Chapter 4, pages 94-100 : CondensationChapter 4, pages 94-100 : Condensation

The formation of dew, fog, and clouds:

removal of atmospheric water vapor

psychrometric chart

Condensation (or deposition) of excess water vapor produces dew, frost, clouds or fogCondensation (or deposition) of excess water vapor produces dew, frost, clouds or fog

Saturated

Unsaturated

T>TdT=Td

T<Td

clouds or dewfall

condensation of excess water vaporcondensation of excess water vapor

Pop quiz: When is dew or frost on grass unlikely ?Pop quiz: When is dew or frost on grass unlikely ?

at night; on a windy, dry day; when it is calm; when the sky is clear;

Condensation (or deposition) can be manmade as wellCondensation (or deposition) can be manmade as well

9-4-94, 07:10: STS-59 launch

ContrailsContrails

wing tip vortices

Contrails are the result mainly of mixing of two air parcels, a really hot one and an ice cold oneContrails are the result mainly of mixing of two air parcels, a really hot one and an ice cold one

contrail dissipates

contrailforms

Saturated

Unsaturated

measured

engineexhaust

cold hot

But most condensation processes on Earth are naturalBut most condensation processes on Earth are natural

Two challenges to rain formation:Two challenges to rain formation:

1. Water vapor needs to condense on nuclei2. These droplets need to increase their volume a million-fold, to become a rain drop.

Condensation NucleiCondensation Nuclei

Giant nuclei 1 - 10 0.01 - 10 Large nuclei 0.1 - 1.0 1 - 103

Aitken nuclei 0.01 - 0.1 103 - 106

Nucleation is the condensation, freezing, or deposition of water vapor in free air (air not in contact with the surface). Almost all nucleation occurs on aerosol (heterogeneous nucleation).• condensation nuclei• ice nuclei

Size, m # density, cm-3

Because the small nuclei have such little mass, they are able to remain suspended in the atmosphere for many days.

Condensation nuclei: sourcesCondensation nuclei: sources

breaking waves and air bubbles inject salt particles and dimethylsulfide aerosols into the air

biomass burning and volcanoes

anthropogenic industrial sources

Condensation nuclei distribution: Condensation nuclei distribution:

Aitken particle concentrations

1. there are plenty of CCN2. therefore condensation occurs at a RH of 100%3. there are more CCN over continents, so continental clouds tend to have more, but smaller droplets.

Dust storm over the MediterraneanDust storm over the Mediterranean

25 Aug 2000, seen by TOMS on SEAWIFS (dust concentration estimated on right)

global winds transport ~200 tonnes of aerosol /y

Condensation nuclei: Hygroscopic particlesCondensation nuclei: Hygroscopic particles

Hygroscopic particle: water vapor can condense on the surface when the relative humidity is «100%.

Salt crystal

cloud droplet formation: the curvature effect

cloud droplet formation: the curvature effect

the saturation vapor pressure is higher around a smaller

droplet

this is because of cohesion forces: the

surface tension is trying to squish the

droplet

cloud droplet formation: the solute effectcloud droplet formation: the solute effect

the saturation vapor pressure over a salty droplet is reduced Kohler curve

the combined solute and curvature effects imply that the ambient RH needs to exceed 100% just slightly, for an embryonic drop to grow.

this is because salt is hygroscopic

cause: nocturnal radiational cooling

(remember the energy budget on clear nights)

Frost: deposition of water vapor

Frost can also be due to the advection of supercooled clouds

Frost can also be due to the advection of supercooled clouds

Ice nucleiIce nuclei

Ice nuclei are less abundant, and more become active at lower temperatures

Droplets can remain liquid (“supercooled”) even below freezing.

Three types of ice nucleiThree types of ice nuclei

freezing nuclei: droplets freeze when a nucleus becomes active

contact nuclei: droplets freeze upon contact

deposition nuclei: water vapor deposits directly as ice

Fog: clouds near the groundFog: clouds near the ground

Fog is defined as visibility to <1 km

a day of widespread fog east of the Rockies

a day of widespread fog east of the Rockies

Fog climatologyFog climatology

I-80 summit webcam

Fog or cloud formationFog or cloud formation

Supersaturation may result from any of 3 processes:

– A. air being cooled to saturation point

– B. evaporation of water vapor

– C. mixing of warm air with cold air.

A

B

C

discussiondiscussion Think of situations in which any of these three processes causes fog

– Land & sea …– Day & night…

Fog by cooling: Radiation fogFog by cooling: Radiation fog

Radiation fog (“ground fog”)Radiation fog (“ground fog”)

clear night sky, light winds, and an air mass that is relatively humid.

ground fog is often confined to valleys: why ?

the layer can grow vertically (radiational cooling from the top of the fog).

