L5 Clouds and Precipitation

Cloud Formation

• Clouds form when air cools adiabatically below its dew point temperature.

• Water vapor condenses into water droplets or ice.

• Condensation requires condensation nuclei (hygroscopic nuclei) like dust, smoke, or salt.

• Condensation nuclei are abundant, especially in industrial areas.

• Cloud formation depends on humidity and cooling processes.

• Cooling occurs through vertical or horizontal air movement.

Vertical Lifting Actions

• Free convection (surface heating)

• Frontal or convergence (differing air masses meeting)

• Orographic (air being pushed up by geographic features)

Cloud Formation Due to Advection

• Advection is horizontal air movement.

Cloud Formation Due to Turbulence

• Turbulence mixes air, resulting in cooling and cloud formation.

Free Convection

• Free convection is caused by surface heating.

• Rising air expands and cools adiabatically, forming cumulus clouds.

• Small cumulus clouds in a stable atmosphere are called "fair-weather" cumulus.

• Fair-weather cumulus clouds are well-spaced and don't grow vertically.

• Estimating convective cloud base:

  • Take the difference between surface temperature and dew point (in °C).

  • Multiply the difference by 400 to get the approximate cloud base in feet.

  • Example: Temperature = +12°C, Dew Point = +6°C; Difference = 6°C; Cloud base = 6 \times 400 = 2400 ft.

Lifting Processes

• Air rising over a large area can form clouds.

• Gentle slopes between air masses form stratus clouds (stable atmosphere).

• Steeper slopes form large cumulus clouds (unstable atmosphere).

• Orographic ascent: Air forced to rise by mountains.

• Stable atmosphere: Orographic lifting forms layered clouds.

• Unstable atmosphere: Orographic lifting forms cumulus clouds.

Convergence Cloud

• Cloud forms where two differing air masses meet.

• Convergence can be widespread (warm and cold fronts) or localized (sea breeze).

• Cumulus clouds formed by convergence may form a 'street' (long line).

Temperature Inversions and Cloud Formation

• Temperature usually decreases with height in the atmosphere.

• Temperature inversion: Temperature increases with height.

• Inversions stabilize the atmosphere and reduce lifting processes, hindering cloud formation.

• Inversions can cause cumuliform clouds to spread out.

• Marine cumulus forms over the sea due to lower sea temperature.

Advection

• Air cools by horizontal movement.

• Warm air moving over a cooler surface (e.g., water) cools below the dew point, forming visible moisture.

Turbulence Mixing

• Turbulent mixing between atmospheric layers can form clouds.

• Warmer air (above dew point) lies below cooler air.

• Turbulence mixes layers, raising warmer air to a level where temperature falls below the dew point.

• Mixing can be caused by rough terrain or surface heating.

• Turbulence cloud may be layered or fractured (stratocumulus-like).

• Turbulence cloud formation is difficult to forecast.

• Clear skies at night, rising sun, and light wind (10-15 kts) can cause turbulence cloud.

• Turbulence cloud is usually dispersed as the sun's energy increases.

• Anticyclones in winter can cause persistent turbulence cloud (winter "anticyclonic gloom").

Cloud Types and Classifications

• Clouds are classified by characteristics and level.

• Two basic classifications: vertical cumulus-type and horizontal stratus-type.

• Classification by level:

  • Low Cloud: 6500 ft and below (stratus, cumulus, etc.)

  • Medium Cloud: 6500 ft to 23,000 ft (prefix alto, e.g., altocumulus)

  • High Cloud: 16,500 ft to 45,000 ft (cirriform clouds, e.g., cirrostratus, or cirrus)

• Levels are approximate for temperate latitudes (30° to 60°).

• Polar regions: Levels are lower.

• Tropical areas: Levels are higher.

• Precipitation-bearing clouds: Prefix nimbo or suffix nimbus (nimbostratus, cumulonimbus).

• Clouds with large vertical extent (cumulonimbus, nimbostratus) extend from low to medium levels, sometimes reaching high levels and the tropopause.

• Tropopause typically stops vertical growth, except for energetic cumulonimbus clouds.

Basic Classification of Clouds

• Stratus (ST):

  • Gray sheet-like layer covering the sky.

  • Low base, poor visibility.

  • Stable, little or no turbulence.

• Cumulus (CU):

  • Well-defined, varying vertical extent.

  • Detached, but can be embedded.

  • Strong vertical currents, rapid vertical growth.

  • Large cumulus: turbulence, unstable atmosphere.

  • Good visibility outside cumulus.

• Stratocumulus (SC):

  • Hybrid cloud type, various forms.

  • Formed by cumulus collapsing, turbulent mixing, or orographic lifting.

  • White or grayish patches or uniform layer.

  • Neither strictly cumuliform nor stratiform.

• Nimbostratus (NS):

  • Thick, dark-gray layered cloud with indistinct base.

  • Precipitation over a wide area.

  • Stable atmosphere, lots of moisture.

  • Associated with warm fronts, prolonged heavy precipitation.

  • Thickness of at least 4000 ft is indicated by anything other than the lightest drizzle.

• Altostratus (AS):

  • Flat, layer clouds, light gray or white.

  • Possible precipitation, little turbulence.

