Aviation Meteorology – Cloud, Precipitation, Icing & Related Phenomena

1. Cloud Classification by Height

In middle latitudes, clouds are broadly classified according to the approximate height of their base above the surface.

Important Points

• Alto- → medium-level cloud.
• Cirro- → high-level cloud.
• -stratus → layer or sheet cloud; generally associated with stable air.
• -cumulus → heap/cumuliform cloud; associated with vertical development.
• CU and CB may begin in the low-level band but extend through multiple cloud levels.
• Nimbostratus (NS) can extend from the surface to about 15,000 ft, therefore spanning the low and medium levels.

2. Stable Air vs Unstable Air

Stable Air

Stable air strongly resists vertical displacement. When air is forced to rise, it tends to spread horizontally rather than continue vertically.

Characteristics

• Produces stratiform/layer clouds.
• Clouds have:
  • Large horizontal extent.
  • Limited vertical development.
  • Sheet-like appearance.
• Typical clouds:
  • ST
  • AS
  • NS
  • CS
• Generally associated with:
  • Smooth air.
  • Continuous or intermittent precipitation from NS/AS.
  • Potentially poor visibility.

Unstable Air

Unstable air favors strong vertical currents and convection.

Characteristics

• Produces cumuliform/heap clouds.
• Typical clouds:
  • CU
  • CB
• Associated with:
  • Updrafts and downdrafts.
  • Turbulence.
  • Showery precipitation.
  • Thunderstorms in sufficiently developed convection.

Quick Comparison

3. Cumulus (CU)

Cumulus clouds are classic cumuliform or heap clouds produced by convection.

Formation

1. Solar heating warms the surface.
2. Air in contact with the heated surface becomes warmer and less dense.
3. The air rises as thermals.
4. Rising air expands and cools adiabatically.
5. When saturation is reached, condensation occurs and CU develops.

Key Points

• CU indicates active vertical air movement.
• Associated with unstable air.
• Updrafts and downdrafts can cause turbulence.
• Larger CU can develop into Towering Cumulus (TCU) or CB.
• Fair-weather CU generally does not produce significant precipitation.

4. Fair-Weather Cumulus — Cumulus Humilis

Cumulus Humilis (Cu Humilis) represents an early stage of daytime convection.

Characteristics

• Small clouds with limited vertical development.
• Form as surface heating begins.
• Produced by rising thermals.
• Indicate a shallow layer of instability.
• Usually do not produce showers or thunderstorms.

Aviation Significance

• Air below and within the clouds can be turbulent or bumpy.
• Dry thermals may produce light to maximum moderate turbulence.
• Air above the tops is generally smoother.

Daily Development

• Formation may begin around 07:00 UTC or soon after sunrise, as surface heating increases.
• With continued insolation, CU may develop into larger CU or CB.
• Maximum instability is indicated around 1500 LMT in the source material.

5. Cumulonimbus (CB)

Cumulonimbus is the ultimate manifestation of atmospheric instability and the most hazardous cloud to aviation.

Vertical Extent

• CB is a cloud of great vertical development.
• It may extend:
  • From near the surface
  • Through low and medium levels
  • Into the high level
  • Up to the tropopause.
• Heights may exceed 45,000 ft and can reach 65,000 ft in some cases.

Composition of a Mature CB

A mature CB can contain all three forms of water:

Supercooled Water

• Large supercooled droplets may occur particularly between 0°C and −40°C.
• These droplets are a major cause of severe clear icing.

CB Hazards

CB can contain virtually the complete spectrum of major aviation weather hazards:

• Violent turbulence.
• Moderate to severe icing.
• Severe clear/glaze icing.
• Lightning.
• Hail.
• Heavy rain.
• Snow.
• Strong updrafts and downdrafts.
• Microbursts.
• Windshear.

Important Memory Point

CB = Great vertical development + severe turbulence + severe icing + hail + thunderstorms.

