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Bài giảng khí hậu học chương 8

G304 – Physical Meteorology and Climatology

Chapter 8
Air masses and fronts

By Vu Thanh Hang, Department of Meteorology, HUS

8.1 Formation of air masses
• The areas where air masses form are called source regions.

• An area must be quite large, many tens of thousands of
square kilometers.
• Horizontal teperature gradient is ~5-7oC/1000km and relative
humudity gradient is ~2-3%/1000km.
• These vertical differences in temperature affect the stability of
the atmosphere Æ affects precipitation.
• Air masses are not permanently confined to their source
regions, they are able to migrate to regions marked by less
extreme weather conditions.

8.1 Formation of air masses (cont.)
• Based on moisture content, air

masses can be considered
either continental (dry) or
maritime (moist).
• According
temperature, they are either
tropical (warm), polar (cold), or
arctic (extremely cold).
• A small letter c or m indicates
followed by a capital letter T, P,
or A to represent temperature.

8.1 Formation of air masses (cont.)

Continental polar (cP) air masses form over large, high-latitude
land masses. In addition to having very low temperatures,
winter cP air masses are extremely dry; summer cP air masses
warmer and more humid than in winter.
Continental arctic (cA) air is colder than continental polar and
separated by a transition zone similar to the polar front called
the arctic front.

8.1 Formation of air masses (cont.)

Maritime polar (mP) air masses are similar to continental polar air masses
but are more moderate in both temperature and dryness. Maritime polar air
forms over the North Pacific as cP air moves out from the interior of Asia.
Maritime polar air also affects much of the East Coast with the circulation
of air around mid-latitude cyclones after they pass over a region. The
resultant winds are the famous northeasters (above) that can bring cold

winds and heavy snowfall.

8.1 Formation of air masses (cont.)
• Continental tropical (cT) air forms during the summer
over hot, low-latitude areas. These air masses are
extremely hot and dry, and often cloud-free.
• Maritime tropical (mT) air masses develop over warm
tropical waters. They are warm, moist, and unstable near
the surface, which are ideal conditions for the
development of clouds and precipitation.

8.1 Formation of air masses (cont.)

8.2 Fronts
• Fronts are boundaries that separate air

masses with differing temperature and
other charactersistics.
• A cold front occurs when a wedge of cold
air advances toward the warm air ahead of
• A warm front represents the boundary of a
warm air mass moving toward a cold one.
• A stationary front differs in that neither air
mass has recently undergone substantial
• Occluded fronts appear at the surface as
the boundary between two polar air
masses, with a colder polar air mass
usually advancing on a slightly warmer air

8.2 Fronts (cont.)

In a typical mid-latitude cyclone,
cold and warm fronts separated
by a wedge of warm air meet
at the center of low pressure.
Cold air dominates the larger
segment on the north side
of the system.

8.2 Fronts (cont.)

Cold fronts typically move more rapidly and in a slightly different
direction from the warm air ahead of them. This causes
convergence ahead of the front and the uplift of the warm air that
can lead to cumuliform cloud development and precipitation.

8.2 Fronts (cont.)
• Five features to determine cold front positions:
- Significant temperature differences between adjacent
- Dew point differences.
- Bands of cloud cover and precipitation.
- Narrow zones where wind direction changes.
- Boundaries separating regions where the atmospheric
pressure changes over the 3-hour period

8.2 Fronts (cont.)
- Warm fronts have gentler sloping
surfaces (1:200).
- Surface friction decreases with
distance from the ground, as
indicated by the longer wind
vectors away from the surface (a).
- This causes the surface of the
front to become less steep through
time (b).

8.2 Fronts (cont.)

Warm fronts separate advancing masses of warm air from the colder
air ahead. As with cold fronts, the differing densities of
the two air masses discourage mixing, so the warm air flows upward
along the boundary. This process is called overrunning, which leads
to extensive cloud cover along the gently sloping surface of cold air.

8.2 Fronts (cont.)
• For identifying warm front positions:
- look for zone where warmer air advances toward cooler
- dew points typically increase behind the position of the
warm front.
- winds commonly shift from southwesterly ahead of the
front to southeasterly behind it.
- cloud cover and precipitation bands are common.
- the zone ahead of the warm front gererally undergoes
decreasing air pressures while the area immediately behind the
front typically has stable air pressure.

8.2 Fronts (cont.)
• Nonmoving boundaries are called stationary fronts.
• Although they do not move as rapidly as cold or warm fronts,
they are identical to them in terms of the relationship between
their air masses.
• The frontal surface is inclined, sloping over the cold air.

8.2 Fronts (cont.)

- The most complex type of front is an occluded front.
- When the cold front meets the warm front ahead of it, that segment
becomes occluded.
- The warm air does not disappear, but gets lifted upward, away from
the surface.
- The occluded front becomes longer as more of the cold front
converges with the warm front.

8.2 Fronts (cont.)

- Eventually, the cold front completely overtakes the warm front, Æ

the entire system is occluded.
- In this occlusion, the air behind the original cold front was colder
than that ahead of the warm front.
- This is an example of a cold-type occlusion.

8.2 Fronts (cont.)

The boundaries separating humid air from dry air are called
drylines and are favored locations for thunderstorm development.
The dryline above (the dashed line) separates low humidity
to the west while to the east humidity is higher as
indicated by the dew point temperatures.

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