecasting wind chill temperatures in south africalearning.ufs.ac.za/lwr314_on/resources/2....
TRANSCRIPT
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