how do the clouds form?. the global water cycle ocean water covers 70% of the earth’s surface
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How Do the Clouds Form?How Do the Clouds Form?
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The global water cycleThe global water cycle
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Ocean water covers 70% of the Ocean water covers 70% of the Earth’s surfaceEarth’s surface
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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)
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Water vapor flows in the atmosphereWater vapor flows in the atmosphere
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Clouds drifts with the winds Clouds drifts with the winds
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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
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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
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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…
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Movie time!Movie time!
Weather: Wet
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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
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• 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
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Forms of Condensation:
• saturation liquid droplets or ice crystals• condensation/deposition dew, frost, fog, clouds
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Different types of fog found throughout the U.S.
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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
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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.
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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
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Environment
ParcelEnvironment
ParcelEnvironment
Parcel
The three types of stability
AbsolutelyUnstable
ConditionallyUnstable
AbsolutelyStable
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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
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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
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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
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• 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?
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• 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