CLOUD Flashcards
How is an air parcel lifted?
- Orography
- Mechanical turbulence
- Convection
- Widespread ascent
- Frontal lifting
Process for formation of cloud?
- Air parcel lifted by 1 of 5 mechanisms
- Parcel subject to less pressure
- Expansion
- Cooling (molecules collide less = less heat released)
- Should lifting & expansion continue, parcel will cool enough to reach saturation
- Saturated air cooled further, means some of the water must be removed by CONDENSATION = CLOUD.
how does a ceilometer work?
Low powered laser beam upwards from the AWS. Beam bounces off of a cloud base back to the station, station calculates height of cloud base.
(beam will also travel through lower layer & bounce off of mid/high layers as well)
How does an AWS account for cloud amounts (SCT, FEW etc.)
Ceilometer measures the length of time it records a base of cloud vs. the time it doesn’t.
I.e. cloud base of 1500ft is measured for 50% of the time = approx 4/8ths = SCT
Limitations of a ceilometer? (5)
Only accounts for cloud immediately above the sensor (therefore a small patch of stationary cloud overhead will be reported as overcast).
Approaching low cloud bases will be missed until overhead.
What does /// after every cloud group in an AWS report mean?
Sensor cannot differentiate between cloud types.
/// indicates that the cloud could be either TCU or CB
List the three groupd of basic cloud types & associated altitudes
- High clouds: 20,000ft to tropopause
- Middle clouds: 6500ft to 20,000ft
- Low clouds: SFC to 6500ft
Define the five types of clouds
- Cumulus: heaped/puffy (convection)
- Stratus: Sheet like (produced by slow gradual lifting of air)
- Alto: Middle (middle troposphere)
- Nimbo: heavy rain
- Cirrus: Streaky appearance (solely ice crystals)
List the 3 types of high clouds
Cirrus (Ci), Cirrostratus (Cs), Cirrocumulus (Cc)
List the 3 types of middleclouds
Altocumulus (Ac), Altostratus (As), Nimbostratus (Ns)
List the 4 types of low clouds
One additional?
Cumulonimbus (Cb), Cumulus (Cu), Stratocumulus (Sc) and stratus (St)
Towering cumulus (TCu)
Describe Orographic lifting
Occurs whenever wind is forced to blow up & over a mountain barrier
Describe convective lifting
Thermals?
Results when low level air is heated via conduction, becomes lighter than surrounding air & starts to rise.
Thermals are exactly the same, but the air is very dry, so has not been able to rise enough to create enough internal cooling for condensation.
Describe how turbulence causes lifting?
Air tumbles over obstacles & buildings, mechanical nature causes tumbling motion with rise & fall of air. Air that rises may cool enough to condense
Describe how widespread ascent causes lifting
As air spirals into the centre of a low, convergence occurs at the surface which leads to lifting of air over a vast area.
Describe how frontal lifting occurs
Cold front undercuts a mass of warmer air, warm air rises up over cold front.
Associated weather with all high cloud types
Nil wx, little turb unless associated with a jet. (CirroCu however indicates presence of turb)
Icing is rare but slight icing is possible.
Any thickening of cirrostratus indicates deteriorating wx,
What do the below cloud types look like?
Ci
Cs
Cc
Ci: hair-like streaky cloud
Cs: transculent layer, halo phenomena
Cc: Thin patch/layer with grains/ripples formed
Altostratus: Description & associated wx (thin vs. thick)
Layer cloud at medium levels, sun barely visible through.
If thin: possible virga or light rain. Light rime ice possible & light turb
If thick: light-mod rain or snow, glaze ice possible in lower levels, light turb unless associated with front
Altocumulus: Description & associated wx
Billowy cloud at medium level
Poss virga, lilght rime icing, lilght-mod turb
Nimbostratus: Description & associated wx
Dark rainy layer cloud
Mod-heavy continuous rain or snow. Mod rime ice, glaze possible in lower levels. light turb, poss mod-sev with front.
Cumulonimbus: Description & associated wx
Rainy heaped cloud with vertical height & anvil.
SHRA, SN or GS/GR, poss TS & lightning
Mod-heavy glaze ice, sev turb in & below cloud
Stratocumulus: Description & associated wx
Billowy or rolled layer of cloud
Generally nil wx, poss light rain.
Light-mod rime icing if low FZL
Light turb, esp if inversion present
Cumulus: Description & associated wx
Heap cloud developing vertically
Poss SHRA or snow from TCu
Light-mod glaze icing just above FZL
Mod-sev turb in & below cloud
Stratus: Description & associated wx
Layer cloud with uniform base
Drizzle & reduced vis, nil icing & light turb especially with inversion.
Additional 3 special cloud types not covered?
mammatus, lenticularis & castellanus
General RoT wrt changes in surface temp/dew point and height of cloud base
the closer together the temp & dew point are, the lower the cloud base with be.
Mathematical RoT for calculating cloud bases;
Cb/TCu
All other
Cb/TCu bases = approx 400 x (T - Td)
All other bases = approx 250 x (T-Td)
(Td = dew point temp)
General RoT for FZL in NZ in winter vs summer
Winter: 5-9000ft
Summer: 9-13,000ft
Why are clouds above the FZL not composed of ice crystals?
Lack of freezing nuclei between 0deg to -40deg.
List the three processes that lead to cloud dissipation
- Sinking of air
- Mixing with clear air
- Direct warming
How does sinking of air dissipate cloud?
Examples of where you would see this
sinking (or subsidence) causes temperature to increase because pressure is also increasing.
Subsidence is found in anticyclones & downwind side of wave crests
How does mixing with air dissipate cloud?
Mixing of saturated air with dry air leads to evaporation of water droplets, evaporation cools the air & it therefore sinks.
Sinking warms the air, further evap takes place
How does direct warming dissipate cloud
Often in arvo, ground level temp increases, dew point temp remains the same. cloud bases therefore rise, and can eventually reach the top of the cloud formation (dissipation from the ground up)
How does convection create clouds?
Heating by conduction > expansion = buoyancy > localised thermals.
Thermals mix with cooler air above & continuously turns itself inside out. This mixing & dragging of air into the base is called entrainment.
When this thermal reaches condensation level, it becomes visible as a cumulus cloud. (lifespan of 20mins unless further heating occurs below)
How does orographic lifting create cloud formations, what kind of clouds would you see?
Air flowing over a mountain range is forced upwards, which causes cooling. If sufficient moisture is present, cloud will develop.
Unstable: convective clouds.
Stable layer above: Lenticularis (in strong winds), otherwise stratiform near ridge line
What causes ascent over a large area?
-Depressions
-Warm fronts
-Cold front
- Turb
What kind of cloud formation would you see as a result of ascent due to depression
Slow lifting process over wide area.
Extensive sheets of layer cloud
Potential for ISOL EMBD Cb
What kind of cloud formation would you see as a result of ascent due to warm front
Slower moving warm front moves up over cold air.
Great sloping cloud sheets formed.
Potential for heavy rainfall from Nimbostratus as bases lower to the ground.
What kind of cloud formation would you see as a result of ascent due to cold front
Fast moving cold air undercuts warm airmass.
Cu clouds (strength & depth of these depends on strength & depth of upward motion)
What kind of cloud formation would you see as a result of ascent due to turbulence
Cloud forms usually as a result of a turbulence inversion, StratoCu acts as a ‘cap’ at the top of the friction layer.
Other cloud types seen as a result of turb is rotor clouds.
How are contrails formed?
In high-flying aircraft, warm moist air from the jet engines mixes with surrounding colder air and condenses to form a trail.
