Ch 6 - Cloud Formation Flashcards
Formation of Cloud
When the temperature of the parcel of air decreases to the dew point through:
- Convection
- Advection
- Conduction
- Radiation
The water vapour condenses to form droplets of water (sometimes ice crystals) which form clouds
Formation of Fog
Exactly the same as the formation of clouds but cannot reach the dew point through convection as Fog is on the immediate surface, not any higher.
Can be formed through:
- Advection
- Conduction
- Radiation (LWR / Terrestrial)
Dissipation of clouds or fog
When the air parcels warms up, most commonly in a high pressure system because of subsidence and compression
Adiabatic Warming
When air parcels are forced to descend, causing compression and warming (it subsides) and thus dissipates
Adiabatic Cooling
When air parcels are forced to rise, then cooling and expanding which forms clouds as the temperature drops slightly below the dew point.
(Ties in with convection)
Adiabatic
There is no gain or loss of energy
No mixing with air from other parcels or the atmosphere
Relative humidity increases as the altitude increases as it is dependent on temperature.
Temp of the air parcel and water vapour will decreases with altitude causing the relative humidity to increase
Cold air holds less moisture
Environment Lapse Rate (ELR)
Lapse rate of the environmental atmosphere
ISA Standard:
-2C/1000ft 0.65C/100m Up to 11km (36090ft)
ELR can vary and can be what ever it likes
Dry Adiabatic Lapse Rate (DALR)
-3C/1000ft -1C/100m
Completely Dry Air ( up to 99% RH)
Saturated Adiabatic Lapse Rate (SALR)
Only used when the air is 100% saturated (in clouds)
-1.8C/1000ft -0.6C/100m
Why the Saturated Adiabatic Lapse Rate is different from the DALR
Saturated due to condensation
- Condensation causes latent heat to be released which keeps the OAT of the parcel of air slightly warmer for longer
- Therefore it cools down at a lesser rate
- Curved gradient as it is dependent upon the temperature, decreased temperature=less water vapour
Therefore at high altitude, where the temp is colder, the temperature of the parcel decreases at a faster rate.
Broken down: Less water vapour -> Less condensation occurring -> less latent heat being released -> temp cools more quickly
High altitude clouds are less thick and more whisky because they are less dense
Stability
A parcel that is forced to rise and continues is unstable.
A parcel that is forced to rise but then tries to sink to its original layer is stable
Neutral or Indifferent Stability
Can occur in:
Dry or SKC (sky clear) conditions, the DALR = ELR (-2C/1000ft)
Or in
Saturated air / clouds have a SALR = ELR (-1.8C/1000ft)
Absolute Instability
When:
ELR > Both DALR and SALR
Absolutely Unstable
Cloud: Convective/ Cumuloform (CU/TCU/CB)
Precip: RA/+RA -> Start/stop quickly SHs of RA and +RA
Visib: Good out of SH
Turbulence: MOD->SEV TURB
Absolute Stability
When
ELR< DALR and SALR
Clouds: Layered/Statoform (ST/AS/CS/NS(RA))
Precip:Drizzle (Dz/+DZ/-DZ)
Vis: Poor-> MOD. Caused by DZ/HZ
Turb: Light->MOD NS (MOD->SEV)
Conditional Stability
DALR
Turbulence Cloud
High pressure traps cool pressure underneath creating an inversion
Wind beneath the inversion creates rotors and turbulent air
Rising air from the land conducts, convects and then condensates at the boundary layer with the unstable air above it creating Stratocumulus clouds
- Very smooth air above and stable
Orographic Cloud
Stable and Saturated
Stable
Fohn Effect - Has to be stable
1 - Low cloud base on windward side, precipitation, cooler temp and strong wind forcing the stable air to rise over the mountain
2 - Lee side has a higher cloud base (due to precip on windward side), a warming temperature and no precipitation
Orographic (Stable and Dry)
Moisture content less
- Dry
- Stable
- Temp can be constant on each side
- Sometimes will have a lenticular cloud on top
Orographic Unstable
Saturated
Unstable so air going up will continue to rise
- May go on to form a cumulus or cumulonimbus - Driven by instability, sucks up air as it needs heat and moisture
- Can sometimes create low pressure systems on the lee side from sucking up the air ( A depression/lee depression/ lee low)
Orographic Unstable
Dry
The dry air may not reach the dew point (no condensation) which means now cloud
Air is still rising over the windward side though so creates what’s known as a blue thermal
Convective Cloud
When the surface temp increases Surface air heats up through conduction Then heats up and rises through convection (unstable - cumulus) - Pressure decreases so hot air rises - Temp of air decreases - Reaches the dew point - Condenses - Convective Cloud
Most likely 2-3 hours after local noon
If it is too dry, it will be a blue thermal as air will still rise without the saturation - clear air turbulence
Non Frontal Convergence Pattern
- Pressure system = Low Pressure
- Convergence and Convection at the surface
Pattern:
- Air rising (pressure difference triggers it)
- Temp decreases (moisture at RH100% - condensation happens)
- Atmosphere stable or unstable
Convective stops at stable layer (isotherm or inversion)
Cloud Base
The lowest zone in which the type of obscurantism perceptibility changes from that corresponding to the clear/haze conditions to that corresponding to water droplets or ice crystals
- Measure above ground level (aerodrome) in feet
Measured by a ceilometer (ground up laser)
Cloud base height = (temp - dew point) x 400
Cloud Ceiling
Height above ground/water of the base of the:
- Lowest layer of cloud
- Below 6000m (20,000ft)
- Covering more than half of the sky (>4oktas)
(Must therefore be BKN or OVC)
