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Surface ridges

Jun 29, 2018

I recently spent a few days in Newport, R.I., briefing clients on weather and strategy for the Newport to Bermuda sailboat race. This race is held every other year, and two years ago in 2016, in the days leading up to the race it appeared that conditions would be quite rough along the route with possible gale-force winds during the Gulf Stream crossing and perhaps for a couple of days afterward. In fact, for some racers, the forecast conditions were strong enough to lead them to withdraw from the race.

This year, the opposite situation prevailed, and participants were looking at the prospects of a very light wind race with some periods of calm winds possible. The reason for this was a rather broad ridge of high pressure that was likely to be present over the race course for several days after the race start. In explaining the weather situation to my clients, several asked questions that amounted to “What is a ridge, anyway?”

Since this is a feature that is often present in the Atlantic during the warm season, I thought it would be of interest for many other ocean voyagers, thus I will dedicate this newsletter to explaining the structure of surface ridges and the winds that they produce.

Here is the definition of a ridge from the Glossary of Meteorology, which is published by the American Meteorological Society: An elongated area of relatively high atmospheric pressure almost always associated with and most clearly identified as an area of maximum anticyclonic wind flow. The locus of the maximum curvature is called the ridge line (or ridge axis).

Figure 1: Eastern Atlantic surface analysis chart valid at 1200 UTC 15 Jun 2018.

The definition is a bit obtuse, so let’s start with something a bit simpler. Those who look at surface weather charts regularly are familiar with the idea that high pressure centers in the Northern Hemisphere feature a clockwise circulation of air around them produced by the combination of the pressure gradient force acting from high toward low pressure — or outward from the center of the high — and the Coriolis force, which deflects large-scale motions of air to the right. Due to friction with the surface, the air flow around a high is not exactly parallel to the isobars defining the high, but rather flows a bit across the isobars from high toward low pressure, or outward from the center of the high. The textbook depictions of high pressure systems almost always show nearly circular isobars surrounding the high, and this makes the clockwise and slightly outward circulation easy to visualize.

Often, however, high pressure systems are not perfectly circular, and in many cases there is an elongation of the circulation extending in one or more directions away from the high center. Figure 1 shows such a high in the eastern Atlantic west-southwest of the Azores. The position of the high center is obvious, and there are several isobars that enclose the center. Wind barbs around the high are generally consistent with a clockwise circulation. There are two regions of the high where the isobars are elongated away from the high center, one to the east-northeast, and another to the west-southwest. I have added brown zigzag lines indicating the ridge lines (or ridge axes) extending away from the high center. These zigzag lines are not present on most surface pressure analysis charts, and are only occasionally seen on 24-hour surface forecast charts (example shown in Figure 2). This leaves the task of locating any ridge axes to the mariner.

Figure 2: Western Atlantic 24-hour surface forecast chart valid at 0000 UTC 11 Jun 2018.

To help in locating the ridge axis, we go back to the definition, which noted that the area of maximum curvature defines the ridge line. The curvature must be anticyclonic for the feature to be a ridge. In other words, the curvature must indicate that the flow is curved as though it is flowing around high pressure. This means when examining the wind flow, if the segment of curved flow is clockwise in nature, then it indicates a ridge. Also, when crossing a ridge axis in a perpendicular fashion, the pressure will be highest along the ridge axis and lower on either side. Looking at Figure 1 or Figure 2, it should be evident that the indicated ridge axes cross the isobars where they have the most curvature, and also by using the labels on the isobars that the pressure will be highest along the axis of each ridge when moving across the axis at right angles.

So why are sailors interested in ridges? It’s because of the winds associated with them, and in some cases, the lack of winds. Because of the sharper curvature of isobars associated with the ridge, the wind direction will shift rather quickly across the ridge axis, much more quickly than in other portions of the circulation of the high. Also, because the isobars are usually farther apart in the ridge axis due to the elongation of the high, wind speeds near the ridge axis are usually lighter.

Figure 3: Western Atlantic surface analysis chart valid at 1800 UTC 18 Jun 2018.

In some cases, a ridge axis may exist without a well-defined high pressure center, and will represent an elongated region of higher pressure. These features are typically weaker than ridges associated with high centers, and therefore produce a larger area of very light winds. The wind direction on either side of weaker ridge axes like this are typically about 180 degrees apart. Such ridges are often more difficult to locate on a surface pressure chart but will still have an anticyclonic wind shift around their peripheries and will also represent higher pressure than areas surrounding them. It was a situation like this that confronted the Newport-Bermuda racers in mid-June of this year.

Figure 3 is a surface pressure chart of the western Atlantic during this year’s Newport-Bermuda race. In this case, while a weak high center is indicated on the chart to the northeast of Bermuda, isobars do not fully enclose the indicated high center. A zigzag line has been added along the ridge axis, and the pressure along this line is higher than areas to its north and south. The curvature of the 1020-mb isobar is sharpest along the Georgia coast, and these factors have been used to locate the ridge axis. Winds were very light near the ridge axis, and the wind direction to its south was generally from the east-northeast, and to its north from the southwest. This can be seen more easily by examining ASCAT wind data observed by satellite (shown in Figure 4), taken at about the same time as the valid time of the chart in Figure 3.

Figure 4: ASCAT wind data observed at about 1430 UTC 18 Jun 2018 for a portion of the western Atlantic (note Cape May, N.J., at the upper left of the image, and Bermuda near the lower right).

This pattern made it difficult for sailboat racers attempting to get to Bermuda, as the region of very light wind was fairly significant and this meant a rather slow race to Bermuda for most boats. Of course for those cruising on a passage to Bermuda, it would mean that the engine would have been kicked on to propel the boat through the ridge and into better wind. Knowing the location and orientation of ridges is important in planning for cruisers, particularly those with limited fuel capacity. For power voyagers, ridges can represent a good place to be with light winds often accompanied by quiet seas, which makes for excellent conditions for that type of voyaging.

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