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 earths 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 highs 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.
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."|
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.
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 highs 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 counterclockwisecourses need to be planned accordingly.
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 oceans 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.
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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