Wind Bends

Accepted Wisdom

The following are direct quotes from well-known yachtsmen, coaches or advisors.

 What does this tell us? Since some of these views are contradictory one implication is that at least some leading sailing experts are wrong in their understanding of wind at coastlines. Another conclusion is that winds at coastlines are very important for yachtsmen, even if they can succeed at the highest level with some serious misconceptions.  

The theory of how wind is likely to respond to passing the coastline is quite subtle. However meteorology is an observational science and over the decades meteorologists have accumulated a wealth of measurements showing what happens in practice. So what does it tell us about the offshore winds the yachting celebrities were talking about?  

In 1970 a colleague, Peter Francis, examined 10 years of offshore wind data on the East Coast;   a database of about 3000 observations. At first glance the change in direction at the coast showed wide variations and there was certainly no consistent change to justify any of the statements above.

When he separated out data into occasions when the sea was noticeably warmer or cooler than the land he did find something significant. Over the 738 occasions when the sea was cooler than the land,  there was a backing of about 6 degrees on average. That is, the wind did not conform with the expectations identified in the table opposite. So, what is happening?

The reason is that a cooler surface creates what is known as an "internal boundary layer" in which cool, dense air stays near the surface and tends to back to run more closely towards low pressure - ie across the isobars. To understand this, remember that at the top of the boundary layer, the Coriolis effect will cause air to move nearly along the isobars - the Gradient Wind. The slower moving air near the surface is less deflected by Coriolis and, so, moves more towards low pressure. This can be seen on a clear night with light winds. Surface winds may blow nearly at right angles to the isobars.

On the other hand if the sea is warmer than the land,  it will generate convection, thus mixing the air near the surface with faster moving air from higher up. Air near the surface will then be moving faster than it was when over the land. Because the air is faster moving, it will be deflected more by the Coriolis effect. This results in air near the surface being deflected more along the isobars.

Sailors will be familiar with these ideas on the larger scale. In the warm sector of a depression warm air from the SW, coming over a cooler sea, will often blow at a quite marked angle across the isobars towards low pressure. In a cold NW airstreams, winds will blow more nearly along the isobars than those in the warm sector.

Clearly the view that, in the northern hemisphere, "...the wind must veer as it moves out over the water" is not a tenable statement. As so often simplistic explanations in meteorology can be misleading.

East Coast Wind Bends - some specific examples

This diagram shows data from a pair of E Coast stations,  for the year 1987. Hemsby, is a coastal site manned by Meteorological office staff. Smiths Knoll light vessel is a few kilometres offshore. There were 42 occasions when the wind was offshore with the isobars reasonably straight and no fronts or similar complications in the region.

The plot shows how the change in temperature is related to the change in direction between the two sites as the wind moves offshore (the horizontal axis on the diagram) . "Goes right" means those occasions when the wind change from land to sea accords with accepted wisdom. "Goes wrong" means those occasions when it does not.

The plot looks confused, which is, itself, a lesson for anyone who thinks meteorology is an exact science. However  a rough trend can be seen. Those occasions when the air is being cooled by the sea tend to be associated with winds backing, consistent with the results of Peter Francis. The points marked with circles were recorded in September and October. For these points,  the data show a slight trend for the winds to veer offshore relative to the directions over land. This is tabulated in the table opposite.

The implication is that for much of the season the wind is as least as likely to bend to the left, the opposite of what the RYA advises. Presumably the reason is because for much of the year the sea is cooler than the air. In September and October the sea is at its warmest;  in 1987 sea temperatures in these months were 13 - 14 degrees.

The data shown here are only a small fraction of a huge amount of good quality data published in meteorological journals relevant to this subject. Although there are exceptions and  there can be no hard and fast rules, it is quite clear that the accepted wisdom is misleading. It would seem that occasions when the offshore wind veers as it moves over the sea are, at best, more likely only when the sea is warmer than the land. That will be more generally when there is a cold NW'ly airstream than in a warm SW'ly.

Cliffs, Headlands and Straits

Winds blowing slantwise onto a steep coast line are deflected along the coast with an increase in the wind.  è

Winds coming over cliffs splash down, often with considerable strength. In the marina at Darthaven in a strong westerly wind coming over Dartmouth, we seemed to be in a very windy spot with this effect   Near the cliffs there may be a back eddy.  ê

Winds blowing through straits are funnelled causing acceleration  ê

There are, of course, many variations on this theme and I make no excuse for the amateurish nature of the diagrams. I have no artistic flair at all!

However, the effects shown are all ones that will not be indicated by many forecasts and definitely not

Winds blowing direct onto steep cliffs may lift off some way off and, also create a back eddy. Yachts have been known to anchor right under the Babbacombe cliffs in an onshore wind.  ê

Complex topography has complex effects.  ê