Background Information

The temperature of the air in winter is not always a reliable indicator of how cold you will feel outdoors. Wind speed, relative humidity, and sunshine also have an effect. What you are wearing, how healthy you are, and your metabolism all affect how cold you feel. "Coldness" is related to the loss of heat from exposed flesh. Hypothermia, a life-threatening condition caused by the lowering of a person's core body temperature, can be prevented by understanding the chilling effect of wind.

In 1939, Paul Siple coined the term "wind chill" to describe the relative discomfort caused by a combination of wind and temperature, but his index did not work for temperatures above freezing or for high wind speeds. In 1941, Siple and Charles Passel developed a new formula to determine wind chill from experiments made at Little America, Antarctica.

Experiments could not be conducted on human skin without permanent injury, so Siple and Passel measured how long it took to freeze 250 grams of water in a plastic cylinder during different combinations of wind and temperature. They assumed that the rate of heat loss was proportional to the difference in temperature between that of the cylinder and that of the surrounding air. They plotted results in kilogram calories per square meter per hour (kg cal/m2/h) against wind speed in meters per second (m/s) to create a nomogram. A nomogram is a graph that has several lines arranged so an unknown value can be read from the intersection of one of these lines and a known value. From the Wind Chill Index Nonogram, scientists create a wind chill table.

The wind chill index or equivalent temperature is based on a neutral skin temperature of 33 Degrees C, or 91.4 Degrees Fahrenheit. It represents how cold the air feels to the skin, not how cold it is! Furthermore, the wind chill effect applies only to human flesh, not houses, cars, or clothing. The index does not take into account all possible heat losses of the body. With physical exertion, body heat production rises, perspiration begins, and evaporation removes heat from the body. A body can lose heat by contact with cold surfaces and by breathing cold air, which removes heat from the lungs.

The next time you want to know how cold it is outdoors, check the thermometer. But keep in mind that other things, such as wind speed, body fat, individual metabolism, and protective clothing, all help to determine how "chilly" you feel.

In the procedure for this activity, you will use the Wind Chill Index Nomogram to find the temperatures needed to complete a wind chill table. The line at four mph on the nomogram approximates the wind speed generated by someone walking briskly under calm conditions. This is the standard wind speed chosen for calculating wind chill equivalent temperatures. Wind chill refers to the temperature which, at a four mph wind speed, cools the skin at the same rate as the observed (ambient) temperature and wind speed.




Procedure

Assume that the air temperature is 20 Degrees Fahrenheit, and the wind speed is ten mph. You are walking to school. What temperature would you actually feel?

To answer this question, perform the following steps.

  1. On the Wind Chill Index Nomogram, shown in Figure 4.1, locate 20 Degrees Fahrenheit on the X (horizontal) axis. Follow the vertical line that starts at 20 Degrees Fahrenheit up until it intersects the ten mph wind speed line.

    Figure 4.1. Wind Chill Index Nomogram.
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  2. Draw a dashed horizontal line from this intersection over to the four mph wind speed line and stop. From the point where you stopped, draw a dashed vertical line down to the horizontal axis. Read the temperature where your line intersects the X axis. This is the wind chill equivalent temperature. If you read 3 Degrees Fahrenheit, then you followed the procedure correctly.

  3. The Wind Chill Table, as shown in Figure 4.2, indicates the wind chill equivalent temperatures for different combinations of wind speed and air temperature.

    Figure 4.2. Wind Chill Table.
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    Notice there are four blank values in the table. Using the same process outlined in Steps 1 and 2 with the Wind Chill Index Nomogram in Figure 4.1, determine the missing values in Figure 4.2, Wind Chill Table, using the wind speed and air temperature pair for each blank.





Questions

  1. What factors, other than temperature, determine how "chilly" you feel?

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  2. Using the Wind Chill Table, find the wind chill equivalent temperature for the following wind speeds and temperatures.

      a. On a cold winter day on your way to work or school or work: The air temperature is 10 Degrees Fahrenheit and the wind speed is 15 mph.