6. Cumulonimbus Capillatus (Cb Cap)

A Cumulonimbus Capillatus is a CB having a distinct fibrous or diffuse cirrus anvil.

Anvil Formation

• The strong CB updraft reaches the stable tropopause.
• Further vertical development is restricted.
• Rising air spreads laterally.
• Strong winds aloft can further spread the upper cloud.
• The resulting fibrous top is the anvil.

Key Point

• Capillatus = CB with anvil.
• The anvil consists primarily of ice crystals/cirrus.

Contrast

• Cb Capillatus: Fibrous/anvil top.
• Cb Calvus: Rounded top without the characteristic fibrous anvil.

7. Hail

Hail (GR) consists of balls or irregular lumps of ice.

Formation

Hail is almost always associated with powerful Cumulonimbus clouds.

Strong updrafts:

1. Carry ice particles above the freezing level.
2. Keep them suspended.
3. Allow them to collect supercooled water by accretion.
4. Produce progressively larger hailstones.

The source associates strong CB updrafts with speeds exceeding 10 m/s.

Aviation Hazard

Hail is one of the greatest thunderstorm hazards.

It may occur:

• Inside CB.
• Below CB.
• Near the anvil.
• Beneath the anvil.
• Several miles from the parent thunderstorm, including in apparently clear air.

Important Rule

Do not fly beneath the overhanging CB anvil, because hail and turbulence may occur there even outside the visible cloud.

8. Showers vs Continuous Precipitation

Showers (SH)

Showers are characterized by:

• Sudden beginning.
• Sudden ending.
• Rapid changes in intensity.
• Convective/heap-cloud origin.

Main Sources

• CU
• CB

CB produces the most intense showers, including:

• Heavy rain.
• Snow showers.
• Hail.
• Thunderstorm precipitation.

Continuous or Intermittent Precipitation

Usually associated with layer clouds, particularly:

• NS
• AS
• ST
• SC

Quick Comparison

9. Nimbostratus (NS)

Nimbostratus is a dense, dark-gray, extensive layer cloud associated with widespread and long-lasting precipitation.

Classification

• Usually considered a low or middle cloud.
• Base may extend from the surface to approximately 6,500 ft.
• Its deep structure may extend to approximately 15,000 ft.
• Therefore, NS may span the low and medium cloud levels.
• It may merge with Altostratus at higher levels.

Formation

NS is strongly associated with:

• Widespread gentle ascent.
• Stable air.
• Overrunning of warm, moist air over colder air.
• Warm fronts and frontal systems.

Warm Front Sequence

The source describes the typical progression as:

Cirrostratus → Altostratus → Nimbostratus

as a warm front approaches.

Precipitation

NS produces:

• Continuous rain.
• Intermittent rain.
• Continuous snow.
• Intermittent snow.
• Generally widespread precipitation rather than showers.

NS is particularly associated with widespread continuous snowfall.

Hazards

• Moderate to severe icing may occur.
• Freezing precipitation may occur in suitable frontal conditions.
• Moderate to severe turbulence is identified in the source.

Important Comparison

NS = layer cloud + widespread ascent + continuous/intermittent rain or snow.

CB = heap cloud + strong instability + showers + severe hazards.

10. Altostratus (AS)

Altostratus is a medium-level stratiform cloud.

Characteristics

• Medium cloud.
• Base generally 6,500–23,000 ft in middle latitudes.
• Gray or bluish sheet/layer.
• Composed of:
  • Water droplets.
  • Ice crystals.
• Associated with stable or gently rising air.

Weather

AS commonly forms ahead of an advancing warm front.

It may produce:

• Continuous rain or snow.
• Intermittent rain or snow.
• Generally light to moderate precipitation.

Memory Aid

Alto = Medium

Stratus = Layer

Therefore:

Altostratus = Medium-level layer cloud.

11. Altocumulus (AC)

Altocumulus is a medium-level cloud.

Height

• General medium-level base range: 6,500–23,000 ft.
• The source notes 7,000–16,500 ft as a typical summer base range in one context.

