AIR MASSES AND FRONTS

This can take place with either a warm or cold
occlusion. If it occurs near a west coast in winter, there
is a good chance the new occlusion is warm. This
formation of a secondary wave cyclone, the dissipation
of the original occluded front, and the rapid
development of a new occlusion is sometimes called
skagerraking, pressure jump, or bent-back occlusion.
EFFECTS OF UNDERLYING SURFACES
The migration of a frontal system from one area
and type of underlying surface to another often has a
great modifying effect. It may cause the front to be
regenerated in some instances or to dissipate in others.
This transition affects cyclones, air masses, and fronts.
Movement Over Land Surfaces
So far, we have established that frontal systems
generally weaken when moving from water to land
surfaces. Once these systems are over land, further
modification can be expected. A front that has just
crossed the mountains and has weakened remains weak
or dissipates unless something occurs to strengthen the
contrast between the air masses. If a cold front has just
moved onshore in winter and encounters ice and snow
cover over the western half of the United States, the
maritime air behind the front quickly takes on colder
continental properties. The cold underlying surface
may totally destroy the cold front, especially if the
associated air mass is moving slowly. On the other
hand, if the front is moving quickly enough that it is not
totally destroyed or modified by the colder surface, it
may quickly regenerate as it approaches a warmer
underlying surface and air mass. These normally exist
over the eastern half of the United States. In this
particular situation, the air behind the front is much
colder than when it started. As the front arrives at the
edge of the snow field, it probably will encounter
warmer moist air from the gulf or the ocean. This
situation quickly results in frontogenesis because of a
sharp air mass contrast. Strong lifting by the wedge of
approaching cold air results in severe thunderstorms
and abundant precipitation along the frontal surface.
If the ice and snow field does not exist over the
western half of the United States, then the weakened
front gradually strengthens as it approaches the warmer
eastern United States. The weather will not be as
intense; however, the cold front will have a much wider
band of clouds and precipitation. With this situation, air
mass contrast is not strong. If the air masses behind and
ahead of the front are weak, the front becomes
4-50
stationary over the extreme southeast United States.
The frontal systems are usually oriented in a
northeast-southwest direction and occur mostly during
the summer and autumn months. Frequently, stable
waves develop and travel along this frontal system,
causing unfavorable weather conditions. When these
waves move out to sea and warmer moist air is brought
into them, they become unstable waves and are
regenerated as they move across the ocean.
As the cold fronts cross the Appalachian
Mountains, they normally weaken once again because
warm moist air is cut off. After passage over the
mountains, warm Gulf Stream waters quickly resupply
the frontal surface with the moisture and warm air
needed for the front to strengthen.
Land to Water Migration
Once a cold front moves offshore, most forecasters
and analysts forget about them and concentrate on the
next approaching weather. When a front moves into the
Atlantic, the weather generally becomes more intense,
especially during fall and winter. While your station
may be relaxing to some degree and enjoying the clear
skies after frontal passage, Bermuda and ships at sea
are most likely bracing for gale force wind and severe
thunderstorm activity.
In middle latitudes, ocean currents carry warm
water away from the equator along the eastern coasts of
continents and carry cold water toward the equator
along the western coasts of continents. The most active
frontal zones of the winter season are found where cold
continental air moves over warm water off eastern
coasts. This situation is noticeable off the eastern coast
of the United States over the Atlantic Ocean. As a cold
front moves off the coast and over the Gulf Stream, it
intensifies, and frequently wave development occurs
near the Cape Hatteras area. This gives the eastern coast
of the United States much cloudiness and precipitation.
This system and its newly intensified front eventually
reaches Bermuda. A similar situation occurs off the
eastern coast of Japan. That area in the Pacific
generates more cyclones than any other area in the
world.
REVIEW QUESTIONS
Q4-20. What two effects cause the modification of
fronts?
Q4-21. What normally happens to a cold front that
moves off the eastern coast of the United
States in the winter?