2008 REED’s East Coast Edition - Weather
Mb. (Note: normal sea level pressure is 1013 chart. Surface low-pressure systems often
Mb). Five hundred Mb occurs at an altitude form near troughs, and surface high-pres-
of approximately 18,000 feet, halfway up the sure systems near ridges.
earth’s atmosphere and at the lowest level of In mid-latitudes, surface lows often form
jet stream activity. downstream (to the east) of jet stream
The solid lines on 500 Mb (and other troughs because the troughs induce an up-
upper-air) charts connect points of equal ward motion to the air. In the Northern
elevation for a specified atmospheric pres- Hemisphere, surface highs are associated with
sure. Upper air wind speeds of 30 knots and upper-air ridges where the air is subsiding
greater are shown using directional “barbs” (in the Southern Hemisphere these shapes
with “feathers” indicating five- and 10-knot are reversed.) Troughs that are sufficiently
increments. Wind speeds above 50 knots are developed to induce the development of sur-
noted with a solid triangular flag. face low-pressure systems are drawn on 500
On a 500 Mb chart, the lines indicate Mb charts with a bold dashed line along their
elevations where the pressure is 500 Mb axis. Troughs and ridges at the 500 Mb level
(see Figure 5). These lines are called height change slowly, often taking days, weeks, and
contours and are similar to contour lines on occasionally months to change their flow pat-
surface topographical maps. (Recall that the tern. This is why they are so useful in making
lines on surface charts, in contrast, indicate surface weather forecasts.
varying pressures at the same, i.e., surface, If a low forms at the surface beneath an
elevation.) On an upper-air chart, the lines upper-level trough, the low system will
are drawn at intervals of 60 meters of alti- usually move in the direction of the upper
tude and are often labeled in decameters. airflow, but at a forward speed of one-third
For example, a line labeled 582 on a 500 Mb to one-half the speed of the upper-air winds.
chart indicates a pressure of 500 Mb at a Normally, surface wind speeds within the
point 5,820 meters above sea level. Gener- low will be half of the 500 Mb wind speed.
ally, the colder the air mass, the lower the For instance, if the wind speed in a trough is
altitude at which the pressure is at the speci- 60 knots at the 500 Mb level, the winds in the
fied amount. The elevations at 500 Mb are surface low will be in the 30-knot range.
higher at the equator and lower at the poles. On a 500 Mb chart, the 564 line (delineat-
The jet stream flows generally from west ing 5,640 meters) often receives special atten-
to east, but can meander north and south. tion because it tends to mark the southern
Meridional flow refers to north-south mo- border (Northern Hemisphere) of Beaufort
tion that forms waves, or undulations, in the Force 7 (28- to 33-knot) surface winds in win-
stream. Meridional flow mixes cold air from ter and Force 6 (22- to 27-knot) surface winds
mid and high latitudes with warm air from in summer. This contour is drawn thicker and
lower latitudes and is a harbinger of unset- darker than other contours on the chart.
tled and possibly stormy weather. When jet-stream winds in the area of a
Zonal flow refers to a west-to-east motion 500 Mb trough exceed 110 knots, there’s a
(in both hemispheres) that has very little strong possibility of a rapidly intensifying
of the undulating, wavy motion found in low at the surface. Conditions that promote
meridional flow. In zonal flow, there is not such development occur when a trough has
much mixing of cold and warm air and, gen- pronounced north-south development that
erally, stable settled weather accompanies it. produces a healthy mix of warm and cold air;
Troughs and Ridges: When you examine a when there are large temperature differences
500 Mb chart, first you should determine if within the atmosphere—again, especially
the height contour lines reflect meridional in the spring and fall—and when an upper-
or zonal flow, because the flow pattern is an level trough is located over a warm ocean
excellent indicator of surface weather. In current.
meridional flow, you should first look for The amplitude (north-south height) and
the troughs (U-shaped dips in the North- breadth (west-east) of troughs and ridges on
ern Hemisphere) and ridges (humps in the a 500 Mb chart indicate the characteristics
form of inverted U’s). Second, you should of the surface highs and lows beneath them.
compare these patterns to a surface analysis Generally, the greater the amplitude, the
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