condensation of clouds droplet (micro-scale)

Question 1

what is the radius of a typical raindrop and a CCN

Answer 1

raindrop- 1000 micrometer/ 1mm

CCN- 0.1 miromemter

Question 2

describe surface tension

Answer 2

H2O molecules are pulled towards one another and downwards, water surface contracts, creating a barrier to external objects

Question 3

describe a particle in a stable state

Answer 3

it will return to its original position after an initial kick

Question 4

describe a particle in an unstable state

Answer 4

the particle will continue to move after an initial kick. does not return to its original position

Question 5

describe a particle in a neutral state

Answer 5

the particle is on a planar surface, does not show vertical movement

Question 6

describe a particle in a conditional instability state

Answer 6

under certain condition it is stable and doesn’t have vertical motion. but it can always become unstable and continue to move vertically.

Question 7

compare homogeneous and heterogeneous nucleation

Answer 7

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

Question 8

in homogenous nucleation, how do droplets grow

Answer 8

-by collision of H2O molecules (condensation)
- quicker growth if there’s higher water vapour availability
grow= more condensation then evaporation

Question 9

in homogenous nucleation, what affects the decay rate of droplets

Answer 9

• decay= more evaporation then condensation
• higher temp= more evaporation
• less surface tension- less force holding molecules tgt–> more evaporation

Question 10

define equilibrium of droplets

Answer 10

evaporation=condensation

Question 11

what equation is the saturation vapour pressure curve based on?

Answer 11

Clausius-Clapeyron equation.

with function of temp to water vapour pressure

Question 12

discuss curvature effect with respect to sizes

Answer 12

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

Question 13

discuss curvature effect on the equilibrium vapour pressure

Answer 13

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

Question 14

true or false:

in homogenous nucleation, RH has to be greater than 115% in order to grow/reach equilibrium

Answer 14

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

Question 15

the Smaller the cloud droplet, the curver it is, the higher the RH needed to maintain equilibrium

Answer 15

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

Question 16

what does the Kelvin equation show

Answer 16

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

Question 17

why is the equilibrium in the Kelvin equation said to be unstable?

Answer 17

• 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.

Question 18

why is homogenous nucleation improbable

Answer 18

because it is rare for supersaturation to happen. hence rain will rarely form

Question 19

facts

Answer 19

supersaturation is not expressed as over 100. it is expressed as saturation value -1
eg. 105% RH= 5% supersaturation

Question 20

saturation ratio

Answer 20

it is the relative humidity but not expressed as a percentage.
so 1.01 = 101%

Question 21

facts

Answer 21

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

Question 22

true or false:

lower concentration of CCN (eg. aerosol) can be found in marine air than in continental air

Answer 22

true. because there are less pollutant found in the ocean than in continents

Question 23

true or false:

despite having a lower CCN concentration in marine clouds, the droplets are larger

Answer 23

true. eg. fog and cloud droplets near coast are large

Question 24

why does the presence of aerosol particle makes the development of cloud droplets more probable?

Answer 24

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

Question 25

give some examples of aerosols

Answer 25

• soot
• sea salt
• dust
• volcanic ash

Question 26

what is the purpose of Raoult’s Law

Answer 26

to find the vapour pressure equilibrium over the solution (water and aerosol)

Question 27

what does a higher solute concentration mean to the condensation of cloud droplets

Answer 27

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%

Question 28

why would a stable equilibrium occur in a solution with aerosol

Answer 28

• 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

Question 29

solute effect favours ________ droplets while curvature effect favours ________ droplets

Answer 29

• small

- large

Question 30

what does solute effect favour small droplets

Answer 30

-increasing radius increases the vapour pressure equilibrium of the solution, hence harder to reach saturation and grow

Question 31

what is the Köhler curve

Answer 31

combination of Kelvin’s equation and Raoult’s law

Question 32

true or false:

in the Köhler curve, condensation can happen below 100% RH.

Answer 32

true. this means rainfall can form below 100% saturation

Question 33

what does the hump in the Köhler curve show

Answer 33

the hump defines a critical radius and the critical saturation ratio

Question 34

what does the area above the Köhler curve mean

Answer 34

• the vapour pressure of the drop is lower than the environment.
• drop grows by condensation
• water vapour can be added to the drop

Question 35

what does the area below the Köhler curve mean

Answer 35

• the vapour pressure of the drop is higher than the environment.
• drop shrinks by evaporation
• water vapour is lost to the surrounding

Question 36

true or false:

smaller droplets grow faster by condensation but always end up having the same radius as other droplets

Answer 36

true

Question 37

droplets growth involves processes other than condensation

Answer 37

true