Weather Charts

By Capt. Jonathan Taylor

Most sailors are well versed in interpreting weather charts that show present or predicted conditions at the earthâs surface. We know that wind is caused by air moving from high to low pressure areas and is bent to the right (in the Northern hemisphere) by the Coriolis effect setting up large circulation patterns, circling clockwise around high pressure areas and counter-clockwise around lows.

What sailors may not know is that the higher levels of the atmosphere also have highs, lows, and circulating winds. For example, the Jet Stream is a high-altitude wind circulating counter-clockwise around a giant low-pressure area high above the northern latitudes.

Why would we surface-bound travelers care about these high-level winds? Due to friction with the air below them, upper level winds create and steer surface level weather. If we knew what the high altitude winds were doing, we could use this knowledge to help predict what would happen at the surface.

Twice each day at 0Z and 12Z the U.S. National Weather Service issues a set of weather charts called 500-mb (millibar) charts that show the winds at around 18,000-20,000 feet. One chart in the set shows present conditions and the others show predictions for various times in the future. The NWS issues weather charts for other altitudes, but the 18,000-20,000 foot height is particularly useful because this is where the upper-level winds interact with surface air.

Let's take a look at the 500-mb chart for the Atlantic Ocean for 0Z Feb 4, 2006 (above, click on it for a larger view). Surface weather charts use lines of equal pressure called isobars to show the pressure at a constant height (namely sea level). 500-mb charts use lines of equal height called height contours to show the height at a constant pressure (namely 500-mb). So the highs (ãHä) and lows (ãLä) on the chart show high and low heights, not pressure. Despite this difference, there are many similarities with surface charts - winds bend counterclockwise around an L, clockwise around an H, and increase in speed where the lines are close together.

The height contours are labeled with height in meters divided by 10. For example, the height contour labeled ã564ä is where one would have to go to 5640 meters height to find 500-mb pressure. The 5640 meter contour is called the ãstorm trackä and is hi-lighted on the chart. High level winds are often strongest near the storm track so surface level storms often follow this contour.

You can see from the chart that winds move generally from west to east with some north-south undulations. Southerly dips are called troughs (pronounced ãtrofsä) and are marked with a dashed line. You can see one trough over the mid United States and another over the mid-Atlantic. As the winds bend to the left around a trough, they often impart a counter-clockwise spin on the air below helping to form surface level low pressure areas.

This article is a very brief intro to 500-mb charts. For more information, I can recommend two books, Weather Predication Simplified by Michael William Carr and Weather Forecasting Handbook by Tim Vasquez.

A useful package of weather charts called The Atlantic Briefing Package can be found on the web at
http://www.opc.ncep.noaa.gov/shtml/A_brief.shtml. If you follow these charts for a few days, you can begin to see the patterns at the upper levels and the relationship with the surface weather.

Jonathan Taylor is a USCG-licensed 100 Ton Master and teaches with the Manhattan Sailing School in New York City and the Caribbean.

All articles are copyrighted by the American Sailing Association

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