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