Notes on Vertical Motion, Clouds, and Contrails

Vertical Motion and Clouds

  • Vertical motion in the atmosphere is generally of small magnitude compared to horizontal wind, but it is crucial for weather development and aviation (turbulence, convergence, divergence, wind shear, etc.).

  • Role in aviation: vertical motions due to turbulence influence flight comfort, safety, and operations (landing/take-off). Localized vertical motion contributes to fog and cloud formation; fractional turbulence is significant for cloud/fog generation.

  • Causes of vertical motion

    • Localised vertical motion

    • Due to friction, terrain, and convection.

    • Sea/Land Breeze, Katabatic Winds, Anabatic Winds, Valley Winds arise from local vertical motion.

    • Local vertical motions are typically confined to a few kilometers, except in severe thunderstorm events.

    • Large-scale vertical motion

    • Driven by pressure systems, frontal systems, upper-air troughs and ridges, and mountains.

    • Can extend across hundreds of kilometers and to great heights; mountain-wave-induced motion may reach the upper stratosphere.

    • Frictional eddies

    • Ground friction disrupts smooth airflow and creates multiple small circulations (eddies) which can be horizontal, vertical, or slant.

    • In an unstable atmosphere these eddies can grow and rise to about 1\ \text{km} height.

  • Terrain effects on vertical motion

    • Air flows across mountains: air ascends on windward side and descends on leeward side.

    • Vertical motions are intensified by atmospheric instability and damped by inversion; when winds are strong, mountain waves are produced on the leeward side.

    • Eddies form on both slopes; disturbances from mountain waves can extend into the upper stratosphere where nacreous clouds form due to transported water vapor from lower levels.

  • Convection

    • Ground heating causes air to rise in convection cells, called thermal eddies or thermals.

    • Gliders exploit thermals for lift.

    • Thermal eddies cause bumpiness and promote convection clouds (Cumulus and Cumulonimbus) with strong updrafts and downdrafts.

  • Pressure systems

    • All pressure systems (low, cyclone, anticyclone, etc.) force air to ascend or descend over wide areas, generating weather and impacting air operations.

    • They also cause convergence and divergence.

  • Frontal zones

    • Fronts provide sloping surfaces along which wind rises.

    • Warm fronts: gradual vertical motion over a large area.

    • Cold fronts: abrupt, sharp upward motion leading to convection, thunderstorms (TS) and squally weather.

  • Wind shear

    • Vertical wind shear produces strong eddies and turbulence.

    • Clear Air Turbulence (CAT) associated with the Jet Stream is due to vertical wind shear and is amplified over mountains due to mountain waves.

  • Convergence and divergence (Key concepts)

    • Convergence

    • Net horizontal inflow of air into a region (velocity convergence).

    • Air accumulates and rises near ground; at upper levels, convergence can cause both upward and downward motion, especially below the tropopause (stratosphere acts as a barrier).

    • Associated with lows, cyclones, depressions, and troughs; leads to upward motion and bad weather but can improve visibility in precipitation-free conditions.

    • Divergence

    • Net horizontal outflow of air (velocity divergence).

    • Example: if wind to the West of a station is 10 kt and to the East is 20 kt, less air enters and more leaves, resulting in divergence.

    • In the upper atmosphere, divergence causes subsidence.

    • Anticyclones and ridges are associated with divergence, leading to fair weather but potentially poorer visibility.

CLOUDS

  • Cloud definition and formation

    • Cloud: an aggregate of visible water droplets or ice particles.

    • Clouds form by adiabatic lifting and cooling of air until water vapor condenses as water drops or deposits as ice particles.

    • Lifting Condensation Level (LCL): the height at which this condensation occurs.

    • Deposition: process by which water vapor changes directly into ice.

    • Clouds are dynamic, continuously evolving and decaying; recognizing cloud characteristics is essential for safe air operations.

    • Aviators should be familiar with cloud classification, appearance, and aviation hazards; photographs are provided in Appendix H.

  • Classification of clouds

    • Two basis for classification:

    • Form: Stratiform, Cumuliform, Cirriform.

    • Height: High, Medium, Low.

    • 10 major genera (genera = structurally similar groups):

    • High clouds form at approx. 6-18\ \text{km}, composed mainly of ice crystals; may cause high-level precipitation.

    • Medium clouds form at approx. 2-8\ \text{km}, contain water droplets and ice crystals; can produce snow or rain.

    • Low clouds form below approx. 2\ \text{km}, consist of water droplets or ice crystals.

