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Bài giảng khí hậu học chương 6

G304 – Physical Meteorology and Climatology

Chapter 6
Clouds and precipitation

By Vu Thanh Hang, Department of Meteorology, HUS

6.1 Mechanisms that lift air
• Four mechanisms lift air so that condensation and cloud
formation can occur:
- Orographic lifting, the forcing of air above a mountain barrier
- Frontal lifting, the displacement of one air mass over another
- Convergence, the horizontal movement of air into an area at low
- Localized convective lifting due to buoyancy

6.1 Mechanisms that lift air (cont.)

The upward displacement of air that leads to adiabatic

cooling is called orographic uplift.
When air approaches a topographic barrier, it can be
lifted upward or deflected around the barrier.
Downwind of a mountain ridge, on its leeward side,
air descends the slope and warms by compression
to create a rain shadow effect, an area of lower precipitation.

6.1 Mechanisms that lift air (cont.)

- Fronts are transition zones in which great temperature differences occur
across relatively short distances.
- When cold air advances toward warmer air Æ cold front (a).
- When warm air flows toward a wedge of cold air Æ warm front (b).

6.1 Mechanisms that lift air (cont.)
West Texas Roll Cloud
Provided by: Cindy
Date: Dec. 28, 2006

A cold front moved into western Texas from New Mexico

6.1 Mechanisms that lift air (cont.)
Approaching warm front
Provided by: Val Vannet
Date: May 31, 2005

The view looks towards Dodd Hill from beneath the pylons
which cross the minor road in the east of the grid box. The
high cirrus cloud heralds the approach of a front and rain

6.1 Mechanisms that lift air (cont.)
• Pressure differences set the air
in motion in the effect we call

• When a low-pressure cell is
near the surface, winds in the
lower atmosphere tend to
converge on the center of the
low from all directions.
• Horizontal movement toward a
common location implies an
accumulation of mass called
horizontal convergence.

6.1 Mechanisms that lift air (cont.)
• Free convection is lifting that results from heating the air
near the surface.
• It is often accompanied by updrafts strong enough to form
clouds and precipitation.
• Free convection arises from buoyancy.

6.2 Static stability and the enviromental
lapse rate
• Statically unstable air becomes buoyant when lifted and

continues to rise if given an initial upward push;
• Statically stable air resists upward displacement and sinks
back to its original level when the lifting mechanism ceases.
• Statically neutral air neither rises on its own following an
initial lift nor sinks back to its original level; it simply comes
to rest at the height to which it was displaced.
• Static stability is closely related to buoyancy.
• When an air parcel is less dense than surrounding air Æ
positive buoyancy Æ floats upward.

6.2 Static stability and the enviromental
lapse rate (cont.)
• Temperatures in the parcel are governed by either the

dry or saturated adiabatic lapse rate, whereas the
surroundings are governed by the environmental lapse
rate (ELR).
• Different types of air with regard to their static stability:
absolutely unstable, absolutely stable, conditionally

6.2 Static stability and the enviromental
lapse rate (cont.)

When a parcel of unsaturated or saturated air is lifted
and the Environmental Lapse Rate (ELR)
is greater than the dry adiabatic lapse rate (DALR),
the result is absolutely unstable air.

6.2 Static stability and the enviromental
lapse rate (cont.)

When a parcel of unsaturated or saturated air is lifted
and the Environmental Lapse Rate (ELR) is less than
the saturated adiabatic lapse rate (SALR), the result
is absolutely stable air and the parcel will resist lifting.

6.2 Static stability and the enviromental
lapse rate (cont.)
• When the ELR is between the dry and saturated adiabatic
lapse rates the air is said to be conditionally unstable, and the
tendency for a lifted parcel to sink or continue rising depends
on whether or not it becomes saturated and how far it is lifted.
• The level of free convection (LFC) is the height to which a
parcel of air must be lifted for it to become buoyant and to
rise on its own.

6.2 Static stability and the enviromental
lapse rate (cont.)

Assume the ELR is 0.7 °C/100 m and the air is
unsaturated. As a parcel of air is lifted, its temperature
is less than that of the surrounding air,
so it has negative buoyancy.

6.2 Static stability and the enviromental
lapse rate (cont.)

A parcel starts off unsaturated but cools to the LCL,
where it is cooler than the surrounding air. Further lifting
cools the parcel at the SALR. At the 200-m level, it is
still cooler than the surrounding air, but if taken to 300 m,
it is warmer and buoyant.

6.3 Cloud types
• Clouds can assume a variety of shapes and sizes and can
occur near surface or at high altitude.
• Most cloud types occur in the troposphere.
• Clouds can contain liquid droplets, ice crystals, or a mixture
of the two.
• Clouds can be thick or thin and have high or low liquid water
or ice contents.
• Four basic categories:
- Cirrus: thin, wispy clouds of ice
- Stratus: layered cloud
- Cumulus: clouds having vertical development
- Nimbus: rain-producing clouds

6.3 Cloud types (cont.)

The ten principal types of clouds that result are then grouped according to
their height and form:
- High clouds: cirrus, cirrostratus, and cirrocumulus
- Middle clouds: altostratus and altocumulus
- Low clouds: stratus, stratocumulus, and nimbostratus
- Clouds with vertical development: cumulus and cumulonimbus

6.3 Cloud types (cont.)

High clouds are generally above 6000 m (19,000 ft).
The simplest of the high clouds are cirrus,
which are wispy aggregations of ice crystals.

6.3 Cloud types (cont.)
Cirrostratus clouds are composed entirely
of ice but tend to be more extensive
horizontally and have a lower concentration of crystals.

6.3 Cloud types (cont.)

Cirrocumulus are composed of ice crystals that arrange
themselves into long rows of individual, puffy clouds.
Cirrocumulus form during episodes of wind shear, a condition
in which the wind speed and/or direction changes with height.

6.3 Cloud types (cont.)

Altostratus clouds are the middle-level counterparts to
cirrostratus. They are more extensive and composed primarily
of liquid water.

6.3 Cloud types (cont.)

Altocumulus are layered clouds that form long bands or contain
a series of puffy clouds arranged in rows. They are often gray in
color, although one part of the cloud may be darker than the rest
and consist mainly of liquid droplets rather than ice crystals.

6.3 Cloud types (cont.)

Low clouds have bases below 2000 m. Stratus are layered
clouds that form when extensive areas of stable air are lifted.
Usually the rate of uplift producing a stratus cloud is only a few
tens of centimeters per second, and its water content is low.

6.3 Cloud types (cont.)

Low, layered clouds that yield light precipitation are called
nimbostratus. These clouds look very much like stratus,
except for the presence of precipitation.

6.3 Cloud types (cont.)

Stratocumulus are low, layered clouds with some vertical development.
Their darkness varies when seen from below because their thickness
varies across the cloud. Thicker sections appear dark, and thinner
areas appear as bright spots.

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