Pockets of Weather
By Elizabeth Rusch
Published in Backpacker magazine, August 1999
The forecast when my husband and I set out for our first backpack of the early spring was remarkable: sunny, dry, and 85 degrees. So why on our first leisurely morning in the Gifford Pinchot National Forest were we still shivering under lumpy layers of long underwear, turtlenecks, and polar fleece?
As we warmed ourselves with hot chocolate, we discounted the altitude. Camping near a feeder to the great Columbia River, we couldn’t have been more than 500 feet above sea level. Perhaps the forecast had changed...
It wasn’t until we set out, fully bundled for a day hike, that we discovered our folly. A mere 100 feet uphill from our tent, it was warm. Toasty, in fact. We peeled off layers and headed back to our campsite to change into shorts. As if entering an ice box, we were blasted by cold air.
The problem: we had set up camp on a flat, shady hollow at the base of a steep canyon, near a creek which carried snow melt from Mt. Adams. "Sounds like a classic example of a chilly microclimate caused by a katabatic flow," says Dennis Thomson, professor of meteorology at Pennsylvania State University. "Cold, denser air draining down the canyon at night probably settled in the hollow where you pitched your tent."
Ignoring microclimates –anomalies in temperature, humidity, and wind measured in inches and feet rather than miles -- can mean reaching for a water bottle in the morning only to find it frozen solid, getting blasted out of slumber by early morning heat, or having your tent blown off a hillside. Here's how to make these localized variations in the climate work for –rather than against –you.
Wanted: Cheap Chill
A frosty hollow, like the one we shivered in, would be welcome relief after a scorching day. Pitch your tent in a depression in the terrain or at the base of a long hillside. Pick a north-facing slope, which receives less afternoon sun. Point your tent window or door up-slope to catch the falling air.
Make camp down-slope of a forest edge, since chilly air seeps out of forests, suggests climatologist Timothy Oke, professor of geography at the University of British Columbia. Trees, by casting shadows and emitting water vapor, cool the air in the woods.
"But if you want a real rush of microclimate, lie in the middle of a field at midnight," says Oke. "The vast open sky sucks up the day's heat, leaving you cold." Stay cool after dawn by avoiding the east-facing side of your clearing. Lying on a shaded rock or compact earth -- both good heat conductors -- may give you the chill you seek.
Woe is the backpacker who makes camp in the lowest ground on a cold, windless day. Move up into a hillside's thermal belt, above where cold air pools at night but below where greater elevation causes temperature to drop, usually 600 feet or so above a valley floor. "It'll feel warmer even 50 or 100 feet up," says Oke.
At night, camping under a forest canopy rather than in an exposed field will likely be warmer because trees cut down heat loss and wind chill. Avoid the forest's perimeter and the cool air that streams out.
If you are stuck in an open area, remember that the southwest side of virtually any rock or tree catches more afternoon rays than other sides. Nestle up to the sunny side to catch the radiation. Bed down on nature's insulation, such as dry leaves or grass.
Dodge The Drizzle
Not much you can do in an drenching downpour, though less rain falls on the leeward side of a hill, says Jeff Renner, meteorologist at Seattle's KING-TV and author of Northwest Mountain Weather. "When moist air is pushed up the windward slope, it cools and drops more rain," he explains. "As the air plunges down the leeward side, it warms and releases less precipitation."
Trees give excellent shelter in light, brief rain. Smooth-barked species, like beech, channel up to half of the rain down their trunks. Rough-barked evergreens disperse drips to the outer edges of their crown. Place your tent in the dry zone, between the crown and trunk. When the rain stops, move to open ground, which dries faster.
Block The Breeze
Flag trees, with all branches pointing downwind, testify to whipping wind. They are often seen on ridgetops, where wind can howl. Pitching your tent behind a rock might not help, Oke warns. "When wind hits a solid object, the air flow is squeezed, strengthening it downwind," he explains.
Better to camp behind something porous, such as a clump or line of trees or shrubs –the broader the better. Indeed, strong wind loses its force in a forest, slowed by friction created by trunks, branches, and leaves. "It can be screaming above an old growth forest and almost silent inside," says Thomson.
Don't make camp in a break of a row of trees or rocks, which can channel the wind right at you. And if you're stuck in a canyon, settle in its widest part.
A breeze can keep biting bugs from landing on your tender skin. Seek open sites and height. "Avoid bottom lands, where air comes to a halt," says Oke. In a canyon, air moves fastest at the narrowest spot and where the sides are steep and long. "Cold air falls like a sled," says Renner. "The longer and steeper the slope, the more velocity it picks up."
While standing water breeds bugs, larger bodies of water breed winds that drive bugs away, says Renner. Differences in the heating and cooling rates of water and earth generate breezes on the shorelines of rivers and lakes, he explains. At 5 to 15 miles per hour, breezes off the water in the day and off the land at night can hinder a mosquito's beeline for your blood.
Watch The Weather
You won't find microclimates everywhere and always. Terrain influences the menu of microclimates. "The more diverse the landscape -- in terms of hills and valleys -- and the more variations in vegetation and soil conditions, the richer selection of microclimates you'll find," says Oke.
And microclimates exist at the whim of the weather, thriving best under stable conditions with clear skies and little wind. Clouds dull the sun's power to create dramatic temperature differences; weather fronts stir the air, obliterating subtle microclimates. "When a storm's on the way, microclimates will be the least of your worries," says Thomson.