|PRESSURE TROUGHS AND SHORTWAVES
METEOROLOGIST JEFF HABY
When analyzing a surface chart you will notice the isobars bend in the vicinity of the warm front and the cold
front. The isobars do not make perfect circles around low-pressure centers because of the pressure troughs created
by the fronts. Pressure can decrease in the atmosphere by:
(1) causing the air to rise
(2) decreasing the density of the air
(3) decreasing the mass of the air (i.e. upper level divergence).
Causing the air to rise counteracts
some the downward force created by gravity. This
lowers pressure just as if someone started pushing up on you when
standing on a scale; your weight would decrease. Fronts force the air to rise. This causes the surface pressure to
decrease in the vicinity of the front. Cold fronts have a more defined
pressure trough than warm fronts because the
average cold front has a steeper slope and stronger temperature gradient than the average warm front. A warm front
raises the air gradually while a cold front lifts the air more quickly in the vertical. The faster the air rises,
the more pressure will lower. A mid-latitude cyclone and a front will both cause the air to rise and pressure to
lower. The stronger the front, the more well defined the pressure trough will be.
Now to shortwaves. A
shortwave is an upper level front or a
cool pocket aloft. Just as a surface front causes the air to rise, upper level fronts can do the
same. First, let's start with a general description of a shortwave:
(1) It is smaller than a longwave trough (shortwave ranges from 1 degree to about 30 degrees
in longitude (the average one
is about the size of two U.S. states put together (Iowa and Missouri put together is a good example)
(2) Isotherms cross the height contours (if it is a baroclinic shortwave). This
creates an upper level temperature gradient and therefore an upper level front
(3) They are best examined on the 700 and 500 millibar charts
(4) They generate positive vorticity (mainly due
to the counterclockwise curvature within the shortwave). This
creates positive curvature and positive shear vorticity. If PVA occurs with the shortwave then the
shortwave will deepen and strengthen due to lift created by upper level divergence.
(5) They can create an environment conducive to surface based convection
or elevated convection due
to the cooling aloft.
It is important to see how much moisture is associated with the shortwave. A shortwave moving over a
warm and moist lower troposphere has a better chance of producing precipitation and strengthening than one
moving over a dry lower troposphere. If the low
level dewpoint depressions are low, the instability and lift associated with the shortwave can enhance
cloudiness and precipitation.
In summary, a pressure trough is associated with a low-level front while a shortwave is associated with an upper
level front or a cool pocket aloft. Both are associated with rising air and can add
instability to the atmosphere.
The diagrams below show an upper level shortwave and a surface pressure trough. The pressure trough is
the kinking along a front.