  • High clouds (Ci, CS, CC)

    • Cirrus (Ci): white, fibrous filaments or patches; ice crystals; no precipitation or ice accretion.

    • Cirrostratus (CS): thin whitish veil; ice crystals; may produce halos around Sun/Moon; when thick, may cause snowfall restricted to high/medium levels; little to no ice accretion.

    • Cirrocumulus (CC): thin, white, wave-like small patches of ice crystals; sky passes between patches; generally no precipitation and minimal ice accretion.

  • Medium clouds (Altocumulus and Altostratus)

    • Altocumulus (AC): white-grey patches, puff-like or rolled forms; arranged in waves or lanes; sky visible between patches.

    • Altocumulus Lenticularis: lens-shaped clouds forming at crests of mountain waves on the lee side of mountains; no precipitation, but possible ice accretion.

    • Altostratus (AS): greyish/whitish uniform cloud sheets; may cover sky; sun visible if thin (ground-glass appearance); Corona phenomena can occur; thick AS can cause continuous rain or snow; ice accretion possible below freezing.

  • Low clouds (Stratus, Stratocumulus, Nimbostratus)

    • Stratus (ST): gray layer with a uniform base; can stretch across the sky; may produce drizzle; ice prisms or snow grains; fog if cloud base touches the ground; ragged patches (Fracto Stratus) may appear in bad weather.

    • Stratocumulus (SC): low, large irregular puffs or rolls; grey or whitish; more regular arrangement than ST.

    • Nimbostratus (NS): dark grey, thick layer, continuous rain or snow; often covers both middle and lower levels; ice accretion possible below freezing; +ice pellets may fall from thick NS during warm fronts.

  • Clouds with vertical development

    • Cumulus (CU): detached, dense with sharp outlines; base dark and nearly horizontal; vertical growth into mounds, domes, or towers; bulging upper part may resemble a cauliflower; sunlit portions are bright white.

    • Towering Cumulus (TCU): well-developed CU with greater vertical extent.

    • Fair Weather Cumulus (CU): limited vertical extent; typically daytime; form from surface heating; develop in forenoon, peak in afternoon, dissipate by evening.

    • Cumulonimbus (CB): heavy, densely developed cloud with large vertical extent; often has an anvil-shaped top; dark base with ragged cumulus below; precipitation may be seen as Virga (raindrops/snow that evaporate before reaching the ground); CB produces all types of precipitation (rain, hail, snow, sleet, etc.) and is the most hazardous to aviation; avoidance is advised.

  • Very high level clouds

    • Nacreous (Mother of Pearl) clouds: rare; observed mainly in Scotland and Scandinavia; resemble cirrus or Altocumulus Lenticularis; exhibit strong irisation (rainbow colours); may be composed of ice crystals or supercooled droplets; form in the upper stratosphere.

    • Noctilucent clouds: resemble cirrus; bluish/silvery, sometimes orange-red; occur in the upper mesosphere (about 80–85 km); visible after sunset and before sunrise between about 72°N and 45°N, most frequently near 55°N in summer (late May–mid August); historically thought to be meteoric dust, but measurements show they consist of ice particles.

  • Reporting cloud base

    • Cloud base reported from above ground level (AGL) in METAR, SPECI, TAF, and Local Forecast.

    • In area forecast charts, cloud base is reported above mean sea level (AMSL).

Altitude and Heights of Cloud Genera (Approximate)

  • The transcript provides altitude ranges by region and cloud level:

    • Polar Region:

    • High clouds: 10{,}000-25{,}000\ \text{ft} \ (3-8\ \text{km})

    • Medium clouds: 6{,}500-13{,}000\ \text{ft} \ (2-4\ \text{km})

    • Low clouds: up to 6{,}500\ \text{ft} \ (2\ \text{km})

    • Temperate Region:

    • High: 16{,}000-45{,}000\ \text{ft} \ (5-13\ \text{km})

    • Medium: 6{,}500-23{,}000\ \text{ft} \ (2-7\ \text{km})

    • Low: up to 6{,}500\ \text{ft} \ (2\ \text{km})

    • Tropical Region:

    • High: 20{,}000-60{,}000\ \text{ft} \ (6-18\ \text{km})

    • Medium: 6{,}500-25{,}000\ \text{ft} \ (2-8\ \text{km})

    • Low: up to 6{,}500\ \text{ft} \ (2\ \text{km})

  • Note: The transcript also indicates a table with “CB Flying Conditions In Clouds” and related notes; some lines are garbled, but the key idea is the altitude ranges above for high, medium, and low clouds by region, and the inclusion of CB and other genera.

