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upwards by the advancing cold air. Winds on each side
of the cold front blow in opposite directions and spiral
upwards in a counterclockwise direction because of
Coriolis force. This rotation establishes a wave or
indentation along the front. Warm air begins to
advance eastward and poleward into the cold air mass
while cold air begins to encircle the warm air from the
west. A V-shaped frontal system develops with warm
and cold fronts at the leading and trailing edges,
respectively. The lifting of warm, moist air generates
low pressure and convective instability as latent heat of
evaporation is released in the upper atmosphere. This
uplift enhances the rotational circulation, increasing
wind flow and producing precipitation over a wide
area. The cold front at the back of the system pushes
up warm air rapidly leading to thunderstorm form-
ation. If the temperature difference at this location
between the two air masses is great enough, tornadoes
can be generated. The cold front, being backed by
denser air, eventually advances faster than the warm
front and begins to lift off the ground the warm air
within the 'V', occluding the fronts. In the final stage of
a depression, the polar front is re-established at the
ground with decreasing rotational air movement
stranded aloft. Without continued convection, cloud
Pola r-front lows
Formation
(Linacre & Hobbs, 1977; Whipple, 1982)
Low-pressure cells developing along the polar front
have cold cores and an effective diameter that is much
larger than that of a tropical cyclone (2000 km versus
400 km, respectively). Their intensity depends upon
the difference in temperature between colliding cold
and warm air masses. They always travel in an easterly
direction, entrapped within the westerlies. Their
location depends upon the location of Rossby waves in
the polar front around 40°N and S of the equator. In
the southern hemisphere, this occurs south of land-
masses, but in the northern hemisphere, it coincides
with heavily populated areas.
The theory for the formation of polar-front lows was
formulated by Bjerknes and his colleagues at Bergen,
Norway, in the 1920s. Outbreaks of cold polar air flow
rapidly towards the equator and clash with much
warmer air masses at mid-latitudes. Coriolis force
dictates that this polar air has an easterly component
while the movement of warmer air on the equatorial
side of the cold front has a westerly component (Figure
3.11). At the same time, the warm air will be forced
Northern hemisphere
Southern hemisphere
1
2
1
2
Low
Low
3
4
3
4
N
Low
Low
Low
Low
5
6
5
6
Low
Low
Fronts
Air movement
Warm
Stationary
Warm
Cold
Occluded
Cold
Stages in the development of an extra-tropical depression as viewed in each hemisphere 1) and 2) initial development of wave form,
3) and 4) mature front development and full extent of low, 5) and 6) occlusion.
Fig. 3.11
 
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