Page 154 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

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▲Figure 5.1 Wind chill in the south polar region. antarctic scientists keep skin surfaces covered in wind-chill conditions reaching –46°C even in summer. [Ted Scambos & rob Bauer, nSiDC.]
What is the temperature—both indoors and outdoors—as you read these words? How is temperature measured, and what does the mea- sured value mean? How is air temperature influencing your plans for the day?
Our bodies subjectively sense temperature and react to temperature changes. Apparent temperature is the gen- eral term for the outdoor temperature as it is perceived by humans. Both humidity (the water vapour content of the air) and wind affect apparent temperature and an
individual’s sense of comfort. Humidity is significant in determining the effect of high temperatures, which may cause heat stress, illness, and loss of life (we discuss heat waves later in the chapter). In cold regions, wind is a more important influence. When strong winds combine with cold temperature, the effects can be deadly (Figure 5.1).
On a cold, windy day, the air feels colder because wind increases evaporative heat loss from our skin, pro- ducing a cooling effect. The wind-chill factor quantifies the enhanced rate at which body heat is lost to the air. As wind speeds increase, heat loss from the skin increases, and the wind-chill factor rises. To track the effects of wind on apparent temperature, the Meteorological Ser- vice of Canada and the National Weather Service (NWS) use the wind-chill temperature index, a chart plotting the temperature we feel as a function of actual air tem- perature and wind speed (Figure 5.2). The goal of the chart is to provide a simple, accurate tool for assessing the dangers to humans from winter winds and freezing temperatures.
The lower wind-chill values on the chart present a serious freezing hazard, called frostbite, to exposed flesh. A downhill ski racer going 130 km·h−1 during a 2-minute run can easily incur this type of injury. Another dan- ger is hypothermia, a condition of abnormally low body temperature that occurs when the human body is losing heat faster than it can be produced. Even without wind, frostbite and hypothermia are potential dangers in the extreme cold found on high mountain summits and in the polar regions. Moreover, hypothermia can arise in any situation where humans become chilled; it is not ex- clusively related to freezing temperatures.
◀Figure 5.2 Wind-chill Index. This index uses wind speed and actual air temperature to determine apparent temperature and was developed by the Meteorological Service
of Canada and the U.S. national Weather Service. windchill.ec.gc.ca/ [Meteorological Service
of Canada.]
Actual Air Temperature (°C)
Calm 0°
8 –2° 10 –3° 15 –4° 20 –5° 25 –6° 30 –6° 35 –7° 40 –7° 45 –8° 50 –8° 55 –8° 60 –9° 65 –9° 70 –9° 75 –10° 80 –10°
–5° –10° –7° –13° –9° –15° –11° –17° –12° –18° –12° –19° –13° –20° –14° –20° –14° –21° –15° –21° –15° –22° –15° –22° –16° –23° –16° –23° –16° –23° –17° –24° –17° –24°
–15° –20° –19° –24° –21° –27° –23° –29° –24° –30° –25° –32° –26° –33° –27° –33° –27° –34° –28° –35° –29° –35° –29° –36° –30° –36° –30° –37° –30° –37° –31° –38° –31° –38°
–25° –30° –33° –35° –37° –38° –39° – 40° – 41° – 42° – 42° – 43° – 43° – 44° – 44° – 45° – 45°
–30° –36° – 39° – 41° – 43° – 44° – 46° – 47° – 48° –48° –49° –50° –50° –51° –51° –52° –52°
–35° – 41° – 45° –48° –49° –51° –52° –53° –54° –55° –56° –57° –57° –58° –58° –59° –60°
– 40° – 47° –51° –54° –56° –57° –59° –60° –61° –62° –63° –63° –64° –65° –65° –66° –67°
–45° – 53° – 57° –60° –62° –64° –65° –66° –68° –69° –69° –70° –71° –72° –72° –73° –74°
–50° – 58° – 63° –66° –68° –70° –72° –73° –74° –75° –76° –77° –78° –79° –80° –80° –81°
Frostbite times: Low risk of frostbite 30 min. 5–10 min. 2–5 min. < 2 min.
Wind Speed (km.h-1)