jet stream, cold, dense air is to the polar side and warm, less dense air is to the
tropical side. These air masses do not mix unless they are energized by a disturb-
ance in the polar front jet stream. As such a disturbance passes overhead an air
aloft diverges (pulls apart horizontally). This creates a surface low pressure zone
into which air converges and rises to make cloudiness andprecipitation. However,
this is not all: the convergence of air into the surface low makes the polar and
tropical air masses push on each other thus creating active fronts. The strength of
an MLC cyclone is largely dependent on the upper air flow. If the flow aloft
strengthens, divergence is favorable for evacuating air from the surface cyclone
and strengthening the storm. If the upper divergence decreases, the surface circu-
lation is damped. The paths of MLCs are readily forecasted because they are
steered in the direction of the air in the polar front jet stream.
A MLC experiences several stages as it crosses the United States. At first, tropi-
cal and polar air masses encounter each other with neither being the aggressor. An
upper disturbance in the polar front jet stream causes a center of low pressure (the
cyclone), a cold front, a warm front, and an incipient wave. With the passage of a
few hours the MLC forms a large wave and there is a circulation established (a
counterclockwise spiral toward the center in the Northern Hemisphere). Within
the wave is, typically, a maritime tropical air mass while pushing the cold front
along is a continental polar or Arctic air mass. It is at this mature wave stage that
the cyclone is strongest and the most severe weather is expected. With the passage
of more time, an occluded front starts to form. The occluded front occurs because
cold fronts travel faster than warm fronts. As cold air overtakes the maritime
tropical air within the wave, the warm air is forced aloft ending the air mass con-
trast along that part of the front. Occlusion progresses southward from the cyclo-
ne’s center in the Northern Hemisphere. As the cyclone occludes its strength
abates and all that remains is residual cloudiness and lightprecipitation. The
cyclone has “died” but, before this, much polar and tropical air has mixed and
the boundary between the polar and tropical air is established in another position.
The air masses, frontal types, and the cyclone center make for a wide variety of
weather. The cold front can create tornadic thunderstorms while there is heavy
snow to the north of the cyclone’s center. As an MLC passes there are significant
weather changes. Indeed, the recognition of existence of MLCs allows some very
good weather forecasting. Suppose we are interested in the weather forecast for
Dayton, Ohio. As a MLC passes, Dayton is in the maritime tropical air ahead of
the strong cold front. As the MLC approaches, the weather will become cloudy
from upper parts of frontal clouds blown along in the upper air. The front will pass
with rain and Dayton will experience a “dry slot” composed of cold air wrapped
around the cyclone’s center. As the cyclone’s center passes Dayton, the city will226 Middle Latitude Cyclones