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Old 04-06-2001
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Understanding High-Pressure Systems


Sailing in light air demands more concentration, but by monitoring larger-scale weather features, a crew can usually avoid the most fickle winds.
The maxi-catamaran Club Med attempted to set a record from NY to the Lizard off the UK in the summer of 2000. On its first day out, winds were very light and the boat’s speed was slow, even coming to a stop at times. An onboard report stated, "The crew is having to apply all their concentration on keeping the boat going. It's good training for The Race where we will also have to cope with areas of high pressure."

High pressure, with its lack of wind, is not the preferred condition for a multihull designed to go fast! All sailors need to understand and cope with areas of high pressure" because under a high-pressure system there is a conspicuous lack of wind. I am not sure there is any good training methodology for dealing with lack of wind on a sailboat.

High-pressure areas exist where air descending from the upper atmosphere meets the earth’s surface and moves outward. High pressure can bring warm or cold temperatures depending where they form, but they always bring clear weather because of their low humidity, which minimizes cloud formation.

Sailors view high-pressure systems warily since they bring light winds or calms. There is, however, a band of dependable wind near a high’s perimeter. These dependable winds are found within a pressure gradient indicated on the surface by tight surface isobar lines as well as flow pattern at the upper air 500-mb level. Visualize a pebble dropped in a pond and the resulting waves propagating outward. There is calm water in the center where the pebble fell and a band of ripples emanating outward. These ripples are the equivalent of winds moving outward around high pressure.


The isobars following this high are close together, indicating that consistent and well developed winds will fill in once the high continues moving offshore.
There is always a strong gradient near a high’s perimeter because as air moves out from a high’s center its motion changes from convective (vertical) to advective (horizontal). An easy way to visualize location of this pressure gradient is to examine a high in cross section. Where the sides of a high-pressure mound are steep is where isobars are close together and winds consistent and well developed.

Every ocean basin has an area of high pressure sitting over it, which grows, shrinks, and shifts its position daily as well as seasonally in response to the influx of air from aloft. Ocean regions each have their own high pressure because ocean water temperatures are stable, allowing upper air to descend and spread outward. Highs over land on the other hand are transient since land heats up and cools down daily and seasonally, and does not always allow air to descend from aloft. In the summertime, for example, it is rare to see strong high pressure over northern hemisphere land masses since the sun is beating down and warming the land, causing surface air to heat and rise. This diminishes the ability of air from aloft to descend.

"Knowing where the high sits is critical in avoiding a hurricane in your ocean."
Northeast and southeast tradewinds in the Atlantic and Pacific Oceans come from the semi-permanent highs inhabiting those waters. These highs normally exist as a single system, but are often split by intruding cold fronts attached to more northerly low-pressure systems. When a strong cold front intrudes into one of these semi-permanent highs, it often splits it into two high-pressure systems. When this occurs it is often possible to sail through what was the middle of the original high without loosing wind.

During the Northern Hemisphere hurricane season (June 1 to November 30) tropical systems must move west under the Atlantic or Pacific high pressure before they can re-curve to the north. Knowing where the high sits is critical in avoiding a hurricane in your ocean.

Using high-pressure systems to advantage requires finding the band of strong winds at its perimeter and staying within its borders. There are several strategies used to accomplish this depending on boat speed and movement of weather systems.


By keeping continuous track of the barometer, a vessel can locate and stay within the more consistent winds along  the perimeter of a high.
A barometer is likely the most important instrument for locating and staying within a high’s wind flow. A recording barometer, preferably one that continuously displays readings, is highly useful. Several models of reliable electronic barometers that display pressure on a screen are available, which is nice because they enable sailors to accurately monitor the location within a high’s wind flow.

Satellite imagery, both infrared and visible, assists in locating high-pressure areas. High-pressure centers have few upper level clouds and scattered puffy cumulus clouds at the surface. Consistent cloud banding exists along a high’s perimeter and this is where wind is found. Perimeter clouds align themselves with wind flow and are called cloud streets. Where distinct and well-defined individual clouds are seen there is certain to be consistent surface winds.

On 500-mb upper-air charts, strong surface winds are found beneath well-formed upper-level ridges. Tight and parallel 500-mb contour lines, also called iso-height lines, support well-developed surface winds. When a high-pressure system contains cold, dense air, its increased tendency to sink permits surface wind speed to average 30 to 50 percent of upper-level winds. For example, a 50-knot upper level wind is reflected in 15 to 25-knot surface wind.

Fast moving boats can often overtake high-pressure systems and sail right through them, whereas weather systems generally move over the path of a slower boat. If a high is stationary, a course optimizing wind speed and direction must be determined as the system is approached. Remember that in the Northern Hemisphere high-pressure systems rotate clockwise, and in the Southern Hemisphere counterclockwise—courses need to be planned accordingly.


Fast-moving boats can overtake high-pressure systems and sail through them, but the rest of us will have to endure the lighter winds as a high moves through our region.
Surface high-pressure systems supported by well-developed upper-level ridges are called blocking highs—the term blocking is used because these strong highs, with central pressures of 1030 mb and often as high as 1050 mb, block the west-to-east flow of weather systems. Blocks essentially prevent the circular flow of air around the earth and can be useful since they provide predictable weather for several days or weeks. But they also dam up warm and cold air, possibly leading to the subsequent development of strong low-pressure systems.

Highs are the weather counterparts to low-pressure systems and are central characters in ocean sailing. Every sailor needs to locate and track the movement of their ocean’s high, be it the Azores-Bermuda High, Pacific High, South Atlantic High, or Indian Ocean High in order to optimize their route. I recommend taking a daily look at the surface and 500-mb analysis charts posted by the Marine Prediction Center (www.mpc.ncep.noaa.gov) and watching The Weather Channel to become more familiar with highs. Their annotated satellite imagery clearly shows where highs are found. Remember, unless you are looking for flat calm conditions without any wind, stay away from the center of a high.


Suggested Reading:

Low-Pressure Systems by Michael Carr

Understanding Weather as a Global Interaction by Michael Carr

Surface Weather Overview by Michael Carr

Buying Guide: Lightning Protection

 

 

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