Clouds and Precipitation Flashcards
When will condensation occur
When e = es and RH =100%
What RH is required for pure water droplet formation?
300%
What is the difference of SVP between a curved surface and a plane surface?
SVP curved surface > SVP plane surface
What is supersaturation?
This is the state of a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances.
What is supercooling?
The is the process of lowering the temperature of a liquid or gas below its freezing point without it becoming a solid.
How is supercooling possible?
A liquid will crystallize below its freezing point when there is the presence of nuclei for the crystal structure to form around. Without nuclei present the liquid phase can be maintained at a much lower temperature.
What is homogeneous nucleation?
Nucleation is the formation of a distinct thermodynamic phase. Nucleation without preferential nucleation sites is called homogeneous nucleation and requires supercooling or superheating to occur.
What are CCNN’s?
These are cloud condensation nuclei - small particles around 2um on which water vapour condenses.
Why are CCNN’s important?
Because water requires a non-gaseous surface to make the transition from gas to liquid.
How can water form with out the presence of hygroscopic nuclei?
When no hygroscopic nuclei is present, water vapour can be supercooled to around -13C for 5-6 hours before water droplets spontaneously form.
How are Phytoplankton linked to global warming in terms of the presence of CCNN’s?
Phytoplankton produce Dimethyl Sulphide (DMS) - DMS helps to form Sulphate Aerosols, (SO4^2-) - Suplphae aerosols act as hygroscopic nuclei - Increased global temperatures can lead to larger algal blooms - Larger algal blooms can lead to more production of Dimethyl Sulphide.
Name the two main paths to saturation
Adiabatic cooling through forced convection and Free convection
How is Radiation fog formed?
This is formed by thermal radiation in calm conditions; air in contact with the ground cools and saturates the air through heat conduction; wind lifts the saturated parcel into the air to be replaced with new air and the process repeats.
How is Advection Fog formed?
This fog is most common at sea; when warm moist air passes over the sea it is cooled. This is common when a warm front passes over the sea or an area with significant snow-pack. Can be common also in warmer areas that experience cold water upwelling such as along the California Coast.
How is frost formed?
This is the solid deposition of water vapour from humid air. Vegetation can act as a condensation nuclei when it is below the freezing point of water. If a solid surface is chilled below the dew point of the surrounding air then freezing frost will form.
What is the frost point?
Similar to dew point - temperature to which a given parcel of air must be cooled at constant barometric pressure for water vapour to be deposited on the surface as ice without going into liquid phase.
What is the Bergeron - Findeisen Theory?
Theory based on the fact that at subzero temperatures the atmospheric vapour pressure decreases more rapidly over an ice surface than over water.
What are freezing nuclei?
These are necessary for ice particles to form - ususually at temperatures of about -15 to -25C.
What is different between freezing nuclei and condensation nuclei?
Freezing nuclei are far less numerous than condensation nuclei
Why do ice crystals readily aggregate upon collision?
Because of their dendritic, (branched) shape. Temperatures between 0 and -5C are favorable to aggregation due to water film that helps to bind crystals.
What are the defining characteristics of cirriform, stratoform and cumuloform clouds?
High cirriform cloud is composed of ice crystals giving a fibrous appearance. Stratiform clouds are in layers, while cumuliform clouds have a heaped appearance with vertical development.
Give examples of hygroscopic nuclei
Sea-salts; fine soil particles; natural, industrial and domestic combustion products raised by the wind. Also conversion of atmospheric trace gas to particles though photochemical reactions.
Why is it important that hygroscopic aeorsols are soluble?
Because the saturation vapour pressure is less over a solution droplet than over a pure water drop of the same size and temperature.
What happens when the size of a droplet increases?
Growth rate by condensation decreases but growth rate by coalescence increases.
What is condensation rate limited by?
By the speed with which the released latent heat can be lost from the drop by conduction to the air.Also competition between droplets for the available moisture.
Explain coalescence
Falling raindrops have terminal velocities directly related to their diameters, such that larger drops can overtake and absorb smaller droplets. Coalescence is initially slow however droplets reach 100-200um radius in 50 minutes.
Explain the seeder feeder mechanism for cloud growth
Low altitude clouds are fed from above and can be typical of mountain regions. Can lead to very heavy short term rainfall. It is when precipitation from upper level clouds falls to the lower level orographic stratus cloud that caps a hill or small mountain.
How do clouds dissipate?
Radiative heating; adiabatic heating; mixing (entrainment); removal of moisture and heat
Explain adiabatic heating
When air sinks it warms adiabatically; warming will induce evaporation and erosion of the cloud. This can occur when dynamic sinking mechanisms increase over a cloud field.
Explain mixing (entrainment)
A cloud doesnt remail perfectly adiabatic - environmental air can mix into the cloud mass. A cloud that is no longer developing and not adding additional condensation moisture will be eroded by the encorperated drier environmental air that induces evaporation.
Explain dissipation in terms of removal of moisture and heat
When temperature increases, the air in the cloud has a higher capacity to evaporate water leading to reduced liquid moisture content of the cloud. The potential for daytime heating to erode a cloud depends on the aspect of the sun, cloud thickness and overall stability and lift in the troposphere.
