Notes on Stability, Optical Phenomena, Precipitation, and Ice Accretion

9. STABILITY AND INSTABILITY OF ATMOSPHERE

  • Atmospheric processes rely on air behaving as a poor conductor of heat; a rising or falling parcel can be treated as insulated from its surroundings (adiabatic behavior if no heat exchange).

  • Processes discussed:

    • Isothermal Process: heat enters or leaves so that the parcel’s temperature remains the same.

    • Adiabatic Process: no heat exchange with surroundings. Etymology: a = not, diabano = pass through. If a parcel ascends adiabatically, it moves to a region of lower pressure and expands; the work done in expansion reduces internal energy, so its temperature falls. Conversely, a descending parcel is compressed, its internal energy increases, and its temperature rises. Adiabatic processes largely determine the vertical temperature distribution in the atmosphere.

  • Lapse Rate (LR): the rate at which temperature decreases with height. Positive LR means temperature decreases with height. Typical value in the troposphere: LR≈6.5 ∘C km−1.LR \approx 6.5\,^{\circ}\mathrm{C} \mathrm{\,km^{-1}}.

  • DALR, ELR, SALR (key lapse rates)

    • Dry Adiabatic Lapse Rate (DALR): the rate at which a dry parcel cools as it rises adiabatically; means DALR≈9.8 ∘C km−1\mathrm{DALR} \approx 9.8\,^{\circ}\mathrm{C\,km^{-1}} (≈ 3°C per 1000 ft).

    • Environmental Lapse Rate (ELR): the actual lapse rate in the atmosphere at a given place/time. In ISA, ELR is stated as ELRISA=0.3 ∘C km−1ELR_{ISA} = 0.3\,^{\circ}\mathrm{C\,km^{-1}} in the transcript (note: ISA often cited as ~6.5°C km⁻¹ in standard teaching, here ELR value is per transcript).

    • Saturated Adiabatic Lapse Rate (SALR): the lapse rate when the air is saturated and lifted adiabatically; the extra latent heat release from condensation reduces the cooling rate. Typically
      SALR≈6 ∘C km−1  (not constant, varies with conditions).\mathrm{SALR} \approx 6\,^{\circ}\mathrm{C\,km^{-1}} \;\text{(not constant, varies with conditions).}

  • Inversion

    • Inversion is a situation where the temperature increases with height (negative lapse rate).

    • Common low-level inversions occur in winter due to nocturnal cooling; inversions indicate stability and reduced turbulence.

    • Inversion layers can suppress surface-to-air mixing; smoke, haze, mist, dust, and fog may stagnate near the ground.

    • Inversion is often found at the top of stratified cloud layers or fog, especially radiational fog.

  • Inversion Layer

    • An atmospheric layer in which temperature increases with height (i.e., a temperature inversion).

    • Vertical motion through such a layer is inhibited due to stability.

  • Instability and Stability (conceptual)

    • A system is unstable if a displaced parcel does not return to its original position (example: a ball on top of an inverted watch-glass rolls away).

    • A stable system returns to its original position after displacement (example: ball in a convex glass oscillates like a pendulum).

    • The stratosphere and thermosphere are generally stable.

  • Stability assessment in a lifting parcel

    • Given an air parcel in a layer, a stability estimate can be made from an aerological diagram (or equivalent thermodynamic analysis) by comparing the parcel’s temperature profile after lifting with the environmental temperature profile (ELR).

    • Tropospheric layers where temperature decreases with height tend to be more unstable; stable or unstable conditions depend on the relative values of DALR, SALR, and ELR.

  • Conditional Instability

    • DALR > ELR > SALR

    • The atmosphere is unstable for saturated air but stable for unsaturated air in this case.

  • Latent Instability (including Convective Instability)

    • When a layer has latent heat release upon lifting, it can enhance instability.

    • Latent instability occurs if the energy release upon lifting exceeds the energy required to lift the parcel; two related concepts mentioned are:

    • Latent instability (real, thermodynamic): when the latent heat release is sufficient to drive instability.

    • Pseudo latent instability: when the energy required to lift is more than the energy released later.

    • Convective instability can arise when lifted layers experience sufficient latent heat release to steepen the lapse rate.

