Transcript Condensation - U Wyoming Atmospheric Science

Chapter 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 fog
condensation of excess water vapor

T<Td
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clouds or dewfall
Saturated
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T=Td
Unsaturated
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T>Td
Pop quiz: When is dew or frost
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at night;
on a windy, dry day;
when it is calm;
when the sky is clear;
on grass unlikely ?
Condensation (or deposition) can be manmade as well
9-4-94, 07:10:
STS-59 launch
Contrails
wing tip vortices
Contrails are the result mainly of mixing of two air parcels, a really hot
one and an ice cold one
engine
exhaust
contrail
forms
measured
Saturated
contrail
dissipates
Unsaturated
cold
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hot
But most condensation processes on Earth are natural
Two challenges to rain formation:
1.
2.
Water vapor needs to condense on nuclei
These droplets need to increase their volume a million-fold, to become a rain drop.
Condensation Nuclei
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
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Giant nuclei
Large nuclei
Aitken nuclei
Size, m
# density, cm-3
1 - 10
0.1 - 1.0
0.01 - 0.1
0.01 - 10
1 - 103
103 - 106
Because the small nuclei have such little mass, they are able to remain
suspended in the atmosphere for many days.
Condensation nuclei: sources
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breaking waves and air bubbles inject salt particles
and dimethylsulfide aerosols into the air
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biomass burning and volcanoes
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anthropogenic industrial sources
Condensation nuclei distribution:
1. there are plenty of CCN
2. 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.
Aitken particle concentrations
Dust 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 particles
Salt crystal
Hygroscopic particle: water vapor can
condense on the surface when the relative
humidity is «100%.
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 effect
the saturation vapor pressure over
a salty droplet is reduced
this is because salt is hygroscopic
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.
Frost: deposition of water vapor
cause: nocturnal radiational cooling
(remember the energy budget on clear nights)
Frost can also be due to
the advection of supercooled clouds
Ice nuclei
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Ice nuclei are less abundant, and more become active at lower
temperatures
Droplets can remain liquid (“supercooled”) even below freezing.
Three types of ice nuclei
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freezing nuclei: droplets freeze when a nucleus becomes active
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contact nuclei: droplets freeze upon contact
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deposition nuclei: water vapor deposits directly as ice
Fog: clouds near the ground
Fog is defined as visibility to <1 km
a day of widespread
fog east of the
Rockies
Fog climatology
I-80 summit webcam
Fog or cloud formation
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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.
B
A
C
discussion
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Think of situations in which any of these three processes causes fog
– Land & sea …
– Day & night…
Fog by cooling: Radiation fog
Radiation fog (“ground fog”)
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clear night sky, light winds, and an air mass that is relatively humid.
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ground fog is often confined to valleys: why ?
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the layer can grow vertically (radiational cooling from the top of the fog).
Advection fog
Radiation & advection fog
Wintertime radiation fog in
California’s Central Valley
summertime advection fog along California’s coast.
Advection fog is persistent in some areas
Welwitschia mirabilis is entirely dependent on the
Namib Desert’s advection fog.
capturing water from the advection fog
fog catcher beetle
(onymacris unguicularis)
fog harvesting net
upslope fog
steam fog
Remember droplet formation mechanisms
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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.
B
steam fog is mainly the result of
mixing of two types of air
A
C
steam fog
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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:
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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 form
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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:
convection
spontaneous
orographic
buoyant
Low-level convergence, OR
divergence aloft, near jet stream
frontal
Calculating the height of cloud bases
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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).
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H (meters) = 125×(T– Td )
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H (feet) = 222×(T – Td )
in ºC
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 ______
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A: in pure air …
in pure air …
the freezing point;
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B: on cloud condensation nuclei …
on ice nuclei …
32 °F;
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C: on cloud condensation nuclei …
on ice nuclei …
some temperature below 32 °F;
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D: on cloud condensation nuclei …
on ice nuclei …
-40 °F
Cloudiness and climate change
Do clouds cool or warm the Earth surface?
cooling effect
warming
effect
(from textbook page 111)
Cloudiness and climate change
The net effect of clouds depends on how high they are
Low-level clouds
block much solar radiation (high albedo)
emit much LW radiation to space (they are warm)
net effect: cooling
High-level clouds
more transparent to sunshine
emit little LW radiation (they are cold)
net effect: warming
Cloudiness and climate
change
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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 manmade: 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