How do the clouds form?
Review of last lecture
• Water Vapor Basics (names of different phase changes,
latent heat)
• Humidity indices (there are 6 total). Saturation vapor
pressure increases non-linearly with temperature
• Two methods of achieving saturation and condensation
(diabatic vs. adiabatic processes). Different types of
condensation - dew, frost, fog (radiation, advection,
upslope, precipitation, steam), clouds.
The most common atmospheric
circulation structure
H
L
Radiation
Cooling
or No
Heating
Convection
Heating
Latent/Sensible
Conduction
H
L
Imbalance of heating
 Imbalance of temperature
 Imbalance of pressure
 Wind
Clouds
• Clouds are instrumental to the Earth’s energy and moisture
balances, and constitute a wild card for climate change
Satellite observation of clouds
• NASA’s International Satellite Cloud Climatology Project (ISCCP)
Combine the measurements of 5 geostationary and 1-2 polar orbiting
satellites. 1983-Now, cloud top height and optical depth.
• NASA’s Earth Observation System including a set of polar orbiting
satellites (A-Train), especially CloudSat (with a cloud radar) and
CALIPSO (with a cloud lidar). Ongoing, cloud particle information,
detailed vertical structure.
Global map of clouds
Vertical structure of clouds
Cloud Properties
1. Cloud top height/pressure
2. Cloud thickness (optical depth)
3. Cloud coverage
•
•
•
•
When clouds comprise more than 9/10th of the sky = overcast
When coverage is between 6/10th and 9/10th = broken
When coverage is between 1/10th and 6/10th = scattered
Cloud coverage less than 1/10th = clear
NASA’s International Satellite Cloud Climatology Project (ISCCP)
Cloud Classification - commonly used in climate research
Why do clouds constitute a wildcard for climate change?
• Clouds are both good reflectors
of solar radiation (cooling effect)
and good absorbers of earth
emitted longwave radiation
(warming effect).
• The net effect (cooling or
warming) depends on the type of
cloud
• In a changing climate, increases
in high thin clouds would promote
warming, while increases in low
thick clouds would cause cooling
• Climate models have difficulties
in simulating clouds, especially
low thick clouds (stratocumulus)
• Conclusion: Clouds cause the
largest uncertainty in model
simulations of future climate.
Stronger
warming effect
Stronger
cooling effect
Video: Convective cloud time lapse
• http://www.youtube.com/watch?v=kapTREk0gX
g
Formation of clouds
• Most clouds form as air parcels in
boundary layer are lifted and
cooled to saturation.
• The air parcels could be lifted by
mountains, meeting of different air
masses, surface convergence, and
local convection
Lifting by local convection
• Static stability – refers to atmosphere’s
susceptibility to being displaced
• Stability related to buoyancy force 
determined by density difference btw parcel
and environment (FB= ρenvg – ρparcelg) 
determined by temperature difference btw
parcel and environment (ρ = P/TR)
• When an air parcel rises, the cooling rate of
the parcel (adiabatic lapse rate or ALR)
relative to the cooling rate of surrounding
atmosphere (environmental lapse rate or
ELR) determines the “stability” of a parcel.
Environment
Parcel
Δp/Δz=ρenvg
ρparcelg
The three types of stability
When comparing the temperature btw parcel and environment, there are
3 possible outcomes:
Environment
Parcel
Parcel
Parcel
Environment
Absolutely
Stable
Absolutely
Unstable
Environment
Conditionally
Unstable
Convection happens when:
(1) boundary layer air is warm and moist
(2) Environmental air above boundary layer is cold
When convection happens:
1. Rising up of air parcel (called
updraft)
2. Formation of clouds and
sometimes precipitation
3. Heating up the environment
because parcel temperature is
warmer than the environment
What stops ‘unstable’ air masses from rising indefinitely ?
1) Entrainment
• Turbulent mixing of ambient air into parcel
• Leads to evaporation along cloud boundaries
• Evaporation uses latent heat, cooling the cloud
 reduces buoyancy
Courtesy Russ Dickerson, U. Maryland
2) Encountering a layer of stable air (inversion)
• a rising parcel may reach a stable upper air
environment
• the parcel cooling rate will exceed that of
the ambient air
• the parcel will slowly cease ascension and
come to rest at some equal temperature level
• three types: radiation, frontal, subsidence
In convection, an updraft is often associated with
a downdraft – Overturning of the troposphere
• Air can be cooled down by
radiation, evaporation of raindrops,
melting of snowflakes, etc.
• Air that is cooler than its
environment tends to sink, leading
to the formation of downdrafts
• Sometimes precipitation drag
enhances the downdrafts
• Downdrafts cool down the
environment (generally the lower
troposphere)
• Downdrafts (also called
downbursts) can cause significant
damage at the ground
Low stratocumulus clouds
• Generated by convection
inside boundary layer
• Convection is driven by
cloud-top longwave cooling
and evaporative cooling
Summary
 3 cloud properties, 9 ISCCP cloud types
 Why do clouds constitute a wildcard for climate change?
Competition between greenhouse effect and albedo
effect
 Convection: 3 types of stability. Two factors limiting the
height of clouds
Works cited
• http://www.atmos.washington.edu/2004Q2/547/www/
• http://capita.wustl.edu/capita/datasets/modis/globfused/g
lob3d.html
• http://www.ssec.wisc.edu/sos/wvsst/wvsst.html
• http://www.arm.gov/news/facility/post/1025
• http://apollo.lsc.vsc.edu/classes/met130/notes/chapter4/
es_temp.html
• http://www.meted.ucar.edu/oceans/currents/print.htm
• http://visibleearth.nasa.gov/view.php?id=57735
• http://www.crh.noaa.gov/lmk/?n=downburst
• http://cde.nwc.edu/SCI2108/course_documents/earth_m
oon/earth/earth_science/convection/convection_advectio
n.htm