Geoscience Reference
In-Depth Information
Migration of a Midlatitude Cyclone
It is now time to visually integrate the various moving compo-
nents of a midlatitude cyclone. View the animation
Migration
of a Midlatitude Cyclone
. This module contains a video that
shows the migration of a typical midlatitude storm system in
North America over the course of several days. As you watch this
system evolve, pay attention to
both
its counterclockwise circu-
lation and its movement from west to east. See if you can identify
the center of the system and associated cold and warm fronts.
After you complete the video, be sure to answer the questions at
the end of the module to test your understanding of this concept.
Evolution of Thunderstorms
Like midlatitude cyclones, thunderstorms have a distinct life
cycle, illustrated in Figure 8.12. The first stage in the develop-
ment of a thunderstorm is the
cumulus stage
(Figure 8.12a),
which begins with convection or the advancement of a cold
front into mT air. This kind of disturbance causes air to rise,
which, in turn, results in the formation of ice crystals and rain-
drops in growing cumulus clouds. Given their small size in this
early stage of thunderstorm development, the ice crystals and
raindrops continue to be uplifted. Condensation adds more la-
tent heat energy to the air during the
developing stage,
and the
atmosphere becomes very unstable with strong updrafts.
As the evolving thunderstorm develops further, it enters
the
mature stage
(Figure 8.12b). In this stage, the atmosphere
is very unstable, and continued convection or frontal advance-
ment results in the massive uplift of air. Winds are blustery dur-
ing this stage, with perhaps some lightning and thunder. As cold
raindrops begin to fall to the ground, however, they pull cold air
toward the surface with them at high speeds in a
downdraft
at
the front of the storm. If you happen to be in this place as a storm
forms, you can often feel a distinct increase in wind speed at
this
gust front
. Where convection is most intense, an anvil head
cloud forms at very high altitude (Figure 8.12c), which may be
6 km to 12 km (20,000 ft to 40,000 ft). This cloud completely
develops when the top of the convection bubble is pulled (or
sheared) downwind by upper airflow. You can see an example
of a forming anvil cloud in Figure 8.12d. If thunderstorm devel-
opment occurs along an advancing cold front, a group of mature
storms and associated anvils will form a feature called a
squall
line
(Figure 8.12e). These features show up clearly on radar
images and are one of the many atmospheric phenomena about
which meteorologists become excited.
Following the mature stage of storm development, thun-
derstorms begin to weaken. This weakening occurs during
the
dissipation stage
and really begins as strong downdrafts
develop when the storm has its maximum intensity. Look for
KEY CONCEPTS TO REMEMBER ABOUT
THE FORMATION OF MIDLATITUDE
CYCLONES
1.
Midlatitude cyclones spin in a counterclockwise fashion
in the Northern Hemisphere as seen from above.
2.
Midlatitude cyclones occur when undulations form in
the polar front jet stream. These undulations are related
to the altitude of the 500-mb pressure surface.
3.
As a cyclone spins in the Northern Hemisphere, it pulls
in mT air from the south on its eastern side. This warm,
moist air encounters cold air as it moves to the north.
4.
As a cyclone spins in the Northern Hemisphere, it pulls
cP air down from the north on its western side. This
cold, dry air encounters warm air as it moves to the
south.
5.
As the system spins, it also migrates from west to east.
Thunderstorms
You have probably experienced a severe thunderstorm at
some point in your life. Maybe you had a sense that a storm
was brewing and then suddenly it hit with heavy rain, strong
winds, booming thunder, lightning, perhaps even some hail.
The storm lasted for a while and then gradually tapered off
as it moved away. If you have ever been impressed by such a
storm, talked about it with your friends, or wondered why it
happened, then this next section will be interesting because it
focuses on the evolution and characteristics of thunderstorms.
It immediately follows the discussion about midlatitude cy-
clones because the most severe storms in the midlatitudes are
most often associated with low-pressure systems. As you work
your way through, just keep in mind that storm development is
related to the way air rises and cools, and its interactions with
latent heat.
Downdraft
A rapidly moving current of cool air that flows
downward in a thunderstorm.