Advection fogAdvection fog

Radiation & advection fogRadiation & advection fog

Wintertime radiation fog in California’s Central Valley

summertime advection fog along California’s coast.

Advection fog is persistent in some areasAdvection fog is persistent in some areas

Welwitschia mirabilis is entirely dependent on the Namib Desert’s advection fog.

Nam

ib D

esert

capturing water from the advection fogcapturing water from the advection fog

fog catcher beetle(onymacris unguicularis)

fog harvesting net

upslope fogupslope fog

steam fogsteam fog

Remember droplet formation mechanismsRemember droplet formation mechanisms

Fog/cloud is formed by:– A. air being cooled to saturation point– B. evaporation of water vapor– C. mixing of warm air with cold air.

A

B

C

steam fog is mainly the result of mixing of two types of air

steam fogsteam fog Forms when cold air drifts over a warmer body of water.

The lowest layer of air becomes both more humid (evaporation) and warmer than the air above (unstable), through contact with the warmer water .

The consequent mixing of warm, humid air with cold, dry air brings the air to saturation, and fog forms.

Pop quiz: The most common type of fog, radiation fog, usually disappears during the day because:Pop quiz: The most common type of fog, radiation fog, usually disappears during the day because:

A: the cloud droplets absorb solar radiation; they then warm, resulting in a higher saturation vapor pressure around the droplets, higher than the vapor pressure in the ambient air; the differential leads to evaporation;

B: a little sunshine trickles thru the fog; that warms the ground, which in turn warms the air; the air then can hold more water vapor, so the cloud droplets are happy to evaporate;

C: the wind picks up during the day and advects the fog up; often the fog can be seen later in the afternoon, reshaped into a cumulus cloud

D: the solar radiation is reflected by the fog, keeping the ground cold, and causing the fog droplets to fall out as dew.

How do clouds formHow do clouds form

Rising air is the key process in the production of clouds: rising air expands and cools.

Humidity increases until it reaches 100%. When this occurs, condensation occurs

Cloud droplets form on condensation nuclei (CCN).

Droplets may grow both by further condensation and by collision/coalescence rain

What makes air rise ???

4 mechanisms responsible for updrafts, clouds, & precipitation:4 mechanisms responsible for updrafts, clouds, & precipitation:

convection orographic

Low-level convergence, OR divergence aloft, near jet stream

frontal

spontaneous buoyant

Calculating the height of cloud basesCalculating the height of cloud bases

Lifting condensation level : level at which an air parcel becomes saturated when lifted (cooled adiabatically)

You can use the following formulas to calculate the height (H) of a cumulus cloud base given surface temperature (T) and dew point temperature (Td).

H (meters) = 125×(T– Td ) in ºC

H (feet) = 222×(T – Td ) in ºF

try out this applet estimate current cloud base

Pop quiz: Cloud droplets form ______ when the air is saturated, and ice crystals form ______when the air cools to ______

Pop quiz: Cloud droplets form ______ when the air is saturated, and ice crystals form ______when the air cools to ______

A: in pure air … in pure air … the freezing point;

B: on cloud condensation nuclei … on ice nuclei … 32 °F;

C: on cloud condensation nuclei … on ice nuclei … some temperature below 32 °F;

D: on cloud condensation nuclei … on ice nuclei … -40 °F

Cloudiness and climate changeCloudiness and climate change

(from textbook page 111)

Do clouds cool or warm the Earth surface?

cooling effect

warming effect

Cloudiness and climate changeCloudiness and climate change

The net effect of clouds depends on how high they are

Low-level cloudsblock much solar radiation (high albedo)emit much LW radiation to space (they are warm)net effect: cooling

High-level cloudsmore transparent to sunshineemit little LW radiation (they are cold)net effect: warming

Cloudiness and climate changeCloudiness and climate change

The effect of clouds on climate can be profound: a 4% increase in low-level cloudiness is sufficient to offset the warming due to a doubling of CO2.

The predicted change in low-level cloud cover and albedo in the next few decades is very uncertain.

The uncertainty relates to aerosol, both natural and man-made: more aerosol more cloud droplets higher albedo stronger cooling

Note: cloud classification is discussed in Chapter 4 , p. 101- 110 (Ackerman & Knox). You will need this for Lab #7 (Wed 14 or Thu 15 Oct).

next topic: adiabatic processes and stability

Note: cloud classification is discussed in Chapter 4 , p. 101- 110 (Ackerman & Knox). You will need this for Lab #7 (Wed 14 or Thu 15 Oct).

next topic: adiabatic processes and stability