  • Clearly defined base, less thick than nimbostratus.

• Altocumulus (AC):

  • Gray or white patches of rolled or bubble cloud.

  • Possible precipitation.

  • Virga: rain or snow evaporating before reaching the ground.

• Cirrus (CI):

  • Thin, wispy clouds of ice crystals.

  • Little change of form or shape.

  • Little moisture or turbulence.

• Cirrostratus (CS):

  • Transparent veil of high cloud.

  • Halo effect around the sun or moon.

  • Indicates an approaching front.

• Cirrocumulus (CC):

  • Not common, shallow convection currents at high altitude.

  • Some turbulence.

  • Regular flakes or ripples, "mackerel sky" visual effect.

• Cumulonimbus (CB):

  • Cloud associated with thunderstorms.

  • Great vertical extent, base near the surface, top at the tropopause (anvil shape).

  • Hazards: turbulence, windshear, intense precipitation, lightning.

Cloud Types and State of the Atmosphere

• Clouds indicate the state of the atmosphere.

• Large cumulus: strong vertical currents, unstable atmosphere.

• Taller cumulus: greater instability.

• Extensive cumulus: "changeable" conditions (sunshine, then rain).

• Turbulence in and around cumulus clouds.

• Stratus: settled weather, stable atmosphere.

• Stratus cloud covers a wide area; rain likely to continue for some time.

• Smooth flying conditions in and around stratus clouds.

Assessing Wind Direction

• Clouds indicate wind direction at altitude.

• Clouds at different levels may move in different directions.

Reporting Cloud Types and Amounts

• Area forecasts indicate cloud type.

• METARS/TAFS record cloud type only if towering cumulus (TCU) or cumulonimbus (CB).

• Cloud cover in oktas (eighths):

  • One okta: one-eighth of the sky covered.

  • Eight oktas: whole sky covered.

• Example: Northern half of the sky covered = 4 oktas (scattered).

• Simplified four-grade classification:

  • Few (FEW): 1 to 2 oktas

  • Scattered (SCT): 3 to 4 oktas

  • Broken (BKN): 5 to 7 oktas

  • Overcast (OVC): 8 oktas

• Cloud amount is measured from a fixed point.

METAR Example

• EGPK 251450Z 18018G30KT 140V210 9999 - RA SCT008 BKN020 OVC080 07/05 Q0992.

  • Scattered (3-4 oktas) cloud base at 800 ft above aerodrome level.

  • Broken (5-7 oktas) cloud base at 2000 ft.

  • Overcast (8 oktas) cloud base at 8000 ft.

Precipitation

• Precipitation: Particles falling from cloud (rain, drizzle, snow, hail).

• Anything other than lightest precipitation (drizzle) indicates cloud at least 4000 ft thick.

• Heavier precipitation implies thicker cloud.

• Stratiform clouds: Light precipitation (drizzle, light rain, light snow).

• Cumuliform clouds: Moderate or heavy precipitation.

• Cumulonimbus clouds: Hail and hailstones (up to 1 lb).

• Nimbostratus: Prolonged rain.

Precipitation Types

• Drizzle (DZ):

  • Water drops between 0.2 mm and 1.0 mm.

  • Light liquid precipitation.

  • Falls from stratus and stratocumulus clouds.

• Rain (RA):

  • Water drops between 1.0 mm and 5.8 mm.

  • Larger clouds with internal updrafts.

  • Coalescence: Smaller droplets merge to form larger drops.

  • Different intensities (slight, moderate, heavy).

  • Drops larger than 5.8 mm break up due to airflow pressure.

• Snow (SN):

  • Ice crystals.

  • Reaches the ground if surface temperature is +4°C or below.

  • Heaviest snowfalls are around 0°C (freezing).

• Snow Grains (SG):

  • Small ice particles, frozen equivalent of drizzle.

• Hail (GR):

  • Hard ice stones from cumulonimbus clouds.

  • Mostly spring or summer occurrence.

  • Small hail (GS).

• Ice Pellets (PE):

  • Pellets of clear ice falling from thick layered clouds such as nimbostratus.

• Diamond Dust (IC):

  • Tiny ice crystals suspended in the air, falling from clear skies in calm arctic conditions.

Virga

• Rain falling from a medium-level cloud but evaporating before reaching the surface.

• Characterized by streaks below the cloud.

Sleet

• UK: rain and snow or snow melting to form rain.

• USA: ice pellets.

• International weather communications: rain and snow (RASN).

Precipitation Intensity

• In METARS and TAFS:

  • =: slight

  • No indication: moderate

  • +: heavy

Operational Risks

• Drizzle: low cloud, poor visibility, high humidity, increased chance of carburetor icing.

• Rain (especially heavy): affects runway surface, difficult aircraft controllability, lengthens take-off and landing distance.

• Snow: reduces visibility, affects surface movements, may close airfield (SNOCLO in VOLMET reports).

Metform Chart Examples

1. 7 kilometers visibility, moderate rain.

2. Isolated 3000 metres visibility, heavy rain.

3. Occasionally 5000 metres visibility, moderate rain or moderate rain and drizzle.

4. 15 kilometers visibility, nil precipitation or slight rain.