What is the 500 MB Chart and why should the Mariner care?
The 500mb chart can be a useful weather tool for the weather savvy mariner along with the more familiar surface pressure charts. The 500mb chart is a constant pressure chart which means that everywhere on the chart the air pressure is the same (500mb). This occurs in our atmosphere, on average, at a height of about 5600 meters or about 18,000ft above sea level but varies from place to place due to the density of the air column.
The heights depicted on this chart represent the level at which the air pressure reaches 500mb or about one half the normal surface pressure of about 1013mb). The lines depicted on the chart are lines of equal height and are given in “tens of meters” above sea level so that the “540 line” on the chart means that the 500mb level is located at a height of 5,400 meters above sea level. Heights increase when the air is warmer and less dense and fall when the air is colder and denser so that the distance between these height lines indicates the slope of the 500mb surface. The 500mb forecast chart is an excellent tool for mariners to estimate where to anticipate the heaviest sea and swell conditions as well as a guide to the expected surface storm tracks and intensities.
On the marine version of the 500mb charts the “564” (5640 meter contour) is highlighted in dark black as it often depicts the southern limit of force 7 or higher westerlies during the winter and force 6 westerlies during the summer. Also the primary surface low storm tracks will tend to lie about 300 to 600 nautical miles northward of this line. A good rule of thumb is that surface lows and fronts will advance at a rate of 1/3 to 1/2 of the 500mb wind speed. Another rule is that the surface winds in the cold southwestern quadrant of surface lows tend to be about one half the intensity of the 500mb wind speed above.
The closer the height contours on the 500mb chart, the faster the upper level wind flow, the stronger the temperature contrasts and the more active is the surface weather below. The development and strengthening of surface lows and the associated bad weather most often occurs on the eastern side of 500mb troughs while surface high pressure and good weather is associated with the western side of these troughs. Without even looking at the surface pressure charts, a mariner can estimate what areas might be best to avoid.
An excellent and detailed look at the use of the 500MB chart can be found in the December 2008 edition of Mariner’s Weather Log. http://www.vos.noaa.gov/MWL/dec_08/milibar_chart.shtml