Composition

• Primarily water droplets.
• May contain ice crystals.

Memory Aid

Alto = Medium level.

12. Altocumulus Castellanus (ACC)

Altocumulus Castellanus (ACC) is recognized by its turret-like or castle-like cloud elements.

Significance

ACC is an important indicator of mid-level instability.

Its presence may indicate:

• Stronger vertical development at middle levels.
• Potential widespread showery weather.
• Possible later development of CB.
• Possible thunderstorm activity.

Aviation Hazards

ACC is associated with:

• Moderate to severe turbulence.
• Icing.

Memory Point

ACC = Mid-level instability → possible later thunderstorms.

13. Stratus (ST)

Stratus is a low-level stratiform cloud.

Appearance

• Uniform gray layer or sheet.
• Resembles fog that has lifted from the surface.
• Large horizontal extent.
• Little vertical development.

Height

• Generally from near the surface to about 6,500 ft in middle latitudes.

Weather

ST commonly produces:

• Drizzle.
• Mist.
• Freezing drizzle.
• Snow grains.

Because vertical currents are weak, ST generally cannot support heavy precipitation.

Important Point

Stratus = low-level + stable air + drizzle.

14. Drizzle

Drizzle (DZ) consists of fine, uniform liquid-water drops falling slowly.

Characteristics

• Drop diameter: generally less than 0.5 mm.
• Source material also identifies approximately 0.2–0.5 mm.
• Usually produced by:
  • Stratus.
  • Stratocumulus.
• Indicates stable air and weak vertical currents.

Operational Significance

Drizzle frequently occurs with:

• Fog.
• Haze.
• Smoke.

Therefore, it can be associated with very poor visibility.

Contrast

• Drizzle → ST/SC
• Showers → CU/CB

15. Cirrus (CI)

Cirrus is a high-level cirriform cloud.

Characteristics

• Height: approximately 16,500–45,000 ft in middle latitudes.
• Thin, wispy, feathery appearance.
• Composed almost entirely/exclusively of ice crystals.

Aviation Characteristics

• Structural icing: Nil or Trace.
• Turbulence: Nil according to the source.
• Visibility within CI: generally 1,000 m+.
• No significant precipitation reaches the surface.

CI can also indicate the approach of a warm front.

Memory Aid

CI = High + ice crystals + nil/trace icing + nil turbulence.

16. Cirrostratus (CS)

Cirrostratus is a high-level, thin, sheet-like cloud composed primarily of ice crystals.

Characteristics

• High-level cloud.
• Cirriform.
• Ice-crystal composition.
• Generally produces nil or trace precipitation.
• Structural icing is nil or trace.
• Associated with optical phenomena, especially halos.

17. Cirrocumulus (CC)

Cirrocumulus is a high-level cirriform cloud.

Height

• High-level cloud base approximately 16,500–45,000 ft in middle latitudes.

Composition

• Almost entirely ice crystals.

Memory Point

CC = Cirro- = High-level cloud.

The source also identifies CI, CS and CC as high-level clouds with predominantly ice-crystal composition.

18. Lenticular Clouds and Mountain Waves

Altocumulus Lenticularis (ACL) is a classic indicator of mountain-wave activity.

Formation

1. Stable air flows strongly across a mountain range.
2. The flow is forced upward over the terrain.
3. Standing/lee waves develop downstream.
4. Air rises and cools adiabatically.
5. Condensation occurs at the wave crests.
6. Lens-shaped clouds form.

Appearance

• Lens-shaped.
• Almond-shaped.
• May occur in vertically stacked layers.
• Appear stationary even though strong winds pass through them.

Location

Usually:

• Above mountain ranges.
• Downwind on the lee side.
• Near wave crests.

Aviation Hazards

Lenticular clouds indicate possible:

• Mountain waves.
• Moderate to severe turbulence.
• Rotor turbulence.
• Severe turbulence in the rotor zone.
• High concentrations of supercooled droplets and possible severe icing.

