Cloud Formation

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

For adiabatic systems gases cool as they expand.

For adiabatic systems gases warm as they contract.

∆ H - Change in heat p - pressure∆V - Volume Change Cv - Specific heat∆ T - Temperature ChangeAdiabatic process - process in which

temperature changes but heat energy is not added or removed.

p . ∆V = - Cv . ∆T

For adiabatic systems gases cool as they expand.

For adiabatic systems gases warm as they contract.

∆ H - Change in heat p - pressure∆V - Volume Change Cv - Specific heat∆ T - Temperature ChangeAdiabatic process - process in which

temperature changes but heat energy is not added or removed.

p . ∆V = - Cv . ∆T

For adiabatic systems gases cool as they expand.

For adiabatic systems gases warm as they contract.

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 Air1) 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 AirRain Shadow DesertsNevadaGobi Desert, China

Mechanisms That Lift Air2) 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 Air2) 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 Air2) 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 AirPressure 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.

StabilityOnce 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 densitylow density air tends to rise upward.high density air tends to sink.

Warmer air is less dense and will tend to riseCooler air is more dense and will resist rising

StabilityIf 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

StabilityAbsolutely unstableAbsolutely stableConditionally unstable

400

300

200

100

0

Hei

ght

(m)

ELR (1.5) DALR (1.0) SALR (0.5)

oC/100m

10oC 10oC10oC

8.5oC

7oC

5.5oC

9oC 9.5oC

8oC 9oC

7oC 8.5oC

StabilityAbsolutely 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

200

100

0

Hei

ght

(m)

ELR (0.3) DALR (1.0) SALR (0.5)

oC/100m

10oC 10oC10oC

9.7oC

9.4oC

9.1oC

9oC 9.5oC

8oC 9oC

StabilityAbsolutely 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

200

100

0

Hei

ght

(m)

ELR (0.7) DALR (1.0) SALR (0.5)

oC/100m

10oC 10oC10oC

9.3oC

8.8oC

8.1oC

9oC 9.5oC

8oC 9oC

7oC 8.5oC

StabilityConditionally 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 dropletsIce crystalsAir

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 TypesFirst widely accepted system for classification of clouds:

Luke Howard (1803) - English naturalist(4 basic categories)

1) Cirrus (Latin for curl) - thin wispy clouds of ice.2) Stratus (Latin for layer) - layered clouds.3) Cumulus (Latin for heap) - clouds having vertical development4) Nimbus (Latin for rain) - rain clouds

Today we use a modified version of Howard’s classification scheme.

Cloud TypesTen major cloud types grouped by their shape and height

A. High Clouds > 6000mCirrusCirrostratusCirrocumulus

B. Middle Clouds 2000 - 6000mAltostratusAltocumulus

Cloud TypesTen major cloud types grouped by their shape and height

C. Low Clouds <2000 mStratusStratocumulusNimbostratus

D. Clouds with vertical developmentCumulusCumulonimbus

Cloud TypesA. High Clouds

above 6000m 1900 feet)

Almost always composed of iceTemperatures <-35oC

a. Cirrus (Ci)• wispy aggregates of ice crystals• ~1.5 km thickFall streaks - ice crystals in cirrus clouds fall and descend below the cloud.

Usually form in fair weather.

Cloud TypesA. 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 TypesB. Middle Clouds

2000-6000 feetcomposed of liquid dropletsalto - 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 TypesB. Middle Clouds

b. altocumulus (Ac)- layered clouds that form long bands of puffs.- often gray in color- mostly liquid

Usually form by convection in unstable air, as a cold front approaches.

Cloud TypesC. 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 TypesC. Low Clouds

c. stratocumulus (Sc)- low layered clouds with some

vertical development.- thicker sections may appear

dark.

Cloud TypesD. Clouds with vertical development

a. Cumuloform (Cu)- clouds that have substantial vertical

displacement. Several subtypes- cumulus humilis - fair weather cumulus

- cumulus congestus - multiple towers of clouds

Cloud TypesD. 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 TypesUnusual Cloud types

Lenticular cloudslens-shapedform downwind of moutain barrierscan appear to stay stationary

Banner cloudsSimilar to Lenticular but form over mountain tops.

Mamatus Cloudscumulonimbus clouds with portions that hang down below the cloud base.

Nacreous cloudssoft white clouds that are present in the stratosphere (heights of ~30 km)

Cloud Types