Water in the atmosphere

Water content of air

Mass mixing ratio, airdry mass

vapour watermassx

%110~ 1gkg

Saturated vapour pressure, equilibrium over flat surface

Rate of evaporation = rate of condensation

svp is a function of T only

Saturated vapour pressure

Condensation

Condensation when R.H.≥100%

R.H. can increase by

(i) increasing x,

(ii) cooling air

%100T same at svp

pressure vapourR.H. Humidity, Relative

Cooling damp air

• Radiation loss• Ascent (of air mass) • Descent, through inversion

• Advection, over cool region

• Influx of cold dry air• Convection

radiation foghill fog/cloud, frontssteam fogadvection fog, harArctic sea smokecumulus clouds

Warm Rain Process

• Saturation• Nucleation• Growth by condensation• Growth by accretion• Rainfall

Nucleation

For typical molecule, r~0.6nm, would need RH=740% (SS=640%)

In atmosphere maximum SS observed is ~1%

Condensation must be on particles with r~100nm

91 2 1.2 10exp 1

w

svp r

svp r RT r

s.v.p. over curved surface > s.v.p. over flat surface

Radius Name Conc. (cm-3) SS

< 0.1 µm Aitken nuclei 10,000 1%

0.1 - 1 µm large nuclei 100 0.1%

> 1 µm giant nuclei 1 0.01%

Nucleation

Hygroscopic nuclei

Kohler curves

9

3

1.2 101

svp r Bm

svp r r

Fewer water molecules in surface so lower evaporation rate

Sizes of droplets

Growth by Condensation

rdr

To grow by dr require this much water

24w wdm dV r dr

2

1

4w

dr dm

dt r dt

Same flux through every shell, so integrate to find Δp

Mass flux of water vapour diffusing through shell of radius n

24dm dp

F n Ddt dn

2

1

4 4

rr F dn F

p dpD n D r

24w w

dr F Dp

dt r r

1

w

dr D svp SS

dt r

Vapour pressure at droplet = svp (T)Vapour pressure in cloud = svp * RH

1

cloud drop surfacep p p

svp RH svp SS

Growth by condensation

Terminal velocity of

drops

6mg rv

22

9wgv r

343

6 6wr gmg

vr r

For laminar flow, at terminal velocitydrag force=weight

Growth by Collision/AccretionContinuous collection model

v1

v2

r1

r2

2{r1+r2}

2

1 2 1 2 l

dmr Ev

dtwr v

relative volume swept out per unit time

collection efficiency

liquid water content of cloud

22 11 1 2 1 24w s air

drr r r v v x Edt

1 2 1 2

11

if and

4s air

w

r r v v

x Edrv

dt

11

drr

dt

For large drops (>40µm) 1 1v r

Arial view of convection cells

Cloud streets