Memory Point

Lenticular cloud = Mountain wave warning.

19. Precipitation

Precipitation occurs when particles grow sufficiently large that the atmosphere can no longer keep them suspended.

Summer Precipitation

Over land, summer precipitation is strongly associated with surface heating and convection.

Main Clouds

• Cumulus.
• Towering Cumulus/Congestus.
• Cumulonimbus.

CB

CB produces the most intense precipitation, including:

• Heavy rain.
• Snow.
• Hail.
• Showers.

General Relationship

Strong surface heating → convection → CU/TCU → CB → heavy showers/thunderstorms.

20. Precipitation Intensity by Cloud Type

The source gives the following general order:

CB > NS > AS / CU Congestus > ST > CI / CS / CC

Important Distinction

• CB: Most intense and violent precipitation; showers.
• NS: Extensive, continuous moderate to heavy precipitation.
• AS: Continuous/intermittent light to moderate precipitation.
• ST/SC: Light precipitation, mainly drizzle.
• CI/CS/CC: Nil or very limited precipitation reaching the surface.

If CB is not among the answer options, NS may be the cloud associated with the most intensive rain among options such as ST, NS, SC and CI.

21. Freezing Rain and Freezing Drizzle

Freezing rain (FZRA) and freezing drizzle (FZDZ) are severe forms of freezing precipitation.

Formation

1. Rain or drizzle forms in warmer air aloft where temperature is above 0°C.
2. It falls into a shallow sub-freezing layer near the surface.
3. The droplets cool below 0°C but remain liquid.
4. These are supercooled droplets.
5. On striking the aircraft or ground, they freeze immediately.

Result

They form:

• Hard clear ice.
• Glaze.
• Rain ice.

This can produce very severe icing because clear ice accumulates rapidly.

Ice Pellets / Sleet

Ice pellets form when the precipitation freezes before reaching the ground, producing small transparent ice pellets.

Memory Point

Freezing rain = supercooled liquid → freezes on impact → severe clear/glaze ice.

22. Icing

Cumulonimbus Icing

CB is the cloud most strongly associated with moderate to severe/extremely hazardous icing.

Why?

• Strong updrafts maintain large quantities of liquid water.
• Large supercooled droplets are present.
• Droplets freeze rapidly on aircraft surfaces.
• This can result in rapid accumulation of clear/glaze ice.

The source identifies the principal severe icing region in CB as approximately:

0°C to −20°C

Other Clouds

Important Point

Cirriform clouds consist predominantly of ice crystals and therefore lack the supercooled liquid water required for significant structural icing.

23. Halo

A halo is a large luminous circle or arc around the Sun or Moon.

Cause

• Produced by refraction of light.
• Light passes through hexagonal ice crystals.

Associated Cloud

Most characteristically associated with:

Cirrostratus (CS)

Key Points

• Indicates a high-level cirriform cloud.
• Ice-crystal phenomenon.
• Typical halo angular diameters include 22° and 46°.

Halo vs Corona

24. Corona

A corona is a small, prismatically colored ring surrounding the Sun or Moon.

Cause

• Diffraction of light.
• Occurs around small, relatively uniform cloud droplets.

Associated Clouds

The source identifies:

• Altostratus (AS)
• Altocumulus (AC)

as good producers of coronas, particularly when recently formed and containing relatively uniform water droplets.

Memory Point

Corona = diffraction + water droplets.

Halo = refraction + ice crystals.

25. Virga

Virga occurs when precipitation falls from a cloud but evaporates completely before reaching the Earth’s surface.

Appearance

• Streaks or wisps extending downward from a cloud.

Significance

Virga indicates dry air beneath the cloud base.

Evaporation causes cooling, which can:

• Strengthen downdrafts.
• Contribute to microburst development.
• Increase windshear hazards beneath the cloud.

Memory Point

Virga = precipitation visible below cloud but evaporating before reaching ground.

