How Do the Clouds Form?

1. How Do the Clouds Form?

2. The global water cycle

3. Ocean water covers 70% of the Earth’s surface

4. More than 1000 major rivers flow on the seven continents Mississippi river (#6) Amazon river (#1) Yangtze river (#4)

5. Water vapor flows in the atmosphere

6. Clouds drifts with the winds

7. Precipitation drives many atmospheric circulations which in turn transport water around the world Hadley Circulation Monsoons Walker Circulation

8. A significant fraction of the human body is water (~75%) The remaining: fat, protein, carbonhydrate, other solids

9. So the water in our bodies may come from … A stream in the Niagara Fall … A dew drop in the Amazon rainforest … A white cloud over the tropical Pacific … An ice sheet in the Arctic Ocean… Therefore we need to protect the environment because any pollution we put into the environment may someday come back into our bodies

10. Movie time! Weather: Wet

11. Water (H2O ) is unique on earth because it 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

12. 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) Methods to achieve saturation and condensation 1st Law of Thermodynamics  expanding air cools, compressed warms (like a manual hand air pump).

13. Forms of Condensation: • saturation  liquid droplets or ice crystals • condensation/deposition  dew, frost, fog, clouds

14. Different types of fog found throughout the U.S.

15. Clouds • 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

16. Satellite 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.

17. 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 • 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: • absolutely unstable air • absolutely stable air • conditionally unstable air

18. The three types of stability Environment Parcel Parcel Parcel Environment Environment Absolutely Unstable Absolutely Stable Conditionally Unstable

19. What stops ‘unstable’ air masses from rising indefinitely ? • 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 • 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

20. Cloud Properties 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

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

22. Why do clouds constitute a wildcard for climate change? • 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

23. Summary • 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.