      _______________

      b. While skiing with friends: The air temperature is -5 Degrees Fahrenheit and the wind speed is 20 mph, while you are standing still, waiting to start down the slope.

      _______________

      c. On a cold winter day on your way to work or school or work: The air temperature is 10 Degrees Fahrenheit and the wind speed is 15 mph.

      _______________

  3. Although the wind you feel in Question 2c is due to your bike's motion, you will still feel the effect of wind chill. What will it feel like?

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  4. Read the "Safety Information - Hypothermia" section in this activity. What precautions should you take to prevent hypothermia?

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  5. Although frostbite and hypothermia are not directly related, you should know what frostbite is and how to prevent it. Read the "Safety Information - Frostbite" section in this activity. What precautions should you take to prevent frostbite?

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Figure 4.3. Questions Sheet
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Conclusions

Review the problem stated in the workstation screen graphic
at the top of this web page and write your conclusions here.



Figure 4.4. Conclusions Sheet
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Safety Information

    Hypothermia

      It is important to be aware of hypothermia's symptoms when spending time outdoors. Hypothermia is the rapid, progressive, mental and physical collapse that occurs as the inner core of the human body becomes chilled. The internal body temperature slides downward. This slide can lead to stupor, collapse, and death. Most hypothermia occurs when the air temperature is between 30 Degrees Fahrenheit and 50 Degrees Fahrenheit. Because most people consider these temperatures to be normal, the effects of hypothermia catch them off guard.

        Hypothermia can occur in warm weather!

      Furthermore, hypothermia occurs even more rapidly when the person is in the water. Often, drowning victims succumb to hypothermia before actually drowning. Whether in air or water, victims of hypothermia frequently do not realize what is happening. Everyone must be aware of hypothermia and help each other.

      These are the symptoms of hypothermia:

      • shivering;
      • vague, slow, slurred speech;
      • memory laps;
      • fumbling hands;
      • stumbling, lurching walk;
      • drowsiness;
      • inability to get going after a short rest.

      To avoid hypothermia, follow these guidelines.

      • Dress appropriately. Wear a hat. Major heat loss occurs through the scalp.
      • Stay dry. Wet clothes lose their insulating value and water robs heat faster than air at the same temperature.
      • Beware of the wind. A slight breeze carries heat away from the skin faster than motionless air.
      • never ignore shivering. Seek help.

      Even mild symptoms should be treated immediately.

      • Get out of the wind, rain, snow, sleet, or hail.
      • Remove wet clothes.
      • Bundle up in blankets, coats, sleeping bags, or other available covers.
      • Drink warm nonalcoholic beverages.
      • Stay awake.

    Frostbite

      If the skin becomes cold enough it can freeze. Freezing usually occurs on exposed skin such as fingertips, nose, ears, and cheeks. Toes of climbers are often frostbitten, due to the lack of sufficient blood flow to keep them warm.

      • To avoid frostbite, stay indoors during cold windy weather.
      • If you must be outside, cover all exposed skin and keep your circulation going in your extremities.
      • If injured skin changes color, get medical help.




For Your Information

The nomogram is based on a general formula for heat loss. Heat loss occurs by means of radiation, conduction, and convection. Combining all effects, the general formula for heat loss (H) is,

H = (A + Bv1/2 + CV)T [Note: v1/2 = square root of v],

where:

  • H is the heat loss in kg cal/m2/h;
  • v is the wind speed in m/s; and
  • T is the difference in degrees celcius (C) between 33 (neutral skin temperature) and the air temperature.
The constants A, B, and C are 10.45, 10.00, and - 1.00, respectively.

The values of the constants A, B, and C vary widely in formulas presented by different investigators. This is to be expected since heat loss (H) depends on certain properties of the body being cooled. This formula measures the cooling power of the wind and temperature in complete shade and does not consider the gain of heat from incoming radiation, either direct or diffuse. Under conditions of bright sunshine, the heat loss should be reduced by about 200 kg cal/m2/h.