| Lesson
Content
In this chapter we learn how to recognize the face of the
extratropical cyclone and its associated fronts. We also discuss the
tropical cyclone, or hurricane.
Midlatitude
background
In the 1920s, Jack Bjerknes helped discover that cyclones often
follow a stepwise evolution of development.
(see figure 8-10). The cyclone arises
as a
frontal wave along a stationary front separating cold, dry cP air
from warm,
moist mT air. It is called a “wave” because the warm
sector region between
the cold and warm fronts resembles a gradually steepening ocean wave.
In
adolescence, the open wave develops strong cold and warm fronts with
obvious wind shifts as the whole system moves to the east or northeast.
Precipitation (in green) falls in a broad area ahead of the warm
front and
in a narrow line in the vicinity of the cold front.
At full maturity, the occluded cyclone sprouts an occluded front,
which Bjerknes conceived of as the result of the cold front outrunning
the
warm front. Usually, the barometric pressure at the
center
of the cyclone reaches its minimum during this stage, sometimes plummeting
to 960 mb in a few intense cyclones—as low as in the eye of
a
Category 3 hurricane! Because of the strong gradient of pressure
near its
center, the cyclone’s winds are usually strongest during this
stage.
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Flight
Tip
If you are considering going for a short flight and a front
is approaching, fly towards the front until the weather deteriorates.
Your route home will be clear. If you fly away from the approaching front,
the weather may be poor at the airfield when you return
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Hurricane
The Earth’s atmosphere and oceans can interact in all sorts of ways.
Certain weather features develop when certain regions of the tropical
oceans interact with the atmosphere during the summer and fall of each year. From space, they look
like large circular
swirls of clouds. They tend to be several hundred kilometers
in diameter.
Because of the location of their birth and the pattern of their clouds,
these swirls are
given the generic name tropical cyclones. In the tropical regions of North
and Central
America, the most powerful of them are called hurricanes. Residents of
the western
Pacific call them typhoons. In most other parts of the world, such as the
Indian Ocean,
they are simply called “cyclones.”
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