I sat on the ridge high above the Hoh River at High Divide looking across the gorgeous valley to Mount Olympus. This was my reward after a very steep hike. I slowly ate my lunch while enjoying the afternoon clouds. I glanced at my altimeter, which told me that I was climbing fast despite having been sitting on a rock for half an hour. I jumped up and hiked quickly back down to the lake basin. I got back to camp in time: I was in the tent when the downpour started.
Why did I dash for camp? My altimeter had given me a clue that a storm was rolling in. Altimeters use barometric pressure - the weight of the column of air above you, pressing down on you - to calculate your elevation; or more specifically, they use the CHANGE in pressure. Weather forecasters also use the change in barometric pressure. Rising pressure means clear weather ahead, falling pressure means storms. (The lowest pressures ever measured are in the eyes of hurricanes!)
My altimeter was unaware I had stopped hiking. Since I knew I hadn't actually gained any elevation over my lunch break, I judged that the weather was changing instead -- a storm was coming up the Hoh Valley.
In a good old tube barometer, barometric pressure is measured in millimeters of mercury. Normal sea level is 760mm, meaning that the weight of the air at sea level can lift mercury, which is pretty heavy, 76 centimeters (it can lift water 10.3 meters!). Other familiar sea level measures of barometric pressure are: 29.92 inches of mercury, 14.696 Pounds Per Square Inch and 1013.25 millibars; all equal to 760mm.
Have you ever drained a soft plastic water bottle on a peak, and once you got down to the trail head, hundreds or thousands of feet below, the water bottle looked like it had been crushed by a giant's hand? It is actually crushed by the air outside the bottle. The pressure inside is from elevation.. That's because the air pressure is lower at higher elevations, that column of air above you is not as thick. That's why it is harder to breathe on a high mountain!
For the altimeter to work correctly, you have to tell your altimeter where you are starting. Altimeters detect the change in the air pressure and calculate your elevation change based on a KNOWN SETTING, called the calibration. Before leaving home I calibrate mine to my home on Capitol Hill which I know is at 322 feet. I often calibrate it again at the trailhead or elsewhere when there is a sign indicating the elevation. The more often you calibrate the more accurate the reading is.
In an altimeter an altitude reading works AGAINST the barometer reading. That is, when there is a change in the pressure it is EITHER a change in the elevation of the unit, or a change in the weather. Modern altimeters tend to be very good at guessing if a change in pressure is weather, or elevation, but it is still just a guess. So you have to think about it when you look at the readout. For example, I woke up one morning during an extremely rainy night in the North Cascades and discovered that, according to my altimeter, my tent had gained 280 feet while I slept. I was pretty sure my tent had not been climbing by itself, usually I have to carry it.
Modern GPS units have an elevation component which works by using satellite timing rather than air pressure. They are not as accurate as barometric altimeters, but they are getting better. You can check one against the other. The GPS elevation is not susceptible to changes in the weather but it is less accurate than an altimeter using barometric pressure.
An altimeter can be pretty handy thing to have in the in the backcountry. I use the altimeter when reading a map to get an idea of which topo line i am actually standing on; it's like having a third dimension for the map! Hikers (and cyclists) use them to gauge the amount of climbing on a day's hike or ride, and therefore the difficulty. Skiers use altimeters to determine how far they have descended on a run, and even to count how many runs they've made. I've even used it to report the location of an injury when a party i encountered needed a rescue.
Just for fun, here are some calibration points around Washington for you: (all are in feet above sea level)
- 33 Lake Samamish State Park (boat ramp)
- 39 Puyallup Fair Grounds
- 112 Tatoosh Island (lighthouse base)
- 117 Washington State Capitol (north door)
- 117 Pike Place Market (Rachel the Pig)
- 272 Rainbow Falls State Park
- 481 Hanford (main building)
- 482 Salish Lodge
- 520 Space Needle (observation deck)
- 569 Maryhill (Stonehenge)
- 1,047 Pot Holes Reservoir
- 1,310 Grand Coulee Dam (top)
- 1,768 Winthrop (main street)
- 2,368 Mt Constitution (lookout)
- 4,314 Johnston Ridge Visitor's Center (Mt St Helens)
- 4,551 Upper Lena Lake
- 4,790 Rainy Lake (SR 20)
- 5,858 Mt Spokane Lookout
- 6,409 Sunrise Ranger Station
- 14,410 Columbia Crest (Mt Rainier)
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