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Meteorology > Ch 6 - Cloud Formation > Flashcards

Ch 6 - Cloud Formation Flashcards

1
Q

Formation of Cloud

A

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

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2
Q

Formation of Fog

A

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)
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3
Q

Dissipation of clouds or fog

A

When the air parcels warms up, most commonly in a high pressure system because of subsidence and compression

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4
Q

Adiabatic Warming

A

When air parcels are forced to descend, causing compression and warming (it subsides) and thus dissipates

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5
Q

Adiabatic Cooling

A

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)

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6
Q

Adiabatic

A

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

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7
Q

Environment Lapse Rate (ELR)

A

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

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8
Q

Dry Adiabatic Lapse Rate (DALR)

A

-3C/1000ft -1C/100m

Completely Dry Air ( up to 99% RH)

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9
Q

Saturated Adiabatic Lapse Rate (SALR)

A

Only used when the air is 100% saturated (in clouds)

-1.8C/1000ft -0.6C/100m

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10
Q

Why the Saturated Adiabatic Lapse Rate is different from the DALR

A

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

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11
Q

Stability

A

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

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12
Q

Neutral or Indifferent Stability

A

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)

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13
Q

Absolute Instability

A

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

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14
Q

Absolute Stability

A

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)

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15
Q

Conditional Stability

A

DALR

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16
Q

Turbulence Cloud

A

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

17
Q

Orographic Cloud

Stable and Saturated

A

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

18
Q

Orographic (Stable and Dry)

A

Moisture content less

  • Dry
  • Stable
  • Temp can be constant on each side
  • Sometimes will have a lenticular cloud on top
19
Q

Orographic Unstable

Saturated

A

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)
20
Q

Orographic Unstable

Dry

A

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

21
Q

Convective Cloud

A 
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

22
Q

Non Frontal Convergence Pattern

A
  • 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)

23
Q

Cloud Base

A

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

24
Q

Cloud Ceiling

A

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)

Meteorology (21 decks)

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