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A Closer Look

The Windchill Factor
he term windchill is attributed to the an average height of 1.5 m (5 ft) above the you feel cooler. How much cooler depends
TAntarctic explorer Paul A. Siple. During ground, which is closer to the height of a hu- on how fast the air is moving and the outside
the 1940s, Siple and Charles F. Passel con- man face than the height of an anemometer temperature—which is what the windchill
ducted experiments on how long it took a can and takes advantage of advances in science, factor tells you. Thus, windchill is an attempt
of water to freeze at various temperatures and technology, and computers. The new chart to measure the combined effect of low
wind speeds at a height of 10 m (33 ft) above also highlights the danger of frostbite. temperature and wind on humans (Box
the ground (typical height of an anemom- Here is the reason why the windchill Figure 22.1). It is just one of the many fac-
eter). They found that the time depended factor is an important consideration. The tors that can affect winter comfort. Others
on the air temperature and the wind speed. human body constantly produces heat to include the type of clothes, level of physical
From these data, an equation was developed maintain a core temperature, and some of exertion, amount of sunshine, humidity, age,
to calculate the windchill factor for humans. this heat is radiated to the surroundings. and body type.
In 2001, the National Weather Service When the wind is not blowing (and you are There is a windchill calculator at the
changed to a new method of computing not moving), your body heat is also able to National Weather Service website, www.nws.
windchill temperatures. The new windchill warm some of the air next to your body. This noaa.gov/om/windchill/. All you need to do is
formula was developed through a year-long warm blanket of air provides some insula- enter the air temperature (in degrees Fahren-
cooperative effort between the U.S. and Ca- tion, protecting your skin from the colder heit) and the wind speed (in miles per hour),
nadian governments and university scientists. air farther away. If the wind blows, however, and the calculator will give you the windchill
The new standard is based on wind speeds at it moves this air away from your body and using both the old and the new formula.

Wind Temperature (˚F)
(mph) 40 35 30 25 20 15 10 5 0 –5 –10 –15 –20 –25 –30 –35 –40 –45

5 36 31 25 19 13 7 1 –5 –11 –16 –22 –28 –34 –40 –46 –52 –57 –63
10 34 27 21 15 9 3 –4 –10 –16 –22 –28 –35 –41 –47 –53 –59 –66 –72
15 32 25 19 13 6 0 –7 –13 –19 –26 –32 –39 –45 –51 –58 –64 –71 –77
20 30 24 17 11 4 –2 –9 –15 –22 –29 –35 –42 –48 –55 –61 –68 –74 –81
25 29 23 16 9 3 –4 –11 –17 –24 –31 –37 –44 –51 –58 –64 –71 –78 –84
30 28 22 15 8 1 –5 –12 –19 –26 –33 –39 –46 –53 –60 –67 –73 –80 –87
35 28 21 14 7 0 –7 –14 –21 –27 –34 –41 –48 –55 –62 –69 –76 –82 –89
40 27 20 13 6 –1 –8 –15 –22 –29 –36 –43 –50 –57 –64 –71 –78 –84 –91
45 26 19 12 5 –2 –9 –16 –23 –30 –37 –44 –51 –58 –65 –72 –79 –86 –93
50 26 19 12 4 –3 –10 –17 –24 –31 –38 –45 –52 –60 –67 –74 –81 –88 –95
55 25 18 11 4 –3 –11 –18 –25 –32 –39 –46 –54 –61 –68 –75 –82 –89 –97
60 25 17 10 3 –4 –11 –19 –26 –33 –40 –48 –55 –62 –69 –76 –84 –91 –98

Frostbite times 30 minutes 10 minutes 5 minutes

BOX FIGURE 22.1 Windchill chart.

differences occur together, and neither is the cause of the other. ture of the land increases more rapidly than the water tempera-
This is an important relationship in a working model of air ture. The air over the land is therefore heated more, expands, and
movement that can be observed and measured on a very small becomes less dense. Cool, dense air from over the water moves
scale, such as between an asphalt parking lot and a grass field. inland under the air over the land, buoying it up. The air moving
It can also be observed and measured for local, regional wind from the sea to the land is called a sea breeze. The sea breeze along
patterns and for worldwide wind systems. a coast may extend inland several miles during the hottest part of
A local wind pattern may result from the temperature the day in the summer. The same pattern is sometimes observed
differences between a body of water and adjacent landmasses. around the Great Lakes during the summer, but this breeze usually
If you have ever spent some time along a coast, you may have does not reach more than several city blocks inland. During the
observed that a cool, refreshing gentle breeze blows from the water night, the land surface cools more rap idly than the water, and the
toward the land during the summer. During the day, the tempera- air moves from the land to the sea (Figure 22.10).

550 CHAPTER 22 The Atmosphere of Earth 22-10