Sunday, November 21, 2010

wind chill

Just in case you're wondering how wind chill works...

(This is a repost from January 2008.)

If you live somewhere in Canada, the northern US, or anywhere there are frigid temperatures you're probably aware of something called wind chill. This is the colder temperature the weatherman will give when he tells you how cold it feels versus the actual thermometer reading.

The idea of wind chill has interested me ever since I heard about it as a kid. And so in keeping with the blogland focus on weather this January (thanks to Rebecca Writes, who lives in the Yukon, where wind chill is a big deal), I thought I'd check it out for myself. Here are some things I discovered.

A layer of warm air up to several millimeters thick surrounds human skin. In exposed skin wind thins that layer. The stronger the wind, the thinner the insulating layer of warmth guarding the skin.

We humans don't sense the temperature of the air, but the temperature of our skin. When the wind robs our skin of its layer of insulation it feels closer to the temperature of the air. That's why exposed skin feels colder on a windy day than a calm one and we experience the phenomenon called wind chill.

Wind chill is calculated by a complicated formula. It takes into consideration engineering correlations of wind speed and heat transfer rate, and used to be expressed in 1000 - 3000+ watts per square meter. But how cold is 1500 watts per square meter? In order to make the wind chill number meaningful, scientists found a way to convert it into an equivalent temperature.

Thus you'll hear your weatherman say something like, "Bundle up. It's -10 but the wind chill factor will make it feel more like -21" This simply means that your face when exposed to the 40 km/hr wind that is predicted along with the -10C temperature will feel like it would on a day when the temperature is -21C with no wind.

Well, not exactly no wind. The lowest measure on an anemometer is a wind of 5 km/hr and also factored into the calculation is the assumption that you are walking (into the wind) at 3 km. per hour.

Some wind chill facts:

*Wind chill doesn't affect objects to make them freeze. If, for example, the wind chill factor is -4C (25F), water will not freeze if the air temperature is 2C (35F).

*Wind chill controversies: Should the method for calculating wind chill be based on:
- whole body cooling while naked
- or whole body cooling while wearing appropriate clothing
- or on cooling of the most exposed skin, such as the face?

*The coldest wind chill in Canada was at Pelly Bay Nunavut, January 13, 1975 when 56 km/hr winds made the temperature of -51C feel like -92C (3,357 watts per square meter).

*Wind chill impacts life in many ways, as in deciding what to wear when going outdoors and whether kids get to play outside at recess.

*Frostbite comes as a result of skin freezing. Symptoms: swelling, redness, tingling, burning --> white and waxy skin --> infection and loss of extremities.

*Hypothermia happens when body temperature falls below 35C (normal is 37C). Symptoms: drowsiness, bad coordination, weakness.

*This wind calculation chart also gives frostbite risk. (The chart is from Environment Canada.)

*If you know the temperature and wind speed, you can calculate the wind chill where you live. (This is an English system calculator. For those using metric - here are links to conversion tables: Km. to miles & celsius to fahrenheit).


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