Where can sailors obtain weather information that will provide insight into upcoming events and allow us to analyze and predict upcoming weather? Our answer lies in the use of a particular "upper air chart," known in the weather business as the 500 millibar (mb) chart.
This chart, which shows strength and direction of upper atmosphere or jet stream winds, allows us to determine location, strength and movement of surface troughs, ridges, lows and highs. Upper air-500 mb-charts take away the guesswork of analyzing surface weather systems and patterns, dramatically improving racing strategy and cruising safety.
Why, and how, do upper-air winds influence surface conditions? Think of the earths surface as the bottom of a stream bedsprinkled with boulders, rocks, and other impediments resembling the earths topographyand think of upper level winds as the water flowing over this stream bed. Now think of each twist, turn, and eddy within the stream as weather featureshighs, lows, and fronts. Each feature is dependent on the streams strength, direction of flow, and mixing of water layers.
Sailors use wind information at the 500 mb (18,000 ft) level since winds at this lower level of the jet stream have the most pronounced effect on surface weather. Remember, normal sea level pressure is 29.92 inches of mercury or 1013 mb. So the 500 mb level is approximately half way up into our atmosphere (500 mb is about half of 1013 mb).For comparison, a pressure of 50 mb is found at 67,000 ft and 0 mb at 600 miles.
Winds at the 500 mb level, and for that matter everywhere in the jet stream, blow generally from west to east. Since there is little friction and disturbance from land topography at upper altitudes, jet stream winds follow a regular and predictable pattern. This pattern is continually monitored and mapped by National Weather Service (NWS) Marine Prediction Center computer models, and drawn daily on charts.
These charts are provided to users, such as ourselves, by Radiofacsimile broadcasts and the NWS Internet connection each day.
Upper air, 500 mb, charts are produced in both analysis and prognosis (forecast) format. Analyses are generated twice each day at 00Z (Z is Greenwich time) and 12Z. A 12-hour forecast is often produced along with the analysis, and a 48-hour forecast for both the Atlantic and Pacific is also produced at 00Z and 12Z.
Additionally, there are 60-hour (2.5-day) and 96-hour (four-day) 500 mb forecasts. Upper air charts are produced in conjunction with surface charts with the same forecast period (12, 48, 60, 96 hour) so surface and 500 mb charts can be used in conjunction with each other for verification and comparison.
As an aside, sea state analysis and satellite imagery, which are valid for the same synoptic time as 500 mb and surface charts can assist in rounding out the weather picture. Most yachts entered in around the world races such as the Vendee Globe, Around Alone, and Whitbread, make extensive use of 500 mb flow patterns and satellite imagery.