Wednesday, June 9, 2010

Waves


It was our second attempt to cross the Columbia River bar in three days. Rough weather had kept us tied up in Astoria, Oregon for several days but it looked like we might just have a long enough weather window to make the passage up to Cape Flattery before the next front came in. From the west came an eight-foot swell that first built, then broke up with a crash as it came into the shallower waters of the bar. Meanwhile, a strong wind from the southwest pounded the hull with four- to six-foot waves in quick succession, rattling the ship and making even experienced mariners a little seasick. As we clawed our way out over the bar toward deeper -- and we hoped calmer water -- Chief Mate Lostie turned to me and said, "I'll bet this all looks very picturesque from that hill over there."

Waves come in several categories. Most common are wind waves that start to form when the wind on the water blows as little as two knots. These small waves, called ripples or wavelets, are caused by the friction between air and water breaking the water's surface tension. If the wind dies down, the ripples disappear. Once the wind gets above two knots, the waves become more stable, although they are still driven before the wind. If the wind is strong enough and lasts long enough, the waves are no longer reliant on the wind at all and can continue to travel long after the weather that produced them is gone. These waves are called swells they can travel for hundreds of miles.

Many factors effect the size of wind waves: the speed of the wind, the distance the wind has blown over the water (called fetch), and the depth of the water. Although these waves can theoretically reach as high as 200 feet, waves more than 5o feet high are rare. The highest scientifically measured waves on record -- 95 to 98 feet for more than twelve hours -- were observed by the British research vessel Discovery in the North Atlantic in 2000 (mariners are notoriously inaccurate at estimating wave height by eyeball alone. On one north Pacific passage, the vessel I was on consistently recorded a northwesterly swell height of six to eight feet. A friend of mine on a smaller vessel recorded the same swells as 10 feet or more).

Another characteristic of waves is called period, the time it takes the crests of two consecutive waves to pass a fixed point. Waves with a short period are sometimes called "steep" and can make for an uncomfortable ride with even a relatively short wave height. On the other hand, even a large swell with a long enough period can be safe and comfortable.

Forecasts. In weather forecasts, wind wave heights are sometimes referred to as seas. Some forecasts will give separate heights for seas and swell, other will refer to combined seas. Wave height predictions can be deceptive: a forecasted height of 10-ft seas does not mean every wave will be ten feet high. Instead, forecasters take the average height of the highest third of the waves expected and use that in their public forecasts. Thus, most waves in a given forecast time and area will actually be smaller than the reported height, but about twenty percent will be higher, some nearly twice as high.

Other waves. Not all waves are wind waves. Seismic activity can produce a tsunami, or tidal wave, that reaches speeds of 400 knots. The highest tsunami ever recorded was in Lituya Bay, Alaska in 1958; it measured more than 1700 feet high. Another type of wave is a storm surge, which occurs in relatively shallow water (like the Gulf of Mexico) during the low barometric pressure preceding a storm. A third type of non-wind wave is the tidal bore, caused the force of the tide pushing against prevailing local current or flows in a narrow channel.

2 comments:

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