| FORECASTING USING THE 500 MILLIBAR CHART
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METEOROLOGIST JEFF HABY
The 500 millibar chart represents
the pressure level in about the middle of the atmosphere (about half the earth's
atmospheric mass is below this level and half is above). The 500 millibar chart is one of the best charts for studying
the following: vorticity advection, the
trough/ridge pattern, and
shortwaves. Vorticity is a spin to the air. A
counterclockwise spin in the Northern Hemisphere is associated with rising air if PVA occurs. Positive
vorticity (shown on 500
mb vorticity charts) is highest where the winds have to follow a curved counterclockwise path and areas with strong
speed shear (wind speed increasing
away from the center of a trough on the 500 millibar plane). The trough / ridge
pattern gives forecasters the following information: which areas are experiencing cooler or warmer weather than normal
(weather tends to be cooler under troughs and warmer under ridges). The trough / ridge pattern can be used to determine
if the upper level winds are
zonal (pretty much west to east flow,
low pressures move quickly within a zonal flow)
or meridional
(atmospheric blocking and unusual cold and warm
weather occur when the pattern is meridional). A short
wave is a small amplitude trough embedding within the longwave troughs and ridges. On a 500-millibar chart they
will show up as a region were the isotherms cross the height contours at a sharp enough angle to produce an upper
level circulation. The current 500-mb analysis chart can be found by clicking below:
500: http://weather.unisys.com/upper_air/ua_man.php?plot=500&inv=0&t=cur
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