Cloud Formation
Formation of clouds requires a
mechanism that results in the uplift of
air.
First, recall
Adiabatic process - process in which
temperature changes but heat energy is not
added or removed.
p . ∆V = - Cv . ∆T
Adiabatic process - process in which
temperature changes but heat energy is not
added or removed.
∆ H - Change in heat
p - pressure
∆V - Volume Change
Cv - Specific heat
∆ T - Temperature Change
For adiabatic systems gases warm as they
contract.
For adiabatic systems gases cool as they
expand.
p . ∆V = - Cv . ∆T
Adiabatic process - process in which
temperature changes but heat energy is not
added or removed.
∆ H - Change in heat
p - pressure
∆V - Volume Change
Cv - Specific heat
∆ T - Temperature Change
For adiabatic systems gases warm as they
contract.
For adiabatic systems gases cool as they
expand.
p . ∆V = - Cv . ∆T
For adiabatic systems gases warm as they
contract.
For adiabatic systems gases cool as they
expand.
dry adiabatic lapse rate (DALR) –
rate at which a rising parcel of dry (unsaturated) air cools.
about 1oC/100m (5.5oF/1000 feet)
Lifting Condensation level (LCL) –
the height at which a rising parcel of air undergoes condensation or deposition.
Saturated adiabatic lapse rate (SALR) –
rate at which a rising parcel of wet (saturated) air cools.
about 0.5oC/100m (3.3oF/1000 feet)
Environmental Lapse Rate (ELR) - rate at temperature in the troposphere
decreases with height.
Mechanisms That Lift Air
1) Orographic Lifting
2) Frontal Lifting
3) Convergence
4) Convection
Mechanisms That Lift Air
1) Orographic Lifting - the forcing of air above a
mountain barrier.
-air flowing toward a topographic high (hill or
mountain) will be deflected over the barrier and
displaced upward.
-downwind of the mountain, air descends the
slope and warms by compression
Rain Shadow - an area on the downwind side of a
mountain barrier having relatively low precipitation.
Mechanisms That Lift Air
Rain Shadow Deserts
Nevada
Gobi Desert, China
Mechanisms That Lift Air
2) Frontal lifting - displacement of one air mass
over another.
Front - A transition zone between two dissimilar
air masses with differing temperatures and/or
moisture. Results in clouds in two ways.
Mechanisms That Lift Air
2) Frontal lifting - displacement of one air mass
over another.
Front - A transition zone between two dissimilar
air masses with differing temperatures and/or
moisture. Results in clouds in two ways.
a) Cold Front - cold air mass advancing toward a
warmer air mass.
The denser cold air displaces the lighter warm air
ahead of it.
Results on cumulonimbus clouds.
Mechanisms That Lift Air
2) Frontal lifting - displacement of one air mass
over another.
b) Warm Front - warm air mass advancing
toward a cooler air mass.
The warm air rides over the wedge of cold air as
if it is rising over a mountain barrier.
Overrunning - warm air flows upward over the
cold air.
Results in nimbostratus clouds.
Mechanisms That Lift Air
Pressure differences in the atmosphere set air in motion
causing winds.
When there is a center of low pressure, winds tend to
converge toward the center from all directions
3) Convergence - horizontal movement of air toward a center
of low pressure.
4) Convection - heating air near the surface causes the air to
become less dense and rise upward.
Stability
Once given an initial boost air may continue to rise.
Other times it may resist rising
Stable air - resists rising.
Unstable air - air that will continue to rise once given it
initial boost.
Stability is related to density
low density air tends to rise upward.
high density air tends to sink.
Warmer air is less dense and will tend to rise
Cooler air is more dense and will resist rising
Stability
If a parcel of rising air cools at a rate that makes it colder than
its surrounding air, it will become relatively dense - this
inhibits rising.
If the lifted air cools more slowly than its surrounding air, it
will become warm and will continue to rise upward.
The cooling of air is governed by the DALR or the SALR
Stability
Absolutely unstable
Absolutely stable
Conditionally unstable
400
300
5.5oC
7oC
8.5oC
200
7oC
8oC
9o C
100
8.5oC
9oC
9.5oC
0
10oC
10oC
10oC
ELR (1.5)
DALR (1.0)
oC/100m
SALR (0.5)
Stability
Absolutely Unstable Air
Once lifted it continues rise upward
Rule 1: Whenever the ELR exceeds the DALR, the
air is absolutely unstable and will continue to rise
once lifted, regardless of whether it is saturated or
not.
400
300
9.1oC
200
9.4oC
8oC
9o C
100
9.7oC
9oC
9.5oC
0
10oC
10oC
10oC
ELR (0.3)
DALR (1.0)
oC/100m
SALR (0.5)
Stability
Absolutely Stable Air
Lifted air that has a negative buoyancy and
sinks back down to its initial position.
