Transcript Cloud Formation - Texas A&M University

Cloud Formation
Ten Basic Types of Clouds (Genera):




High: Ci, Cs, Cc
Middle: As, Ac
Low: St, Ns, Sc
Clouds of Great Vertical Extent: Cu, Cb
Cumulus: heaped
Strato: layered
Nimbo: rain
Alto: middle
Cirro: wisp
Clouds may then be further classified
according to species and varieties terms.
Species: Castellanus, lenticularis, fibratus,
etc.
Varieties: Undulatus, translucidus, opacus,
etc.
Cloud Development
Cumuliform Clouds: May form from thermals
(convectively unstable air warmed by the surface
and rising (convection lifting)) or by vertical
turbulence motion in statically stable air.
Cumulus, Cumulonimbus form by air warmed by
the surface and which becomes unstable.
Altocumulus, Cirrocumulus form by small scale
vertical turbulent motion and not from warmed
surface air.
Cumulonimbus
Cirrocumulus

Cumulonimbus
Stratiform Clouds: Flat layered appearance.


Form in statically stable environment where
vertical motion is suppressed.
Must have an external lifting to cause the air to
rise (forced lifting).
• Frontal lifting
• Orographic lifting
• Convergence lifting

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Cirrus, altostratus, stratus, nimbostratus
Note: Cirrocumulus and Altocumulus should be
classified as cumuliform (heaped), even though
they form in a statically stable environment.


Stratus
Nimbostratus
Stratocumulus: May be caused by either
thermal updrafts or turbulent eddies or both.

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The vertical thickness of the cloud is less than it’s
horizontal extent.
The turbulent eddies
are relatively small
and/or the thickness of
unstable air above the
cloud base is small.
Processes causing saturation
1. Cooling: Temperature cools to Dew Point

Adiabatic Cooling by lifting of air parcels.
• Forced lifting
– Frontal
– Orographic
– Convergent
• Convective lifting (buoyancy)

Cooling by being in contact with cold ground.
• Advection fog

Cooling by radiation. Ground temperature cools by
radiation.
• Radiation fog.
2. Adding moisture: Dew Point increases
to Temperature.


Evaporation of liquid water into unsaturated air.
Forms precipitation or frontal fog.
Steam fog. Cold air moves over warm, humid
surface (Cold air moves over warm lake or
ocean).
3. Mixing of two unsaturated parcels.
mX  mB  mC

The temperature, vapor pressure, mixing ratio,
specific humidity are the weighted averages of
the original parcels.
mB  TB  mC TC
TX 
mX
3 mB  eB  2  mC  eC
eX 
mX
mB  rB  mC  rC
rX 
mX
Example problem, pg. 149

Why cannot table 5.1 be used to determine
r and rs?
Clouds and upslope fog
Air is forced upward or ground rises and air
is forced to flow up over ground.
Rising air cools at dry adiabatic lapse rate
until saturation (T = Td). Base of the cloud,
is the LCL.
Other Fogs
Advection Fog: Caused by advection of warm,
moist air over cold surface. Cold surface cools
the air to saturation. As air moves across the
surface, it becomes colder by heat being
conducted out of it into the ground, or water,
surface. The potential temperature of the air
depends on the time spent over the ground which
 C

is a function of the wind speed.
  H x 
   sf c   0   sf c e


zi


zi = thinkness of layer
Q0 = initial potential temperature of air
CH = heat transfer coefficient (value
between 2x10-2 and 2x10-3
Qsfc = potential temperature of surface.
Distance air must travel before fog begins


forming:
T

T
z
0
sf c
x

 ln 


CH
Td  T sf c 
i
Radiation Fog: Infrared radiation at night
from the ground surface lowers the temperature
of the ground and results in lowering of the air in
contact with the ground. If the temperature
drops to, or below, the dew point temperature,
fog forms.
The depth of the fog will be the height to which
temperatures have dropped to the dew point.
The time the radiation fog begins to form can
be approximated by the empirical formula:
a M
2
t0 
where,

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

32
 TRL  Td 
2
 FH 
2
(7.10)
a = 0.15m1/4 s1/4
TRL=residual layer temperature (Temp. in
statically neutral layer above near surface See pg.
151). The temperature of air unaffected by ground
cooling.
M = wind speed in residual layer
FH = average surface kinematic heat flux. (pg. 52)
Residual Layer
Consider the structure of the Atmospheric
Boundary Layer. Lowest (on the average) two
km of the atmosphere.
FA: Free atmosphere
layer
EZ: Entrainment Zone
MI: Mixed Layer
zi: Mixed-layer depth
SL: Surface Layer
CI: Capping Inversion
RL: Residual Layer
SBL: Stable Boundary
Layer
M = Wind Speed
r = Mixing ratio
Within Atmospheric Boundary Layer
During daytime: Statically-unstable mixed layer
(ML). Separated from free atmosphere by the
strongly stable entrainment zone (E Z) of
intermittent turbulence. Mixed layer depth (zi) is the
distance from ground to the middle of the EZ.
During night: Statically stable boundary layer
forms under a statically neutral residual layer. The
residual layer contains the pollutants and moisture
from the previous mixed layer, but is not turbulent.
At night: Turbulence in the EZ ceases, leaving a
non-turbulent separation layer called the capping
inversion (CI), which is strongly statically stable.
Bottom 20 to 200 meters of ABL is the Surface
Layer (SL). Here, frictional drag, heat
conduction and evaporation from surface cause
substantial changes in wind speed, temperature
and humidity with height. Also known as the
constant flux layer.
The depth of the radiation fog layer can be
determined by:
 1 2 
t
34
12

 (7.11)

z  a  M t ln 
  
t0  
Problems
N1 (a, c, e, g)(show thermo-diagram), N2
(a, c), N5(b, c, e), N7(a), N8(a,b)(use
spreadsheet for plots), N9(a, b) (use
spreadsheet for plots)
SHOW ALL EQUATIONS USED AND
CALCULATIONS