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Study shows wind to be more variable than steady

Jan 1, 2003

We've all read about those passages with nothing but weeks of wind aft of the beam and no need to adjust a sheet. No doubt they happen, and when they do, they are also doubtlessly noteworthy. I'm still waiting for one of those myself. In fairness, I guess I should admit that the wind can be consistent when it is strong and my destination is upwind.

Over the last couple of years I have been distributing weather observation forms to cruising yachts, and I've gathered enough data to present a short summary of findings.

I summarized the logs by first pulling out the wind observations and listing them. Then each observation is compared with the next one. The idea is that if wind speed is the same, then one probably doesn't need to change sails. If wind direction is the same, one may need a sail change, but not to alter course. If both are the same, that is the "classic" tradewind passage, at least for as long as it lasts.

Heart of Gold, a Schumacher 50, on a passage between Mopelia and Rarotonga in July 1993, however, showed no constant wind direction over a 72-hour period. It backed or veered constantly between south and west, with excursions through north. Wind speed was constant less than half the time. In this data, wind speed varies continually.

Looking at all of the passage records in which I have compiled for the tropics, watch-to-watch wind speed stayed the same within one Beaufort force factor, or about five knots, 35% of the time; direction, 68% of the time; and both 18% of the time.

This compares with a winter passage back to San Francisco from Hawaii on which there were 14 occasions in 25 days (about one half of the observations) which had wind direction consistent between two or more observations. Variation on one watch of 20 or more degrees occurred in about 20 percent of the observations. The wind speed was equal between observations on seven occasions, comprising about 30% of the data.

The greatest wind speed variation between observations was three Beaufort force factors, but it also varied that much during one observation period 20 percent of the time, and two of the occasions of equal wind speed were force eight. In other words, the variables are more variable.

But on a coastal California passage in summer 1993, with observations every four hours, wind direction was constant in 50% of the observations. The most common direction was NW, 33% of the time. Wind direction and speed were constant in 37% of observations. Richard Henry Dana in Two Years Before the Mast did characterize the northwesterlies of California as a form of tradewind. I suppose that there are rare occasions when one could make a good tradewinds passage without touching a sheet. Normally, though, the passage would be pretty slow because one would be wandering off course a lot of the time, and to make up for the heavy stuff, one would be reefed even when it was light. I knew some people who did a 33-day passage from Moorea to Hawaii that way.

All of this can be put into perspective by looking at some data from the Atlas buoy chain. Atlas buoys were deployed in an array along the equator as a part of the effort to measure El Niño, and to formulate climatic models for tropical and global climate. The Tropical Ocean/Global Atmosphere (TOGA) project is a 10-year international effort started in 1985. TOGA Notes is published quarterly and contains articles with titles like "Assimilation of Sea Level Data into a Primitive Equation Model of the Tropical Pacific."

In one of the more basic pieces, average buoy-observed wind speed for 1987-88 for the eastern tropical Pacific was approximately 11 knots, the bottom of Beaufort force 4, with a standard deviation of about three knots. In other words, two-thirds of the observations are within range of 5 to 17 knots, force 2 to force 5. These data are consistent with the yacht observations. For that matter, the October Pilot Chart of the South Pacific shows a wind rose at two degrees north and 132 degrees west that indicates no calms, and wind from the southeast 59% of the time, and from the east 34% of the time.

For the purpose of navigation by sail, the time scale of variation is more interesting and important than the simple average. The bad news is that unless one is either lucky or very slow, the chances are excellent that one will be changing sail constantly, even in the Trade Winds. The good news is that there are fewer gales and fewer radical changes in wind direction. Those changes in wind speed and direction that one can expect will be about half as frequent as those in the variable wind belt.

Charles Warren lives in San Francisco and has voyaged in the Pacific.


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