Geoscience Reference
In-Depth Information
S U M M A R Y O F K E Y C O N C E P T S
1.
An air mass is a large body of air that forms in specific
geographic regions and thus has distinctive character-
istics. Five principal air masses affect North America.
Continental air masses include continental Polar (cP),
continental Arctic (cA), and continental Tropical (cT). The
maritime air masses are maritime Tropical (mT) and mari-
time Polar (mP).
the polar front jet stream at the 500-mb pressure level.
As a cyclone spins, it pulls warm (mT) air up from the
south on its eastern side. This warm, moist air encoun-
ters cold air as it moves to the north. The cyclone also
pulls cold air (cP) down from the north on its western
side. This cold, dry air encounters warm air as it moves
to the south.
2.
Air masses have distinct boundaries called fronts. At a
stationary front, contrasting air masses are flowing par-
allel to one another. A warm front is a place where warm
air is advancing into relatively cool air. Given that warm
air slowly slides over the top of the cooler air along a
warm front, rainfall is slow and steady. A cold front is a
place where cold air is advancing into relatively warm air.
Given the higher density of colder air, rainfall is intense
and of short duration along the front because warm air
cools quickly when it is rapidly forced aloft.
4.
In general, the most severe midlatitude storms form along
strong cold fronts when warm, moist (mT) air ahead of
the front is rapidly forced aloft. Thunderstorms evolve in
predictable stages, including the cumulus stage, mature
stage, and dissipating stage, that are related to the up-
ward and downward flow of air. The strongest storms as-
sociated with midlatitude weather are tornadoes, which
are localized bodies of intense low pressure that develop
in association with supercell thunderstorms.
5.
The strongest tropical storms are hurricanes, which
develop when easterly waves strengthen beyond the
depression and tropical storm phases to produce sus-
tained winds greater than 63 knots (74 mph).
3.
A midlatitude cyclone in the Northern Hemisphere spins
in a counterclockwise fashion as seen from above. These
atmospheric features develop when undulations form in
C H E C K Y O U R U N D E R S T A N D I N G
1.
Define an air mass.
7.
What is the basic difference between a warm front and
a cold front? Why is the term
front
used in association
with these concepts?
2.
What are the specific characteristics of an mT air mass,
and how do they differ from the characteristics of a cP
air mass?
8.
Precipitation along a warm front is gradual and long-
lasting, whereas it is short-lived and often violent along
a cold front. Why does this difference exist?
3.
Which air mass is most likely to be associated with
precipitation—an mT air mass or a cP air mass? Why?
9.
Describe the evolution of a thunderstorm and the vari-
ous stages it goes through during its life cycle.
4.
Discuss the evolution and migration of a midlatitude
cyclone.
10.
What is a downdraft, and why is it the first step in the
dissipation of a thunderstorm?
5.
Why are midlatitude cyclones a mechanism through
which contrasting air masses are mixed?
11.
Discuss the evolution of a hurricane in the Northern
Hemisphere, including its various stages, movement,
and relationship with ocean temperature.
6.
How does the formation of an upper air trough at the
500-mb level result in the development of a midlatitude
cyclone?
