Basics of the 500-millibar chart


One of the more useful weather charts in additional to surface analysis and forecast charts is the 500-millibar chart. The 500-millibar surface is a constant pressure surface approximately midway up in the troposphere (the lowest layer of the Earth atmosphere). Roughly half of the atmosphere is above and half is below the 500-millibar level. The pressure exerted by the air column above this level is exactly 500 mb, but the height of this surface varies. The 500-mb constant pressure surface generally averages 18,000 feet (5,600 meters) in height but can vary from between 15,000 feet (4,700 meters) in a cold and dense atmosphere, to nearly 20,000 feet (6,000 meters) in a very warm and less dense atmosphere.

The solid contours shown on a 500-mb chart represent geopotential height  — basically, the height in whole meters. The number 5640 means 5,640 meters. In a sense, you are looking at a topographical map of the 500-mb pressure surface. Since the heights are lower in colder air masses and higher in warm air masses, in the Northern Hemisphere the heights are generally lower to the north and higher to the south.

The closer the 500-mb height contours, the stronger the horizontal and vertical temperature contrasts and the faster the wind speed at the 500-mb level (the wind at this level is generally parallel to the height contours). A simple rule of thumb is that the tighter the height contours, the higher the wind speed, and the stronger the temperature difference below 500 mb.

Winds at the 500-mb level are generally not the actual jet stream, which is generally between 200 and 300 mb. Jet streams exist due to horizontal temperature contrasts. In an extremely cold atmosphere, the Arctic jet stream can extend down to 500 mb. However, meteorologists do call wind speed maxima at 500 mb “jets” or “jet streaks.” On the 500-mb charts distributed by the National Weather Service, only 500-mb heights and winds are depicted. The distribution of height contours, and therefore the strength of the 500-mb winds, implies the strength of horizontal temperature contrasts.

Example of a 500-millbar chart product from the National Weather Service.

The "L" and "H" labels represent areas of relatively higher and lower heights. An L or H with a closed contour around it implies that the high or low has a closed circulation with the wind circulating around it.

Troughs are areas of relatively lower heights and are U- or V-shaped contours.

Ridges are areas of higher heights; these are shaped like an upside down U or V and are indicated by a zigzag line.

On 500-mb charts, the 5,640-meter height contour is enhanced in bold. Some basic rules of thumb used by marine meteorologists concerning the 5640 contour and the 500-mb wind maxima are:

• In wintertime, the 5640 contour is an excellent indication of the southern extent of surface winds of Force 7 westerlies or greater. In summer, the 5640 height contour is more representative of Force 6 surface westerlies.

• The surface storm track is usually 300 to 600 nautical miles north and parallel to the 5640 height contour.

• Fronts (cold fronts in particular) and surface storm centers move at approximately one-third to one-half of the wind speed.

• The surface wind speed, especially in the west to southwest quadrant of a surface low (in the cold air) is approximately 50 percent of the 500-mb wind speed.  

By Ocean Navigator