Science Lessons from Bitter Cold
The strong winds and bitter cold that the new year 2010 brought to large parts of the United States has put wind chill in the news.
This, like any outbreak of cold, windy weather, offers high school physics teachers in places where people are talking about wind chill an opportunity to relate the laws of thermodynamics to everyday life.
Elementary and middle school teachers aren’t likely to use the word “thermodynamics” in class, but they can make two important points:
- Wind chill is a good guide to the dangers of cold weather
- It affects only people and other animals
Most people are aware that they feel colder when the wind is blowing, but this is because the wind carries away the layer of air around you that your body’s heat has warmed.
The big danger of air temperatures below 32 degrees Fahrenheit is that the water in the cells of your skin can freeze to cause frostbite. When the air is just a little below freezing and no wind is blowing, heat from your body warms the air around you enough to prevent frostbite.
The laws of thermodynamics apply to both machines and animals that burn “fuel” to supply energy. Machines use this energy to do things such as accelerate your car when the traffic light turns green and you press on the accelerator pedal.
Food is the fuel for animals, including us. One of the basic principals of thermodynamics is that turning fuel into energy is never 100% efficient. The energy that’s not used escapes as heat.
Your car uses its cooling system to get rid of this excess heat to keep it from destroying the engine. Your body gets rid of its excess heat by radiating it away from your skin and expelling some when you exhale.
When the air’s temperature drops into the 60s you need to have at least some clothing on to hold in enough of your body’s heat for comfort.
This is another law of thermodynamics at work: heat flows from warm objects to cold objects–such as from your body to surrounding cool air, cooling the warm object and warming the cold object. The greater the temperature difference, the faster heat moves from warm to cold.
As the air grows colder you need more clothing to hold enough of your body’s heat next to your body, first for comfort and then, as the temperature drops, to hold in enough heat to prevent not only frostbite but also hypothermia.
While the air and skin temperature have to be colder than 32 degrees for frostbite, hypothermia can occur at warmer temperatures. Hypothermia refers to the loss of enough core body temperature that internal organs and the brain no longer function normally. While frostbite can cause serious injuries, it’s rarely deadly.
The first signs of hypothermia, such as shivering, begin when the body’s core temperature drops below approximately 95 degrees. If the victim isn’t immediately warmed, he or she will die.
Wind increases the danger of cold air because it carries away the heat your body produces. The faster the wind, the faster it carries away heat. But, no matter how fast the wind blows, it can’t cause anything to become colder than the air.
The law of thermodynamics that heat flows only from warm to cold explains this. No matter how much the wind huffs and puffs, if its temperature is 33 degrees, it can’t cool anything such as your car’s radiator or your bare skin to 32 degrees.
But if your skin is cooled to 33 degrees and you don’t stop heat from escaping, you will quickly become a victim of hypothermia.
Since wind carries warm air away from your body even when you are wearing warm clothing, it has the same affect on cooling your body’s skin and core that a lower temperature would.
For example, the U.S. National Weather Service wind chill chart tells you that if the temperature is 20 degrees F and the wind is blowing at 35 mph, you will cool off as quickly as you would in zero-degree air with no wind
The Weather Service’s wind chill chart starts with 40 degrees F, and gives a “wind chill temperature” of 25 degrees in a 60 mph wind. If you were out in such conditions you wouldn’t have to worry about frostbite even on exposed bare skin since even with a 60 mph wind your skin would never cool below the air’s 40 degrees. But, you’d be in real danger of hypothermia unless you were wearing very warm clothing.
Chapter 11 of [amazon-product text=”The AMS Weather Book: The Ultimate Guide to Americas Weather” type=”text”]0226898989[/amazon-product] has more on wind chill, frostbite and hypothermia, including the story of how Paul Siple (1909-1968), a pioneer Antarctic explorer came up with the idea of wind chill.
Siple first went to Antarctica in 1928 after winning a national competition to be the Boy Scout who went with Admiral Richard Byrd’s first Antarctic expedition in 1928. He conducted the experiments that led to his developing the wind chill index while on Byrd’s 1939-1941 Expedition.
He led the eight scientists who spent the first winter at the new U.S. South Pole station in 1957-58. Navy Lieutenant (junior grade) led the eight Navy men who operated the station. He tells the story of the wind chill index and the first winter any humans had spent at the South Pole in his book, 90 Degrees South.
For more on why the NWS adopted the current wind chill index, see a story I did for USA TODAY in February 2005.
The National Weather Service has more information about wind chill online:
A good source of information for how to avoid frostbite and hypothermia when you are in a place with extremely low temperatures is in the sections on these in Chapter 24 of the Field Manual of the U.S. Antarctic Program, It’s available for download as PDF files on the U.S. Antarctic Program Web site.
Ned Rozell’s “The Physics of Life at Forty Below” on the Alaska Science Forum Web site describes how people who live in the colder parts of his state go about daily life when the alcohol in the thermometer reads 40 below–alcohol is used because mercury freezes at 38 below.
More on using weather to teach science