WITH MID-LATITUDE CYCLONES
METEOROLOGIST JEFF HABY
A mature mid-latitude cyclone is often a merging point
of 2 or more air masses. The air masses north of the
mid-latitude cyclone are continental polar
or maritime polar air while the air masses to the south of the mid-latitude
cyclone are continental tropical and/or
maritime tropical. When these air masses
are advected toward and into a
mid-latitude cyclone they produce what are called "conveyor belts". Since these air masses have different moisture
and temperature characteristics (different densities) they advect over the top or underneath each other. The
advection of different air masses underneath or over the top of each other is termed
Rule: The denser air mass will advect underneath the less dense air mass. This same principle applies to geology:
the denser oceanic crust advects under the continental crust (albeit slow!). Also, at the same temperature, moist
air will advect over the top of dry air since moist air is less dense.
Using density, continental polar air is
relatively dense while maritime tropical and continental tropical are relatively less dense. Because temperature
is more important than moisture content at determining density, maritime polar air is more dense than continental
tropical air. Therefore, from largest to smallest density the air masses are: continental polar, maritime polar,
continental/maritime tropical. Continental tropical air can be less dense than maritime tropical air although the
maritime tropical air has a higher moisture content. This is because the continental tropical air generally warms
to a greater temperature than maritime tropical air during the day.
As air masses merge near a mature mid-latitude
cyclone, the tropical air will lift above the other air masses due to it being the least dense. Since the maritime
tropical air has the most water vapor content, raising mT air will produce condensation, clouds and precipitation.
The continental polar air mass is dense and wants to hug right along the earth's surface. Since this cold air mass
generally wants to sink, the atmosphere becomes increasingly stable behind a cold front
(termed cold air advection).
A mid-latitude cyclone can also tap a continental tropical airmass.
When this happens, a dryline develops. In the U.S.
Plains, a dryline separates continental tropical from maritime tropical. The continental tropical air sits between
the cold front and the dryline. This sector of the mid-latitude cyclone is called the dry slot.
Critical point: the
position and strength of a mid-latitude cyclone will determine which air masses will be drawn toward the mid-latitude
cyclone. All a forecaster has to do is look at the air masses in the immediate vicinity of a mature mid-latitude
cyclone to see which air masses will be drawn into the low.
Some rules of thumb: If the warm sector of the
mid-latitude cyclone does not have much moisture, precipitation will not be widespread; To have a well defined
dryline in association with a U.S. midlatitude cyclone, the maturing low needs to be positioned over the central
or southern high plains (in late Spring / early summer); A large temperature difference between the mT and cP air masses
as they are drawn toward a midlatitude cyclone will produce a strong
low level jet in the warm sector and will
intensify the jet stream aloft; When
differential advection results in dry
air being lifted over mT air, the stage is set for
convective instability (thermodynamic instability).