How Do the Clouds Form?How Do the Clouds Form?

The global water cycleThe global water cycle

Ocean water covers 70% of the Ocean water covers 70% of the Earth’s surfaceEarth’s surface

More than 1000 major rivers flow on the More than 1000 major rivers flow on the seven continentsseven continents

Amazon river (#1) Yangtze river (#4)Mississippi river (#6)

Water vapor flows in the atmosphereWater vapor flows in the atmosphere

Clouds drifts with the winds Clouds drifts with the winds

Precipitation drives many atmospheric circulations Precipitation drives many atmospheric circulations which in turn transport water around the worldwhich in turn transport water around the world

Hadley Circulation

MonsoonsWalker Circulation

A significant fraction of the human body A significant fraction of the human body is water (~75%)is water (~75%)

The remaining: fat, protein, carbonhydrate, other solids

So the water in our bodies may come from …So the water in our bodies may come from …

A stream in the Niagara Fall …

A white cloud over the tropical Pacific …

A dew drop in the Amazon rainforest …

Therefore we need to protect the environment because any pollution we put into the environment may someday come back into our bodies

An ice sheet in the Arctic Ocean…

Movie time!Movie time!

Weather: Wet

Water (HWater (H22O ) is unique on earth because it O ) is unique on earth because it

can exist in all 3 states (phases)can exist in all 3 states (phases)

• An H2O molecule

• 3 states (gas, liquid, solid) depending on how the molecules are connected together

• Can change from any state to any other state. Latent heat is consumed or released in a phase change

e.g. Evaporation -> liberation of water molecules, requires energy

• Saturation: equilibrium between evaporation and condensation

• Diabatic processes – add/remove heat– Conduction (e.g. movement of air mass over a cold surface)– Radiation (e.g. cooling of boundary layer air by longwave

radiation)

• Adiabatic processes - no addition/removal of heat– Add water vapor to air– Mix warm air with cold air– Cooling of air parcel when it rises (because air parcel expands

when it rises, like a balloon)

1st Law of Thermodynamics expanding air cools, compressed warms (like a manual hand air pump).

Tcp v

Methods to achieve saturation and condensationMethods to achieve saturation and condensation

Forms of Condensation:

• saturation liquid droplets or ice crystals• condensation/deposition dew, frost, fog, clouds

Different types of fog found throughout the U.S.

CloudsClouds• Clouds are instrumental to the Earth’s energy and moisture

balances

• Most clouds form as air parcels are lifted and cooled to saturation (Lowering temperature to dew point cloud formation)

• Clouds have interesting distributions

Satellite observation of clouds - ProjectsSatellite observation of clouds - Projects

• NASA’s International Satellite Cloud Climatology Project (ISCCP) Combine the measurements of 5 geostationary and 1-2 polar orbiting satellites. 1983-Now, cloud top height and optical depth.

• NASA’s Earth Observation System including a set of polar orbiting satellites (A-Train), especially CloudSat (with a cloud radar) and CALIPSO (with a cloud lidar). Ongoing, cloud particle information, detailed vertical structure.

1) rate of cooling of parcel (unsaturated v. saturated)• unsaturated – dry adiabatic lapse rate (DALR)• saturated – saturated adiabatic lapse rate (SALR)

2) rate of cooling of surrounding atmosphere• environmental lapse rate (ELR): an overall

decrease in air temperature with height• ELR is measured by weather balloon. Long-term

global average ELR = 0.65oC/100m.

Three possible outcomes:1) absolutely unstable air2) absolutely stable air

3) conditionally unstable air

Static Stability and the Environmental Lapse Rate

• Static stability – refers to atmosphere’s susceptibility to being displaced• Stability related to buoyancy function of temperature• The rate of cooling of a parcel relative to its surrounds determines its ‘stability’ of a parcel

Environment

ParcelEnvironment

ParcelEnvironment

Parcel

The three types of stability

AbsolutelyUnstable

ConditionallyUnstable

AbsolutelyStable

1) Entrainment• Turbulent mixing of ambient air into parcel • Leads to evaporation along cloud boundaries• Evaporation uses latent heat, cooling the cloud

reduces buoyancy

Courtesy Russ Dickerson, U. Maryland

What stops ‘unstable’ air masses from rising indefinitely ?

2) Encountering a layer of stable air (inversion)• a rising parcel may reach a stable upper air environment • the parcel cooling rate will exceed that of the ambient air• the parcel will slowly cease ascension and come to rest at some equal temperature level• three types: radiation, frontal, subsidence

1. Cloud top height/pressure

2. Cloud thickness (optical depth)

3. Cloud coverage

• When clouds comprise more than 9/10th of the sky = overcast

• When coverage is between 6/10th and 9/10th = broken

• When coverage is between 1/10th and 6/10th = scattered

• Cloud coverage less than 1/10th = clear

Cloud Properties

NASA’s International Satellite Cloud Climatology Project (ISCCP) NASA’s International Satellite Cloud Climatology Project (ISCCP) Cloud Classification - commonly used in climate researchCloud Classification - commonly used in climate research

• Clouds are good reflectors of solar (shortwave) energy and good absorbers of earth’s (longwave) energy.

• The net effect (cooling or warming) depends on the type of cloud

• In a changing climate, increases in some types of clouds would promote warming, while increases in others would cause cooling

• Climate models are limited in how well they can simulate clouds:• We don’t know all of the processes that cause clouds• Clouds form at scales much smaller than models can resolve

• Conclusion: clouds cause probably the largest uncertainty in model simulations of future climate. Improving this is a major focus of climate researchers

Why do clouds constitute a wildcard for climate change?

• Global water (hydrological) cycle

• Water Vapor Basics (names of different phase changes, latent heat)

• Two methods of achieving saturation and condensation (diabatic vs. adiabatic processes). Different types of condensation - dew, frost, fog (radiation, advection, upslope, precipitation, steam), clouds.

• Formation of clouds: 3 types of stability. Two factors limiting the height of clouds. 3 cloud properties. 9 ISCCP cloud types.

Summary