Tuesday, May 6, 2014

How cold could it get?

My next Appalachian Trail section hike is from the James River Footbridge, to Manchester Center, VT in early June. Hoping to keep my weight down, my summer-weight equipment plan sports a 50 deg top-quilt, a 40 deg under-quilt, a lightweight merino base layer, a goretex rain jacket, light merino glove liners, a merino buff, and a windshirt.

Thought I better make sure that's gonna be sufficient. After all, it's still gonna be spring in the mountains...

Glasgow, Va is close to the footbridge. So I wanted to see their historic temperature averages. By selecting their monthly forecast, then looking out into the future, they give us a table of historic averages and record temperatures.

"But wait!" you say. "You won't be hiking at the same elevation as Glasgow. It's colder up in them mountains. How're you gonna account for that?"

That's where "Lapse Rate" comes in. The Glossary of Meteorology defines Lapse Rate as "the decrease of an atmospheric variable with height. The variable being temperature unless otherwise specified." It varies with humidity. At average humidity, and at elevations below 39,000', it's 3.5 degrees per 1000' of elevation gain or loss - not accounting for shade, wind, clouds ...

Consulting the google, I find Glasgow is at an elevation of 738'. For my first 240 miles, I'll be hiking no higher than 4000', with several potential shelters and campsites around 3500'. The difference between Glasgow's elevation and 3500' rounds up to  2800'. So, 2.8 thousand, times 3.5 degrees per one thousand feet = 9.7 degrees.

Call it an even 10.

So, subtracting 10 degrees from the numbers in the table above, it looks like average lows from 45-49. Easily within the range of the gear I've selected. But what about those historic lows? Looks like the lowest low could be as low as 25.

Yeesh ...

So I'll be watching the forecast in the days before I leave, and may slip a down vest and maybe even a warmer sleeping bag into my pack.

Or maybe I'll just drop down 3500" on cold nights ...

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