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post #1 of Old 03-27-2002 Thread Starter
Michael Carr
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Weather for 'Round-the-World Sailing

Navigator Mark Rudiger (right) and tactician Chris Larson of ASSA ABLOY, assessed the weather correctly on Leg Five. Above they survey the competition in the Volvo Ocean Race somewhere in the Caribbean.

As we move into spring, more and more attention is being focused on the Volvo Ocean Race, which recently finished in Miami, FL this week and resumes on April 14 with Leg Six to Baltimore, MD. Weather in this and other around-the-world races—both for the participants and for those who follow the action closely—is a critical subject and the source of nearly endless discussion. Of course every distance-racing sailor and marine meteorologist has a favorite way of tracking down the best information regarding wind and sea conditions for ocean racing, and it's often the individual approach that defines success in this competitive arena.

A common thread in each approach is locating and tracking temperature differences and changes in the atmosphere. Regions of temperature differences represent regions of wind. But too much wind can be as frustrating as too little wind. With too much wind come large and often breaking seas that affect a vessel's overall stability. Big winds and seas will lose a race just as easily as no wind and flat seas.

This 500-mb chart shows the greatest wind activity—the storm-track line—just between the blue and red lines.

But round the world racing covers a lot of geography, so, how do you find the perfect conditions over such a wide span of area? Most meteorologists start with a global, 500-mb upper air chart, like the one above. The chart shows lines that parallel the wind flow at the lower levels of the jet stream, about four miles above the sea's surface. It is these winds that both control and support surface weather, moving hot and cold air, which produce the surface winds. Where the lines are the closest or tightest is where wind flow is strongest aloft, which also translates to the areas of strongest wind flow on the surface.

Between where the lines turn from red to blue there is a narrow yellow line on this 500-mb chart. This line, often referred to as the storm-track line, always represents the area where the most energy aloft is concentrated. It can be compared to the 50 yard line on a football field, the area where most of the action takes place. Continuing this analogy, end zones can be thought of as the north and south poles, where temperature, or players, are homogenous and so relatively little activity takes place.

Note where the yellow line lies in the northern hemisphere, between 40N and 60N, while in the southern hemisphere the lines lies between 20S and 40S. Why the difference? Because this image depicts summer in the northern hemisphere and so there is more heat than cold. This pushes the confrontation between hot and cold into more northerly latitudes. In the southern hemisphere it is winter time in this image so there is more cold than heat and the confrontation line moves into the lower latitudes.

By using this surface-pressure chart, the author can verify that the high and low-pressure systems line up with the upper level ridges and troughs.

The undulations you see in the upper-level flow are called troughs and ridges. Troughs are equatorial dips and ridges are poleward extensions of the upper airflow. Upper-air ridges support surface high pressure and upper air troughs support surface troughs. So, now that you know this, look at the surface pressure chart and you will see that surface high pressure and surface low pressure line up with the east sides of upper level ridges and troughs. Surface pressure systems always precede upper-level features because upper-level flow pushes development along, just as snowplow pushes the snow it accumulates in its plow.

Now go to the surface-wind flow chart and you can associate the counterclockwise surface winds with low-pressure areas and clockwise winds with high pressure, in the northern hemisphere (and just the opposite in the southern hemisphere). We have a direct connection between upper-level troughs and ridges, surface highs and lows, and surface wind flow.

Surface Wind Flow—Though it's a bit difficult to make out on this scale, this chart indicates surface wind flow for the same time and date as the above charts. which indicates the respective wind strength and direction for the highs and lows.

Now keep going with this theme and look at the primary swell chart. This shows seas produced by significant wind fields, winds which blow across from a consistent direction, at a consistent force, for a duration of days and often weeks. Winds, which are cold, produce the highest seas since the air is heavy and sinks, downward, to the seas surface where its angle of attack is most efficient for producing large and consistent seas.

Adding to this combination of forces is water temperature, as seen on the sea-surface temperature chart (see below). Where water is cold and then has warm moist air move over it, fog and precipitation form. When temperature differences between water and air reaches 20 degrees C and greater strong convective conditions occur, meaning winds are strong and gusty as energy is exchanged rapidly. Note on the sea surface chart location of the Gulf Stream and Kuroshio Current. These regions always experience strong thunderstorms, fog, rough seas and often rapidly changing weather due to the massive amounts of energy moved and exchanged over relatively short geographical regions.

The primary swell chart indicates the running direction of seas created by significant wind fields.

So to win, or place well in an around-the-world race, you must first find the regions of energy, i.e. locate and track the 500-mb storm track line. Then compare the shortest physical route, i.e. a great circle, to this line and determine the best combination of shortest route and best winds.

Navigators in the Volvo Ocean Race will be checking their route regularly by examining surface wind and pressure charts so they can track the weather tendencies and patterns. They'll be asking themselves, are pressures increasing or decreasing? Are the upper level isoheights converging or diverging? Tendencies and patterns tell them as much about the weather as absolute values. It is best to visualize flow patterns, especially the movement of upper-level troughs and ridges.

A look at sea-surface temperature charts like this one tell navigators where the greatest temperature differentials exist, and thus the most likely wind activity.

An ongoing onboard mantra aboard every VOR 60 will likely be "where is the pressure gradient?" When you stay with pressure you stay with wind. But like these racers, every sailor should know his or her (and the boat's) limits so as not to become greedy and by taking too much pressure gradient and end up sailing into large and threatening seas. Steady and consistent movement and anticipation of changes in the upper level flow pattern is the way to win an around-the-world race.

These global weather displays are provided by the ORION weather and routing software system, which is a product of Ocean Routes Inc., a global weather and routing company.

Suggested Reading:

Understanding Weather as Global Interaction by Michael Carr

Surface Weather Overview by Michael Carr

Performance Basics for Routing by Michael Carr


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