Clouds and Precipitation

Question 1

When will condensation occur

Answer 1

When e = es and RH =100%

Question 2

What RH is required for pure water droplet formation?

Answer 2

300%

Question 3

What is the difference of SVP between a curved surface and a plane surface?

Answer 3

SVP curved surface > SVP plane surface

Question 4

What is supersaturation?

Answer 4

This is the state of a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances.

Question 5

What is supercooling?

Answer 5

The is the process of lowering the temperature of a liquid or gas below its freezing point without it becoming a solid.

Question 6

How is supercooling possible?

Answer 6

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.

Question 7

What is homogeneous nucleation?

Answer 7

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.

Question 8

What are CCNN’s?

Answer 8

These are cloud condensation nuclei - small particles around 2um on which water vapour condenses.

Question 9

Why are CCNN’s important?

Answer 9

Because water requires a non-gaseous surface to make the transition from gas to liquid.

Question 10

How can water form with out the presence of hygroscopic nuclei?

Answer 10

When no hygroscopic nuclei is present, water vapour can be supercooled to around -13C for 5-6 hours before water droplets spontaneously form.

Question 11

How are Phytoplankton linked to global warming in terms of the presence of CCNN’s?

Answer 11

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.

Question 12

Name the two main paths to saturation

Answer 12

Adiabatic cooling through forced convection and Free convection

Question 13

How is Radiation fog formed?

Answer 13

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.

Question 14

How is Advection Fog formed?

Answer 14

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.

Question 15

How is frost formed?

Answer 15

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.

Question 16

What is the frost point?

Answer 16

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.

Question 17

What is the Bergeron - Findeisen Theory?

Answer 17

Theory based on the fact that at subzero temperatures the atmospheric vapour pressure decreases more rapidly over an ice surface than over water.

Question 18

What are freezing nuclei?

Answer 18

These are necessary for ice particles to form - ususually at temperatures of about -15 to -25C.

Question 19

What is different between freezing nuclei and condensation nuclei?

Answer 19

Freezing nuclei are far less numerous than condensation nuclei

Question 20

Why do ice crystals readily aggregate upon collision?

Answer 20

Because of their dendritic, (branched) shape. Temperatures between 0 and -5C are favorable to aggregation due to water film that helps to bind crystals.

Question 21

What are the defining characteristics of cirriform, stratoform and cumuloform clouds?

Answer 21

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.

Question 22

Give examples of hygroscopic nuclei

Answer 22

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.

Question 23

Why is it important that hygroscopic aeorsols are soluble?

Answer 23

Because the saturation vapour pressure is less over a solution droplet than over a pure water drop of the same size and temperature.

Question 24

What happens when the size of a droplet increases?

Answer 24

Growth rate by condensation decreases but growth rate by coalescence increases.

Question 25

What is condensation rate limited by?

Answer 25

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.

Question 26

Explain coalescence

Answer 26

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.

Question 27

Explain the seeder feeder mechanism for cloud growth

Answer 27

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.

Question 28

How do clouds dissipate?

Answer 28

Radiative heating; adiabatic heating; mixing (entrainment); removal of moisture and heat

Question 29

Explain adiabatic heating

Answer 29

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.

Question 30

Explain mixing (entrainment)

Answer 30

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.

Question 31

Explain dissipation in terms of removal of moisture and heat

Answer 31

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.