Geoscience Reference
In-Depth Information
feels less cold as time progresses. A new index has been
constructed that is referenced to a human walking at
5-6 km hr
-1
with their face exposed. This index was
established using volunteers walking on a treadmill in a
wind tunnel. Figure 3.23 presents this version of the
wind chill index. For example, a face exposed to a
50 km hr
-1
wind at a temperature of -25°C experi-
ences the equivalent, still-condition temperature of
-42°C. For this scenario, the old index gave a greater
wind chill of -51°C. The chart also indicates the
temperatures and wind speeds at which human flesh
will freeze after 2 minutes, 10 minutes, and prolonged
exposure to the elements. For example, if the air tem-
perature near the ground is -25°C and the wind
speed is 20 km hr
-1
, then frostbite will occur within
10 minutes. Other factors are being incorporated into
this schema. One's flesh cools more quickly at night
than in sunshine, an effect that is, surprisingly, exacer-
bated by precipitation but not by increased humidity.
In addition, nothing substitutes for acclimatization.
Hence, the original inhabitants of Tierra del Fuego at
the tip of Patagonia were discovered walking around
naked - by Europeans who found the cold unbearable.
fall through the colder air mass (Figure 3.20). If this
colder air is below freezing and the rain does not freeze
before it reaches the ground, then it will immediately
turn to ice upon contacting any object with a tempera-
ture below 0°C. If the warm front becomes stationary,
then continual freezing rain will occur over a period of
several days, and ice can accumulate to a thickness
of tens of millimetres around objects. The added
weight on telephone and power lines may snap the lines
where they connect to a pole. Collapsing tree branches
exacerbate the problem and disrupt road transport
(Figure 3.24). The area most susceptible to crippling
ice storms occurs in a band from central Texas, north
over the Appalachian Mountains and along the north-
eastern United States-south-eastern Canada border.
This is the region where low pressure can develop at
the leading edge of mobile polar highs and stall against
the Appalachians or a preceding high-pressure cell.
Severe ice storms occurred in Kentucky and Tennessee
in January 1951, Georgia and South Carolina in
December 1962, Maine in December 1964 and January
1979, Texas and western New York in March 1976,
Iowa in March 1990, Mississippi in February 1994, and
the US Midwest in December 2000.
The worst ice storm occurred on 4-10 January 1998
along the Canadian-US border east of the Great
Lakes. In the United States, federal disaster areas were
declared in 37 counties, 11 people died and 18 million
acres of forest worth $US1-2 billion was damaged by
ice. Analysis indicates that the storm has a return
Free
zing rain
(Environment Canada, 1998; Jones & Mulherin, 1998;
Fell, 2002-2003)
In some cases in winter or early spring, mid-latitude
depressions will develop rain instead of snow along the
uplifted warm front. To reach the ground, this rain must
Temperature (˚C)
Wind
speed
km hr
-1
5
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
10
3
-3
-9
-15
-21
-27
-33
-39
-45
-51
-57
-63
20
1
-5
-12
-18
-24
-31
-37
-43
-49
-56
-62
-68
30
0
-7
-13
-20
-26
-33
-39
-46
-52
-59
-65
-72
40
-1
-7
-14
-21
-27
-34
-41
-48
-54
-61
-68
-74
50
-1
-8
-15
-22
-29
-35
-42
-49
-56
-63
-70
-76
60
-2
-9
-16
-23
-30
-37
-43
-50
-57
-64
-71
-78
70
-2
-9
-16
-23
-30
-37
-44
-51
-59
-66
-73
-80
80
-3
-10
-17
-24
-31
-33
-45
-52
-60
-67
-74
-81
Frostbite possible after prolonged exposure
Frostbite possible after 10 minutes
Frostbite possible after 2 minutes
Fig. 3.23
Equivalent temperature or wind chill for given air temperatures and wind speeds. Wind chill temperatures at which frostbite is possible after
10 minute and 2 minute exposures are shaded (based on Environment Canada, 2002).
