condensation of clouds droplet (micro-scale) Flashcards
what is the radius of a typical raindrop and a CCN
raindrop- 1000 micrometer/ 1mm
CCN- 0.1 miromemter
describe surface tension
H2O molecules are pulled towards one another and downwards, water surface contracts, creating a barrier to external objects
describe a particle in a stable state
it will return to its original position after an initial kick
describe a particle in an unstable state
the particle will continue to move after an initial kick. does not return to its original position
describe a particle in a neutral state
the particle is on a planar surface, does not show vertical movement
describe a particle in a conditional instability state
under certain condition it is stable and doesn’t have vertical motion. but it can always become unstable and continue to move vertically.
compare homogeneous and heterogeneous nucleation
homo- consists of pure water drops only, without the presence of condensation nuclei (CCN). it further grows by condensation
heter- consists of water and other particles eg. aerosols
in homogenous nucleation, how do droplets grow
-by collision of H2O molecules (condensation)
- quicker growth if there’s higher water vapour availability
grow= more condensation then evaporation
in homogenous nucleation, what affects the decay rate of droplets
- decay= more evaporation then condensation
- higher temp= more evaporation
- less surface tension- less force holding molecules tgt–> more evaporation
define equilibrium of droplets
evaporation=condensation
what equation is the saturation vapour pressure curve based on?
Clausius-Clapeyron equation.
with function of temp to water vapour pressure
discuss curvature effect with respect to sizes
larger drop- less curved (more like planar). with neighbouring particle hence stronger surface tension. easy to form droplets
small drops- more curved, easier for molecule to leave near the surface as fewer supportive neighbouring particle. harder to form droplets
discuss curvature effect on the equilibrium vapour pressure
larger drop- despite less curved, supersaturation needed for H2O to collide and grow and form droplets
small drops- more curved, a even higher supersaturation is needed for H2O to collide and form droplets
true or false:
in homogenous nucleation, RH has to be greater than 115% in order to grow/reach equilibrium
true. it is very hard for pure H2O molecules to collide without help. so the vapour pressure has to be very high in order for them to collide and remain at equilibrium
the Smaller the cloud droplet, the curver it is, the higher the RH needed to maintain equilibrium
true. has to be supersaturated to maintain equilibrium.
- If more RH is present than required for equilibrium
Cloud droplet grows by
condensation
- once a droplet has grown enough, it can grow bigger easily due to less curation
what does the Kelvin equation show
the relationship between the cloud droplet’s radius and the relative humidity required to reach equilibrium. the greater the particle, lesser the RH required for droplets to grow
why is the equilibrium in the Kelvin equation said to be unstable?
- because it causes a positive feedback. if evaporation occurs, then the droplets will continues to shrinks easily. if condensation occurs, the droplet will continue to grow easily.
- it will not return to its original positive.
why is homogenous nucleation improbable
because it is rare for supersaturation to happen. hence rain will rarely form
facts
supersaturation is not expressed as over 100. it is expressed as saturation value -1
eg. 105% RH= 5% supersaturation
saturation ratio
it is the relative humidity but not expressed as a percentage.
so 1.01 = 101%
facts
at a given RH, a droplets must be at its corresponding critical radius in order for it to remain stable.
-to grow, the radius has to exceed its critical radius
true or false:
lower concentration of CCN (eg. aerosol) can be found in marine air than in continental air
true. because there are less pollutant found in the ocean than in continents
true or false:
despite having a lower CCN concentration in marine clouds, the droplets are larger
true. eg. fog and cloud droplets near coast are large
why does the presence of aerosol particle makes the development of cloud droplets more probable?
aerosol can be
- wettable- attracts water
- hygroscopic- dissolvable in water
- presence of a dissolved solute lowers the equilibrium vapour pressure –> harder for water droplets to be evaporation.
- condensation is occur when RH< 100%, easier to reach saturation
give some examples of aerosols
- soot
- sea salt
- dust
- volcanic ash
what is the purpose of Raoult’s Law
to find the vapour pressure equilibrium over the solution (water and aerosol)
what does a higher solute concentration mean to the condensation of cloud droplets
higher solute conc. means a stronger solute effect.
- droplet can reach a equilibrium with the environment at a lower supersaturation compared to pure water droplet of same size
- condensation can happen when RH <100%
why would a stable equilibrium occur in a solution with aerosol
- aerosols attracts more water molecules, this dilute the aerosol concentration.
- evaporation increases due to the presence of more water vapour, droplets then goes back to equilibrium size
solute effect favours ________ droplets while curvature effect favours ________ droplets
- small
- large
what does solute effect favour small droplets
-increasing radius increases the vapour pressure equilibrium of the solution, hence harder to reach saturation and grow
what is the Köhler curve
combination of Kelvin’s equation and Raoult’s law
true or false:
in the Köhler curve, condensation can happen below 100% RH.
true. this means rainfall can form below 100% saturation
what does the hump in the Köhler curve show
the hump defines a critical radius and the critical saturation ratio
what does the area above the Köhler curve mean
- the vapour pressure of the drop is lower than the environment.
- drop grows by condensation
- water vapour can be added to the drop
what does the area below the Köhler curve mean
- the vapour pressure of the drop is higher than the environment.
- drop shrinks by evaporation
- water vapour is lost to the surrounding
true or false:
smaller droplets grow faster by condensation but always end up having the same radius as other droplets
true
droplets growth involves processes other than condensation
true
