|WHY AREN'T COLD FRONTS AND
BOTH LIFTING MECHANISMS?
METEOROLOGIST JEFF HABY
If low level CAA is a sinking mechanism, then why does a cold front produce a lifting of air? The answer
is because each is an independent dynamic process.
A cold front is the immediate divide between synoptic scale air masses where colder air is separated from
warmer air and the colder air is advancing. The density discontinuity created at the boundary can lead to
synoptic scale uplift. Usually
this uplift occurs along and ahead of the cold front boundary.
Low level CAA is a condition in which temperatures decrease over time at a point location due to the bringing
in of colder air about a horizontal plane. This not only occurs just behind a cold front boundary but also
extends to hundreds of miles behind cold front passage.
In an ideal case, precipitation occurs along the cold front boundary. After the cold front boundary passes
a location, the temperature continues to cool and skies begin to clear or the precipitation comes to an end.
Several hours after cold frontal passage the skies clear. The front itself helps contribute to synoptic
uplift along the frontal boundary. As this lifting mechanism moves away over time, the precipitation
ends and skies clear. The CAA pattern behind the cold front helps contribute to the sinking of air and
clearing skies. Drier air behind the cold front can also contribute to the decrease in precipitation
and clouds. Additional lifting and sinking mechanisms such as
differential thermal advection,
jet streak divergence or convergence, etc. can contaminate this ideal case.
On a weather map, the lifting from a cold front will tend to cover a linear region while the sinking from
low level CAA will tend to cover a large area behind the cold front. Lifting combined with greater
low-level WAA out ahead of a cold front contributes to most precipitation being in the warm sector ahead
of the cold front.