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Twisting in the Wind

On windchill, temperature, and waiting for summer

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My apologies for the slow postings last week. I was waiting for the news wires and the National Park Service Morning Report to echo some Easter weekend mayhem so I’d have survival cases to highlight, but apparently –shockingly – not much happened.

So, bored with day-to-day hamster wheel training like runs and bike rides, I spent Sunday on an epic sandstone scramble around Fern’s Nipple in Capitol Reef. It was the usual gorgeous but committing country, a route along rugged shelves in the middle of nowhere (see photo). It was a beautiful and refreshing escape, but one thing the photo doesn’t show: Fast-moving thunderstorms whipped cold gales into my face the entire trip.

Spring can be a blustery time in most areas of the country, greatly affecting the actual conditions you deal with, in spite of the season’s rising thermometer. Southern Utah is notorious for spring wind, and for the last three weeks, it’s been a nonstop 10 to 20 mph force of God, blasting out of the West. Frankly, it’s getting old – the chilled fingers, blowing sand, stolen hats and wind-torn maps.

As I scrambled through the gusts, I thought about how I’d be facing a lot more wind in the coming months on assignments to Alaska, Utah and Montana. And that got me thinking about windchill.

Windchill factor is a term that attempts to describe what happens to human tissue when it’s being cooled by both low temperatures and wind.As wind increases, heat is carried away from the body at a faster rate due to convection, driving down skin temperature (which can lead to frostbite) and internal body core temperature (leading to potentially fatal hypothermia). Simply put, windchill can be explained as the apparent temperature felt on exposed skin. The opposite affect in hot/humid temperatures is called the heat index.

For most purposes, wind chill is not expressed for temperatures above 50 degrees Fahrenheit, or wind speeds below 3 mph. It’s also worth noting that bright sunshine can increase wind chill temperatures by 10 to 18 degrees Fahrenheit.

Windchill cooling can radically affect living tissue, but wind does not cool dry objects lower than the actual air temperature; it just cools them more quickly. For example, with an air temperature of 0 Fahrenheit and a wind speed of 20 mph, windchill might be -22 Fahrenheit. Your fingers would freeze just as quickly as if it were -22 Fahrenheit, but metal objects (and your skin) will not cool lower than 0 Fahrenheit.

The firstwind chill tables were developed just prior to the Second World War. They were based on the cooling rate of a small water-filled plastic bottle suspended in the wind. The National Weather Service didn’t begin regularly using windchill indices in pubic announcements until the 1970s. In 2001 they revised the wind chill tables to more accurately reflect the cooling effect experienced on a human face – the most common spot where people get frostbite.

The net effect was to increase estimated wind chill cooling at slow wind speeds, below 15mph, while reducing the estimated effect of higher wind speeds. For example: Standing in freezing air temperatures and a 10mph wind creates a windchill of 24F. The same conditions with a 20mph wind only lower the windchill to 20F.

While the science and numbers are intriguing, for the backcountry traveler it all boils down to a simple rule: Anytime you want to stay warm, whether it’s in a survival situation or just a lunch break, find a sunny spot out of the wind. Even with a mild 10mph breeze, it can increase your perceived temperature by 30 degrees or more.

And until summer truly arrives, staying out of the wind, and in the sun, is the quickest way to warm up.—Steve Howe

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