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Paddling Articles In the Same Boat

The Restless Air

Under Pressure

By Tamia Nelson

Rain drummed against our sagging tarp, sending a sheet of water cascading down from the peak. Low, gray clouds obscured the surrounding hills. It was warm—in the low 70s—and there was barely a breath of wind. That was just as well, we thought. From the shelter of the tarp, we peered out through the rain at an Adirondack lake that looked like hammered pewter.

Farwell checked his watch. Silently, he reached into the chest pocket of his anorak and removed our elderly Thommen Everest baro-altimeter, kept safe and dry in a plastic bag. He tapped it gently on its face, then made a pencil entry in our logbook. I looked at him inquiringly, not needing to voice my question. "No change," he said. I grunted. Sweet-smelling steam from a pot of simmering pea soup hung in the air under the tarp. We sat silently, looking out at the rain-shrouded lake. A common merganser and her eight nearly full-grown chicks worked the near-shore shallows. Now and again one would bring up a fish. We gnawed on slightly soggy Vermont Country Crackers and waited for our soup to be done.

It was the fifth day of our trip. Each afternoon and evening for five consecutive days it had rained hard and long. Occasionally a thunderhead would bubble up from the low-hanging cloud and dump still more rain on us, while sudden lightning lit up the dark sky. Except for brief, cold gusts that accompanied the thundershowers, the wind—when there was any wind—was southerly and light.

And every two hours throughout all five days, whether we were in our boat, on the portage trail, or in camp, Farwell repeated the same reassuring ritual. From first light to the last thing at night, he consulted our Swiss-made oracle eight times a day, noting the position of its needle each time. And for five days the needle hardly budged.

On the morning of our last day, however, Farwell's first gentle tap on the dial sent the needle jumping higher. A northerly breeze now raised ripples on the lake. The ever-present clouds had lifted. They'd lifted, but they hadn't gotten any lighter. Ragged, dark tendrils scudded below the cloud base. We didn't waste time. We got our gear together and tossed it into our boat. In less than ten minutes we were paddling toward our take-out, helped on our way by a strengthening northwest wind.

By the time we'd reached the take-out and started to load our gear for the drive home, a squall line was moving through. Two- and three-foot-high rollers crashed against the the shore. Cold, wind-driven rain stung our skin. Thunder echoed against the hills. We were glad to get in the truck and head for home. Before our hour-and-a-half drive was over, blue sky was visible through rents in the cloud, and the temperature had dropped fifteen degrees. A final check of the barometer showed us that the pressure was still rising.

Not many paddlers bother with barometers. I got along without one for years myself, in fact. Nor is a barometer an infallible weather prophet. If you've ever read Jerome K. Jerome's Victorian classic Three Men in a Boat you may remember the scene in which the hopeful narrator relies on a hotel barometer's misleading indications—it rises steadily, from "set fair" to "very dry" to "much heat"—throughout a cold, three-day drizzle, with predictably hilarious results.

Nevertheless, I was glad we took our barometer along when we went paddling in the Adirondacks. If we hadn't brought it with us, and if we hadn't paid attention to the sudden early-morning change, we might well have persuaded ourselves to linger over breakfast, only to be caught out in mid-lake in a full-blown squall. Our barometer saved us from an "adventure" we'd just as soon have missed, in other words.

OK. A barometer won't guarantee that you'll alway be one step ahead of changes in the weather, but it can certainly improve your chances. So just what does a barometer do? That's an easy one. Barometers simply measure atmospheric pressure—they weigh the air that presses down on us, in other words. And they've been around for quite a while. The first barometer was designed by Evangelista Torricelli, a student of Galileo's and his successor to the post of mathematician to the grand duke of Florence. The earliest barometers—you'll still find their direct descendents in laboratories around the world today—were four-foot-long tubes filled with mercury. As atmospheric pressure rises and falls, the column of mercury rises and falls in concert. That's why barometric pressure is often reported in inches (or centimeters) of mercury.

But a four-foot-long tube filled with mercury wouldn't be a very welcome companion on a canoe trip, would it? Happily, the "metallic manometer" patented by E. Bourdon in 1849 made the aneroid barometer possible. Handy as a pocket watch, the aneroid uses a sealed, partially-evacuated metallic cell to measure pressure. As atmospheric pressure increases, the walls of the cell collapse. As it decreases, they rebound. The resulting tiny movements are magnified by a mechanical linkage, and a needle moves around a graduated dial, indicating the pressure. The mechanism is simple, reasonably accurate, and durable. Our Thommen Everest is just such an aneroid barometer.

Not that technology hasn't moved on since 1849. Today you can even buy watches which incorporate electronic pressure sensors. We own one of these, too—a Casio 950. It not only measures atmospheric pressure, but it also doubles as a barograph, displaying a record of pressure changes over the last twenty-four hours. If you have a watch like this, you don't even need to keep a logbook. One glance will tell you all you need to know.

Owning a barometer is one thing, but knowing how to make the best use of it is another. First, remember that cold air is heavy and warm air is light. At any given time, the planet is covered with irregularly-shaped masses of warm and cold air. And these masses are constantly in motion. They overtake one another, jostle for position, and even engage in fierce pitched battles for territory. The result is the phenomenon we call weather.