Rule 2: Whenever the ELR is less than the SALR,
the air is absolutely stable and will resist lifting,
regardless whether it is saturated or not.
400
300
8.1oC
7oC
8.5oC
200
8.8oC
8oC
9o C
100
9.3oC
9oC
9.5oC
0
10oC
10oC
10oC
ELR (0.7)
DALR (1.0)
oC/100m
SALR (0.5)
Stability
Conditionally Unstable Air
The tendency of the air to rise depends on
whether it is saturated or not.
Rule 3: Whenever the ELR is between the DALR
and the SALR, the air is conditionally unstable and
whether it rise or sinks depends on whether it
becomes saturated or not.
Composition of Clouds
Water droplets
Ice crystals
Air
Cloud Types
General Characteristics of Clouds
• Assume a variety of shapes
• Most occur within the troposphere
• Contain a combination of liquid droplets and ice
crystals
Cloud Types
First widely accepted system for classification of clouds:
Luke Howard (1803) - English naturalist
(4 basic categories)
1)
2)
3)
4)
Cirrus (Latin for curl) - thin wispy clouds of ice.
Stratus (Latin for layer) - layered clouds.
Cumulus (Latin for heap) - clouds having vertical
development
Nimbus (Latin for rain) - rain clouds
Today we use a modified version of Howard’s classification
scheme.
Cloud Types
Ten major cloud types grouped by their shape and height
A. High Clouds > 6000m
Cirrus
Cirrostratus
Cirrocumulus
B. Middle Clouds 2000 - 6000m
Altostratus
Altocumulus
Cloud Types
Ten major cloud types grouped by their shape and height
C. Low Clouds <2000 m
Stratus
Stratocumulus
Nimbostratus
D. Clouds with vertical development
Cumulus
Cumulonimbus
Cloud Types
A.
High Clouds
above 6000m 1900 feet)
Almost always composed of ice
Temperatures <-35oC
a. Cirrus (Ci)
• wispy aggregates of ice crystals
• ~1.5 km thick
Fall streaks - ice crystals in cirrus clouds fall and
descend below the cloud.
Usually form in fair weather.
Cloud Types
A.
High Clouds
b. Cirrostratus (Cs)
• Composed entirely of ice
• Moon or sun can be viewed through these
clouds but will have a whitish appearance
• Sometimes light from the sun or moon will be
bent by the ice and create a halo around the sun
or moon.
c. Cirrocumulus (Cc)
• Ice crystals arranged in rows of individual
puffy clouds
• Rows form from wind shear - wind changing
speed or direction.
• Mackeral sky
Cloud Types
B. Middle Clouds
2000-6000 feet
composed of liquid droplets
alto - middle
a. altostratus (As)
- similar to cirrostratus but at middle
level in the atmosphere.
- composed of liquid
- scatter back a large percentage of
incoming solar radiation, reducing the
sunlight reaching the surface.
- No shadows
- No halo as in cirrostratus
Cloud Types
B. Middle Clouds
puffs.
b. altocumulus (Ac)
- layered clouds that form long bands of
- often gray in color
- mostly liquid
Usually form by convection in unstable air, as a cold
front approaches.
Cloud Types
C. Low Clouds
cloud base below 2000m
a. stratus (St)
- layered clouds.
- form from extensive areas of
unstable air
- can exceed several states in extent
b. nimbostratus (Ns)
- low, layered clouds that yield
precipitation.
- Usually light steady precipitation
Cloud Types
C. Low Clouds
c. stratocumulus (Sc)
- low layered clouds with some
vertical development.
- thicker sections may appear dark.
Cloud Types
D. Clouds with vertical development
a. Cumuloform (Cu)
- clouds that have substantial vertical
displacement.
Several subtypes
- cumulus humilis - fair weather cumulus
clouds
- cumulus congestus - multiple towers of
Cloud Types
D. Clouds with vertical development
b. Cumulonimbus (Cb)
- clouds that have substantial vertical
displacement that produce precipitation.
- can extent from a few hundred meters
from the ground to the top of the troposphere.
- anvil shaped top composed of ice.
- hailstones - form in the anvel
- updrafts and downdrafts causes
turbulence.
Unusual Cloud Types
Unusual Cloud Types
Unusual Cloud Types
Cloud Types
Unusual Cloud types
Lenticular clouds
lens-shaped
form downwind of moutain barriers
can appear to stay stationary
Banner clouds
Similar to Lenticular but form over mountain tops.
Mamatus Clouds
cumulonimbus clouds with portions that hang down below the
cloud base.
Nacreous clouds
soft white clouds that are present in the stratosphere (heights of
~30 km)
Cloud Types