26. Weather Radar

Airborne and ground-based weather radar are primarily designed to detect and measure precipitation.

Radar Echo Strength

The strength of a radar return depends mainly on:

• Size of precipitation particles.
• Number/concentration of particles.

The source emphasizes particle size as the more important factor.

Hail

Hail produces the strongest radar echoes.

Wet hail is particularly reflective because:

• Hailstones are large.
• A film of water can cover the hailstone.
• The water-covered hailstone behaves as a very large radar-reflective target.

Radar Does Not Detect Fog Properly

Weather radar generally does not detect minute cloud droplets.

Therefore:

• Fog.
• Mist.
• Ordinary cloud droplets.

do not provide the same radar returns as precipitation.

Operational Point

Radar is primarily used to help pilots identify and avoid:

• Thunderstorms.
• Heavy precipitation.
• Hail.

However, a radar display does not guarantee avoidance of fog or other non-precipitating instrument weather.

27. Contrails — Mintra and Maxtra Levels

A contrail forms when hot, moist engine exhaust mixes with cold ambient air and the resulting mixture reaches saturation.

Mintra Level

Mintra Level = minimum altitude at which contrails can form.

Below the Mintra Level:

• Ambient conditions are generally too warm or insufficiently humid.
• The exhaust-air mixture does not reach the required saturation.
• No contrails below Mintra Level.

Maxtra Level

Maxtra Level = maximum altitude at which contrails can form.

Above the Maxtra Level:

• Temperature/humidity conditions become unfavorable.
• Exhaust mixing no longer produces the required saturation.
• No contrails above Maxtra Level.

Memory Aid

Contrail formation depends on the combination of temperature and humidity, not altitude alone.

28. Cloud Formation over Flat, Dry Land

On flat, dry land, the primary mechanism of cloud formation is convection.

Process

Solar heating → surface warming → warm air → rising thermals → adiabatic cooling → saturation → condensation → CU/CB

Detailed Process

1. Dry surfaces such as rock, sand and dry soil absorb solar radiation efficiently.
2. The surface becomes a hot spot.
3. Heat is transferred to adjacent air.
4. Air expands and becomes less dense.
5. The air rises in thermal bubbles.
6. Rising air cools at the Dry Adiabatic Lapse Rate (DALR) while unsaturated.
7. If saturation is reached, water vapor condenses.
8. Convective clouds such as CU or CB form.

Important Point

Dry air requires:

• Greater heating.
• More lifting.

before saturation is reached.

29. Cloud Dissipation and Subsidence

Clouds dissipate when the air becomes warmer or drier and relative humidity falls below saturation.

Subsidence Process

1. Air sinks.
2. Descending air is compressed.
3. Compression causes adiabatic warming.
4. Temperature increases relative to dew point.
5. Relative humidity decreases.
6. Cloud droplets evaporate.
7. Cloud dissipates.

High-Pressure Systems

Subsidence is strongly associated with high-pressure/anticyclonic systems.

A subsidence inversion may form and act as a lid, suppressing vertical development.

Result

• Cloud dissipation.
• Generally clearer skies.
• Stable conditions.
• Haze/fog may sometimes remain trapped below the inversion.

Memory Point

Subsidence → compression → warming → lower RH → cloud dissipation.

Conversely:

Uplift/convection → cooling → saturation → cloud formation.

30. Cloud Ceiling

In aviation meteorology, cloud ceiling is the height above aerodrome level of the lowest cloud layer covering more than 4 oktas of the sky.

Cloud Amount

Therefore, the ceiling is determined by the lowest BKN or OVC layer.

Key Points

• Cloud amount is reported in oktas.
• Ceiling relates to the lowest layer with more than 4 oktas.
• Reference height is above aerodrome level.

31. Cloud of Operational Significance

A cloud of operational significance has specific aviation reporting implications, including CAVOK assessment.

Height Criterion

The cloud base is considered operationally significant when it is below:

1,500 m (5,000 ft) or the highest MSA, whichever is greater.