Near the surface of the earth, cold air and high pressure go together, as do warm air and low pressure . Of course, the terms "cold" and "warm" are relative. A cold air mass in summer won't often usher in snow or freezing rain. It's just colder than the neighboring air. Likewise, "high" and "low" pressure are also relative terms. A high-pressure system is just a mass of air that presses down harder—one that weighs more, in other words—than the air around it.

Most weather happens when masses of cold and warm air butt up against each other. The usual result is what we've come to call a "low" or low-pressure system. We've all seen satellite images of hurricanes or winter storms on the evening news. Photographed from orbiting satellites, these look like huge whirlpools, rotating counterclockwise around a center of low pressure. (That's in the northern hemisphere. South of the equator, low pressure systems rotate clockwise.)

Highs, on the other hand, are the mirror images of lows. In the northern hemisphere, the winds in a high blow clockwise around the high pressure at the center. You can plot highs and lows on a map. When you connect all the points of equal pressure, you end up with something that looks a lot like a topographic quadrangle. You'll see peaks (highs) and depressions (lows), valleys (troughs), and ranges of hills (ridges). Unlike topographic maps, however, weather maps are never content to stay as they are. The peaks and valleys are forever on the move, their shape changing with every hour.

It's a pretty complex picture, isn't it? How do you put it all together? Let's look back at our rainy Adirondack canoe trip.

When Farwell read the barometer and wrote down the pressure every two hours, he wasn't just passing the time. He was looking for change. He didn't just write down the reading, in fact—he plotted it on a graph. That's one of the reasons we keep our trip logs in engineers' field books. Not only do their orange covers make them almost impossible to lose, but every other page is divided into squares, like the graph paper you probably used at school. These squared pages are ideal for plotting barometric pressures—or for sketching an unknown mushroom to scale, to be looked up in the guide-book when you get home. There's an added bonus. The paper itself is resin-impregnated. You can write on it without tearing even after a week of almost continuous rain. I'm always surprised that so few paddlers use these books.

Why bother graphing pressures? Because, while it's always good to know what the pressure is now, it's the trend in pressure that's of most importance to paddlers. On our Adirondack trip, the barometric pressure was more or less steady. The needle of our Thommen Everest had been hovering around one point on the dial for days. When it suddenly jumped up, we realized that something was brewing. In our case, this "something" was the passage of a long-delayed, stalled cold front. We knew that this would ultimately mean the rain would stop and the skies would clear. First, though, we suspected we'd get a stiff northerly wind, with a good chance of a line squall or two to liven things up out on the water. How did we predict this? Just by knowing a few basic principles.

In the northeastern United States, a rising barometer typically signals the passage of a trailing cold front associated with a low-pressure system. The wind—which has usually been blowing out of the south or southwest for some time—now veers round to the northwest or north, bringing in drier, cooler air.

But our cold front was moving into a well-entrenched mass of warm, moist air, and our barometer showed that it was really muscling in. The result was predictable. The heavy, cold air behind the front kicked the warm air upstairs, triggering the line of squalls that swept the lake just as we were loading our truck. Once the cold front was well beyond us, however, the cooler, drier air behind it provided welcome relief from the week-long rain and muggy weather.

But suppose that your graph shows that the barometer is falling, not rising? What does that mean? In general, if the pressure trends gradually and steadily down, you can expect clouds to move in, eventually bringing rain (in the summer). The wind will back round to the south, and both humidity and temperature will rise. After a longer or shorter period of more-or-less steady pressure, the system will pass on eastward and the barometer will begin to rebound, ushering in—you guessed it—cooler and drier air. A sudden, sharp drop in pressure, on the other hand, may well signal the approach of a deep low-pressure system, bringing strong winds and heavy rain with it. So, if your pressure graph starts looking like the wall of the Grand Canyon, you'll be smart to look around for a good place to weather a blow. In all probability, you're about to have a run-in with a storm that you'll long remember. When barometric pressure drops quickly, you can expect a short, sharp shock—stiff southerly winds, sudden downpours, and then a quick wind shift to the west and north.

On the other hand, slow and gentle changes in pressure, whether they're up or down, usually mean that you'll experience relatively benign weather, though you may have prolonged rain. But don't stop plotting pressures. A sudden change at any time—particular a sudden drop in pressure—should put you on your guard. Keep your head down!

Back to our canoe trip again. The barometer was steady. The pressure was low, but it wasn't very low. We were between weather systems, in other words—wallowing in an extended shallow trough. The southerly winds associated with this trough pumped moist air up from the southern states. The result? Persistent low cloud and light but steady rain, augmented by occasional "instability" thundershowers in the warm afternoons.

The barometer warned us what to expect before we left on our trip. Atmospheric pressure had been falling gradually for more than a day. The early high clouds had thickened and lowered. By the time we got to our put-in, the barometer had leveled out and low-lying cloud blanketed the sky. As we put our canoe in the water at the start of the trip, we knew we were in for a wet time. An old mariner's jingle goes, "Long foretold, long last...." But we were ready.

And, despite the rain, it was good week. We were alone in a corner of the Adirondacks that's usually busier than a city park. Well, no, that's not entirely true. We weren't completely alone. We had the mergansers for company, after all. And very good company they were, too. You know what they say about water running off a duck's back, don't you? The mergansers were in their element, and they were having the time of their lives. So were we. Our barometer helped see to that.

Copyright 2000 by Verloren Hoop Productions. All rights reserved.

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