Pequot Lakes School
[ School Calendar ]   [ Staff Webmail ]   [ Student Webmail ]
Meteorology «
Directions: Copy and paste the worksheet found at the end of the reading into a wordprocessing document.  Read and answer the questions using the chart, reading, and graph. Print your work and turn it in with your name, period and the date on it.

The Wind Chill Factor
Adapted from USA TODAY

Wind chill is an important factor that many outdoor enthusiasts fail to take into consideration before departing on an extended winter expedition.  With dog mushers in Alaska, the general rule of thumb is at -30 degrees with a 30 mph wind, your flesh will freeze in 30 seconds. If that doesn't get your attention, then you've never been really cold.

Cold weather is stimulating to some while others avoid it like the plague. For the serious outdoor enthusiast it's just another part of what you have to deal with when you plan a trip or outing. Unfortunately, not everyone realizes the seriousness of the wind chill during extended exposure. In extreme conditions, even a quick sprint to the mailbox can mean frost bite if your skin is not properly protected.

The “Wind Chill Factor” is described as a "feels-like number." It is based on the fact that wind carries heat away from the body, causing the body to cool more quickly. Wind chills are designed to indicate the dangers of different combinations of wind and temperature on the bodies of humans and animals. 

The Problem
Theoretically, the wind chill index is supposed to measure the rate at which the body loses heat when exposed to cold and wind. This index was created as a public health tool to reduce hypothermia, frostbite and other cold-related ailments.  As a practical matter, the wind chill index is supposed to tell people how warmly to dress, a crucial decision for people who spend long periods outdoors such as construction workers or skiers.  But scientists say the wind chill index is at best a rough estimate. It doesn’t take into account big differences between heat loss in the sun and heat loss in the shade, for example.

The term "wind chill" goes back to the Antarctic explorer Paul Siple, who coined it a 1939 dissertation, "Adaptation of the Explorer to the Climate of Antarctica." During the 1940s, Siple and Charles Passel conducted experiments on 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. The original formula used to calculate wind chill was based on those experiments.   But human skin freezes at a different rate than water. Even different parts of the body – the face versus the hands, for example – freeze at different rates.

A few years ago mechanical engineer Maurice Bluestein went outside to shovel snow on a bitterly cold day, a -25 degree temperature and a -65 degree wind chill factor.  While standing in the driveway, Bluestein noticed that his exposed skin wasn’t freezing in 15 seconds, as it should have if the wind and the cold were the equivalent of –65 degrees.  The wind chill index used by the National Weather Service from 1973-2001 significantly over-stated how cold it feels when the wind blows.  Bluestein and his colleague, Jack Zecher, went on to devise a new wind chill formula that would  make use of a more accurate estimate of "the thermal properties of the skin" and "modern heat transfer theory."  Bluestein admits he’s no weather expert but, as a mechanical engineer, he does know a lot about heat transfer.   "It didn’t take me long to realize it wasn’t as cold as the chart claimed it was," says Bluestein, a professor at Purdue University-Indiana University in Indianapolis.

A Revamped Wind Chill
In the fall of 2001 the National Weather Service, finally began using a revised wind chill formula based on the one created by Bluestein, as well as other models created by half a dozen other researchers rather than equations based on 1940s experiments.  In addition, wind speeds used in the new formula are from winds 5 feet above the ground. The wind at 5 feet is about one-third the speed of wind at 33 feet, where official wind speeds used to be measured. 

The new formula in its final form was developed by representatives of seven government agencies, including the U.S. National Weather Service, the Canadian weather service and university scientists after testing in a chilled wind tunnel at Canada's Defence Civil Institute of Environmental Medicine in Toronto. In the experiment, faces of several men and women were exposed to various temperatures and winds. The researchers measured how fast the temperatures of the exposed skin dropped to devise the new formula.

The new wind chills aren't nearly as scary-sounding as the old ones. For example, at 5 degrees with a 30-mph wind, the old formula determined a wind chill of minus 40 degrees. The new formulas says the chill would be minus 19 degrees.

"This information will help people make sound decisions about how to dress for the weather," Weather service Director Jack Kelly said in a statement. "Exposure to cold, biting air for long periods of time is dangerous," Kelly said. "Our main goal was to use modern science in revising the index so that it's more accurate and makes the human impact more prominent."