Special Rule

Cumulonimbus (CB) and Towering Cumulus (TCU) are always clouds of operational significance, regardless of their base height.

Important Points

• Threshold: 1,500 m / 5,000 ft.
• Height reference: above aerodrome level (AGL).
• Compare 1,500 m with the highest MSA.
• Use the greater value.
• CB and TCU are always significant.

32. Ceilometer

A ceilometer is an instrument used to determine cloud-base/cloud height.

Operating Principle

• Usually uses a laser/light pulse based on the LIDAR principle.
• Sends a light pulse upward.
• The pulse is scattered by the cloud base.
• The instrument measures the return time.
• Cloud-base height is calculated.

Operational Use

Ceilometers are particularly useful for determining:

• Cloud base.
• Height of operationally significant low cloud.
• Vertical visibility when the sky is obscured.

Reporting

When the sky is obscured, vertical visibility (VV) is reported.

Distinction

Ceilometer → cloud base/height

Transmissometer → Runway Visual Range (RVR)

The source identifies cloud height as typically reported in hundreds of feet above aerodrome level.

33. Tropical Downpour and Visibility

Intense tropical downpours, generally associated with strong convective activity such as CB or TCU, can produce extremely poor visibility.

Visibility

The source identifies visibility during extreme tropical downpours as potentially falling to:

Less than 100 m

Causes

• Very high concentration of rainwater.
• Severe meteorological visibility reduction.
• Water on the windscreen can further reduce cockpit visibility.

Associated Clouds

• Cumulonimbus.
• Towering Cumulus.

Operational Hazard

Extremely poor visibility combined with convective hazards creates a significant aviation risk.

34. Important Cloud–Weather Relationships

35. High-Yield Aviation Meteorology Facts

Cloud Levels

• Low: Surface–6,500 ft.
• Medium: 6,500–23,000 ft.
• High: 16,500–45,000 ft.
• CB/CU: Great vertical development and may cross multiple levels.

Cloud Type and Stability

• Stable → Stratiform
• Unstable → Cumuliform

Precipitation

• CU/CB → Showers
• NS/AS → Continuous or intermittent rain/snow
• ST/SC → Drizzle
• CI/CS/CC → Little or no significant precipitation

Most Important Hazards

• CB → Most hazardous cloud
• Hail → CB
• Severe turbulence → CB
• Severe icing → CB
• Mountain-wave turbulence → Lenticular clouds
• Mid-level instability → ACC
• Halo → CS/ice crystals
• Corona → AS/AC/water droplets
• Virga → Dry air below cloud
• Freezing rain → Severe clear/glaze icing

Contrails

• Mintra → Minimum contrail level
• Maxtra → Maximum contrail level

Instruments

• Ceilometer → Cloud base/height
• Transmissometer → RVR
• Weather radar → Precipitation

36. Quick Revision Table

37. Final Exam Memory Map

ST

Low + stable + layer + drizzle

AS

Medium + layer + frontal + continuous/intermittent rain/snow

NS

Deep layer + warm front + widespread ascent + continuous rain/snow + icing

CU

Convection + thermals + unstable air + turbulence

ACC

Mid-level instability + possible CB development

CB

Maximum instability + all levels + showers + hail + severe icing + violent turbulence

CI

High + ice crystals + nil/trace icing + nil turbulence

CS

High + ice crystals + halo

ACL

Mountain waves + rotor turbulence + possible severe icing

Radar

Precipitation detection; hail gives strongest echo

Ceilometer

Cloud base/height

Mintra / Maxtra

Lower / upper limits of contrail formation

Halo / Corona

Halo = ice crystals + refraction

Corona = water droplets + diffraction

Virga

Precipitation evaporates before reaching ground

Freezing Rain

Supercooled liquid + freezes on impact + severe glaze/clear ice

Showers

CU/CB + sudden start/stop + rapid intensity changes

Continuous Precipitation

NS/AS and other layer clouds