4. Cloud Physics Flashcards
Describe how the Raoult effect influences droplet formation
Water vapour condenses on a droplet when its pressure is greater than the SVP of the droplet (air will always be supersaturated above a droplet).
A high concentration solute (like sea water) lowers the SVP of a droplet (compared to pure water). This makes it easier for condensation to occur on the droplet.
The larger the droplet, the easier it is for condensation to occur (larger surface area)
Due to the fixed number of solute particles, as the droplet grows, the Raoult effect reduces.
Describe how the Kelvin effect influences droplet formation
Water vapour condenses on a droplet when its pressure is greater than the SVP of the droplet.
Kelvin states that the SVP over a curved surface is higher than that over a flat surface. This discourages condensation on droplets, with a greater effect when droplets are smaller.
Surface tension effect tends to make the droplet evaporate at an ever increasing rate as its size decreases
As water vapour condenses into the nucleation site, the latent heat is released (increasing the droplet temperature) which increases its SVP too.
Therefore as droplets grow, the Kelvin effect encourages evaporation over condensation.
Draw a graph of droplet diameter against saturation ratio, mark r* and S*. What do these correspond to?
Graph. with r* and S* corresponding to the peak.
For a droplet to grow, the super saturation of the air must be greater than S. Once past radius of r, the saturation can be lower for the droplet to continue growing.
Define saturation ratio
Saturation ratio is the existing vapour pressure divided by the vapour pressure at the same temperature over a flat surface of water.
What is a deliquescent particle
A particle (solid like salt) that absorbs water and dissolves, vapour pressure surrounding the droplet then drops, countering Kelvin
Draw and annotate a graph of Diameter against Relative humidity for a deliquescent particle becoming a droplet and vice-versa
Graph: ............./ droplet ............/ |------ / drop formation |...... / |---- crystallisation | | solid particle
What is saturated vapour pressure (SVP)
Its the pressure of water vapour above a flat surface of water at equilibrium (strong function of temperature, surface topology and nature of solution, ie strength of solute)
What effect does surface topology have on SVP
Vapour pressure over a convex water surface (sphere) is higher than that over a flat one
Why is it almost impossible to start condensation on a water molecule?
Characteristic length is too small
Requires a nucleation site (salt, soot, etc.)
What is the overall effect of Raoult’s Law and Kelvins Law combined?
Relatively easy to start to grow the droplet, under Raoult’s law
As the size (radius) increases the curvature and Kelvin’s law dominate and make it harder for condensation
What effect does ice formation have?
Ice has a lower SVP than super cooled water, meaning ice crystals can grow faster than water droplets
(Often water droplets start out as small ice crystals which then thaw and melt)
Give some natural and man made examples of aerosols
Natural: Volcanoes, Dust storms, Sea spray, pollen
Man-made: Fossil burning, agri-activities, industry
Why are aerosols important?
Act as nucleation sites for droplet formation
Scatter and reflect sunlight (increasing albedo of planet and reducing global warming) (sulfates)
Absorb solar and long wave radiation (acting as a thermal blanket, increasing global warming) (dust and soot)