Despite its improved accuracy, one forecaster fears that the public may not immediately embrace the new wind chill formula.   "It sounds like they're factoring in things that actually affect the human body, important factors that are more realistic," said CNN meteorologist Jill Brown. "But everybody hates change, and some people are not going to like it."   Brown also said that people in colder climates will have to adjust themselves mentally to the newly calibrated temperatures.  "You'll have to kind of recalculate your feeling of how cold it actually is out there," she said, "I think for everyone it's going to be a little bit confusing -- not that it's a bad thing. It's probably an improvement, but it's going to take a while to get used to a new standard."

Some odds and ends related to wind chill:

Cold-related Ailments
Hypothermia  results when body temperature falls below 95oF.  Symptoms include drowsiness, impaired coordination and weakness. It can also be fatal.

Frostbite  is the result of skin freezing. It causes swelling, redness, tingling and burning. Skin turns white and waxy as the frostbite progresses. Loss of extremities and infection can result.

Frostnip  is a condition where ice crystals form under the skin.

Chilblains  occur when bare skin is exposed to cold water, or when wet skin cools. The skin itches and swells. Chilblains can lead to gangrene.

Freeze Tolerance
The wood frog (Rana sylvatica) has what's called freeze tolerance. In winter, the wood frog hibernates on land, usually with their only shelter being a pile of leaves. Because this leaves it exposed to the cold, frost penetrates their skin and they freeze. Ice forms around its internal organs. Its blood stops flowing. Breathing stops. The heart stops beating and muscles stop moving. The wood frog's body functions return to normal when it thaws.

Briskly Walking Naked
The following passage is word for word from a National Center for Atmospheric Research document on windchill published when the original wind chill formula was first adopted:  "The wind is blowing at 20 mph and the temperature is 20 deg F. If I were out naked, how would this make me feel? The answer (from the formula!) is that you'd feel like you were naked, and walking briskly through calm air at approximately -11 deg F. You might also feel pretty silly for having forgotten your coat and pants! The goal of the windchill temperature is to relate (perhaps extreme) conditions to something we have all likely experienced, namely: briskly walking naked through calm air at certain carefully calibrated temperatures! [If naked, would you walk anything but briskly?]."

Name; __________________________________ Date;______  Period: ______

Wind Chill Factor
Reading Worksheet

1. Wind chill is the rate at which...

2. Who coined the term “wind chill”?

3. Describe the 1940’s experiments that yeilded the original wind chill formula.

4. What was one problem with those original experiments when it comes to applying them to human skin?

5. During what time period did the National Weather Service use the original wind chill formula?

6. What was the name of the Purdue University professor that devised a new wind chill formula?

7. What two things did he take into account that allows his formula to be a more accurate?

8. When was the new formula put into place by the National Weather Service?

9. The new formula uses wind speeds at 5 feet, Where were earlier wind speed measurements taken?

10. How much slower are winds at 5 feet?

11. How was the new formula tested?

12. Name and describe one cold related ailment:

13. What property does Rana sylvatica have that allows it to freeze solid and then return to normal when it thaws?

14. If naked, would you walk anything but briskly? (Just a little joke from the NWS!)

15. Using the NWS Windchill Chart at the top of the page, determine what the wind chill factor would be on a day when it is -30oF and the wind is blowing 15mph:

16. Using the chart, determine what the wind chill factor would be on a day when it is 0oF and the wind is blowing 55mph:

17. Using the chart, determine how fast you would get frostbite if the wind chill was -69oF:

18. Using the chart, determine how fast you would get frostbite if the wind chill was -19oF:

19. Using the Wind Chill Temperature Comparison graph at the end of the reading determine what the wind chill reading would have been using the old formula if the wind were blowing 25 mph and the air temperature was 5oF:

20. How many degrees different is the wind chill reading using the new formula compared to the old formula if the wind were blowing 25 mph and the air temperature was 5oF?

21-22. Use the wind chill formula found on the bottom of the wind chill chart at the top of the page to answer this question:  If you were in the northern tundra and the wind were blowing 65 mph and the air temperature was -50oF, what would the wind chill factor be (where T = air temperature and V = wind speed)?