|SMALL SCALE PRESSURE CHANGES
METEOROLOGIST JEFF HABY
Air pressure changes can be seen on the mesoscale and microscale as well as the synoptic scale. The
decreases by either decreasing the mass above an area or causing a mass of air to rise. For example, a midget will
weight less than a football lineman because the midget has less mass. If someone is pushing down on the midget's
shoulders, he/she will weight more. This is synonymous with high pressure and sinking air. If someone began to lift
the midget from the scale, he/she would weight less. This is synonymous with low pressure and rising air.
run into the terms mesohigh and mesolow. In a thunderstorm or thunderstorm complex, the mesohigh is associated with
the downdraft while the mesolow is associated with the updraft. A downdraft contains dense rain cooled air that is
accelerating toward the surface. The higher density and the fact the air is sinking cause the surface pressure to
rise in this region of a thunderstorm. An updraft contains lower density warm and humid air that is rising. The
updraft becomes a region of relative low pressure. Air enters the
updraft region of a storm and exits in the downdraft region. Some air exits
in the thunderstorm's anvil.
The upper levels are impacted by the surface mesolow
and mesohigh. A thunderstorm complex can create a
shortwave in the upper atmosphere (i.e.
500-mb) over time. The
dense air associated with the downdraft increases the surface pressure but will often times cause height falls in
the upper levels. This same effect can be seen on the synoptic scale. Underneath a shallow arctic air mass which
moves into the U.S. is high pressure at the surface BUT there is a longwave
trough in the upper levels. The cold
air at the surface causes the
thickness of the atmosphere to decrease
because cold air is denser. This causes height
falls in the upper levels. Pressure changes can be used to analyze frontal positions,
outflow boundaries, and
micro and macro bursts.