  • Flying conditions by cloud type

    • Stratiform clouds (AS and CS): generally smooth flight; thick AS and NS during monsoons can still be smooth; NS reduces visibility due to continuous precipitation; ice accretion possible above freezing levels in AS and NS.

    • Cumuliform clouds (CC, AC, CU, TCU, CB): can cause turbulence; fair-weather CU/SC cause bumpiness; well-developed CU/TCU/CB pose serious hazards (turbulence, hail, strong updrafts/downdrafts, gusts, icing, lightning); avoid flying through CB; turbulence can extend 10–20 km around an active CB; squalls associated with CB can disrupt landings and takeoffs; avoid during thunderstorms; hail may occur in clear air below the anvil.

  • Cloud of Operational Significance

    • A cloud with base below 1500 m (5000 ft) or below the highest minimum sector altitude, whichever is greater.

  • Cloud amount (how the sky is partitioned)

    • Sky is divided into eight octas; corresponding sky condition and ceiling:

    • 0/8: Nil → SKC (Sky Clear)

    • 1-2/8: FEW → Fine

    • 3-4/8: SCT → Partly cloudy

    • 5-7/8: BKN → Cloudy (more than half sky covered)

    • 8/8: OVC → Overcast

  • Height of cloud base (definitions)

    • Height is the distance from the surface to the base of the lowest cloud layer.

    • In weather reports, cloud base height is given AGL; in area forecasts, it is given AMSL.

Cloud Species (Based on form and structure)

  • Fibrous: Clouds in the form of filaments without tufts or hooks.

  • Lenticularis: Elongated lens-shaped CC, AC, or SC clouds with sharp margins; typically of orographic origin or on the lee side of CB/TCU.

  • Castellanus (Castellanous): CI, CC, or AC cloud with cumuliform protuberances; usually connected by a common base, often arranged in lines.

  • Fractus: Ragged ST or CU cloud.

  • Congestus: Clouds with strong sprouting and great vertical development; bulging resembles a cauliflower.

  • Calvus: CB with no cirrus at the top but with protuberances beginning to change to a fibrous structure.

  • Capillatus: CB with a distinct anvil; usually accompanied by showers, tall convective activity, and Virga; well-defined Virga is seen.

Condensation Trails (Contrails)

  • Condensation trails are visible streaks of condensed water vapor formed in the wake of a moving aircraft.

  • Types of contrails

    • Wing Tip Trails: Thin, transient, short-lived contrails forming near wing tips and propeller edges due to aerodynamic pressure drop, expansion, and adiabatic cooling leading to condensation of atmospheric vapor.

    • Exhaust Trails: Form from condensation of moisture in aircraft exhaust at about 9\ \text{km} and above; long, persistent, and visible; reveal position and track of the aircraft.

    • Distrails (Dissipation Trails): When aircraft passage through a cloud leads to clear lanes or contrail dissipation; caused by heat from exhaust evaporating the cloud in the wake.

  • Levels related to contrail formation (Mintra, Drytra, Maxtra)

    • Mintra Level (ML): Above this, condensation trails form when the air is saturated; below ML, no contrails form. The flight level where the ambient temperature is about -45^{\circ}\text{C} provides a good estimate of ML at 100% RH for jet aircraft.

    • Drytra Level: At very low temperatures, contrails can form even when relative humidity is 0% due to the moisture from the exhaust; Drytra level is typically about 2\ \text{km} above the Mintra level for saturated air.

    • Maxtra Level: Lies in the stratosphere, above which no contrails form.

    • Tropopause considerations (Trapopause): The contrail likelihood is influenced by proximity to the tropopause; the provided diagram labels regions where contrails are No Contrails, Contrails Possible, Contrails Certain, etc., relative to ML, Drytra, and Maxtra and the troposphere/tropopause boundary.

  • Practical takeaway for operations

    • Contrail formation indicates humidity and temperature regimes; persistent contrails can imply high-altitude moisture and influence visibility and weather interpretation for flight planning.

Note: Some numerical values in the original transcript are garbled or malformed (e.g., unit formatting or misspellings). The key ideas and ranges have been preserved where interpretable (e.g., cloud altitude bands by region, octas scale for sky cover, common contrail height references). If you need a cleaned, table-like reproduction of the altitude ranges by region or a corrected version of the garbled table, I can format that separately.