  • Stability Criteria (summary)

    • For a dry atmosphere:

    • If \mathrm{DALR} > ELR\, → absolutely stable

    • If DALR=ELR \mathrm{DALR} = ELR\, → neutral stability

    • If \mathrm{DALR} < ELR\, → unstable

    • For a saturated atmosphere:

    • If \mathrm{SALR} > ELR\, → absolutely stable

    • If SALR=ELR \mathrm{SALR} = ELR\, → neutral stability

    • If \mathrm{SALR} < ELR\, → unstable

    • Conditional instability: \mathrm{DALR} > ELR > \mathrm{SALR}

  • Normand Theorem and LCL (Lifting Condensation Level)

    • The Normand Theorem states that if one draws upwards from dry bulb temperature ( TT ), SALR from wet bulb/wet-bulb ( TwTw ), and isohydric from the dew point ( TdTd ) temperatures, the respective lines meet at a point called the Normand Point.

    • The first Normand Point from the surface is called the Lifting Condensation Level (LCL) and indicates the height at which cloud formation may begin.

  • Notes

    • The Stratosphere and Thermosphere are described as stable layers.

10. OPTICAL PHENOMENA

  • Optical phenomena provide information about clouds, turbulence, lapse rate, ice accretion, and are important for aviation.

  • Rainbow

    • A group of concentric colored arcs produced by sun or moonlight refracted, reflected, and dispersed by raindrops or other water droplets.

    • Primary rainbow: circle/arc with center opposite the luminary; subtends an angle of ≈42∘\approx 42^{\circ} at the observer.

    • Secondary rainbow: concentric and fainter, radius about ≈52∘\approx 52^{\circ}, red on the outside, violet on the inside.

    • The colors show the sequence VIBGYOR (violet, indigo, blue, green, yellow, orange, red).

  • Halo

    • A halo is a circle of light around the sun or moon produced by refraction through ice crystals in cirrostratus clouds.

    • Small halo: typical radius 22∘22^{\circ}; often appears white; halo can indicate ice-crystal-rich high clouds and potential icing conditions.

    • Large halo and other halo forms may be observed; red on the outside, violet/blue inside for some halos.

  • Corona

    • Luminous rings around the sun or moon caused by diffraction of light passing through mist, fog, or a cloud composed of very small water drops or ice particles (often in Altostratus or Cirrostratus).

    • Corona rings are often small (not more than 5∘5^{\circ}) and show color ordering with red on the outside and violet/blue on the inside.

    • Diffraction is due to the boundary interaction of light with small droplets; coronae indicate moderate icing if cloud is above freezing.

  • Mirage

    • Light rays bend due to refractive index changes with height as density decreases with altitude.

    • Result: distant objects appear at altered positions; mirages are common when surface temperature is much higher or lower than the air above.

  • Bishop's Ring

    • A mirage-related phenomenon: a whiteish ring around the sun or moon, with bluish tinge inside and reddish-brown outside; radius ~22∘22^{\circ}; due to diffraction by fine dust particles in the upper atmosphere (dust may be volcanic).

  • Twilight colours and Irisation

    • Sunset/sunrise colours arise from refraction and selective absorption of light in the atmosphere.

    • Irisation: pink colours in certain diffractive patterns near clouds; bands can appear parallel to the line of sight.

  • Glory

    • One or more colored rings around the observer’s shadow, seen in fog or mist when light diffracts from many tiny droplets.

    • Similar to corona but seen around the observer’s shadow; common around aircraft shadows in flight.

  • Crepuscular Rays

    • Dark bluish rays radiating from the sun; shadows cast by clouds near the horizon.

  • Aurora

    • Atmospheric electricity phenomenon occurring high in the atmosphere (heights approx. 70–1000 km; peak around 100 km).

    • Displays include streamers, rays, arches, curtains; colors typically greenish-white, red, or yellow.

    • Caused by excitation of atmospheric gases in the rarified upper atmosphere due to bombardment by energetic particles from magnetic storms; can disrupt radio communications.

    • Northern Hemisphere: Aurora Borealis (Líghts) around high latitudes; Southern Hemisphere: Aurora Australis.

  • Lightning and atmospheric electricity (summary)

    • In fair weather, the Earth is negatively charged with a typical potential around ~100 V relative to the air.

    • During a thunderstorm, this direction reverses locally, with precipitation carrying net positive charge toward the ground.

    • Types of lightning:

    • Ground discharge (thunderbolts): cloud-to-ground; often with a main channel and sometimes a luminous ball following the discharge.

    • Cloud discharge (sheet lightning): occurs within a cloud.

    • Air discharge: from cloud to air, not striking the ground.

  • Saint Elmo’s Fire

    • A bluish or greenish luminous discharge observed when flying in cumulonimbus clouds under strong electrostatic fields; accompanied by crackling sounds; intensity ranges from weak to moderate.

11. PRECIPITATION

  • Definition: Liquid water droplets or solid water particles falling from clouds to the ground.

  • Forms and typical diameters:

    • Drizzle (DZ): droplets of diameter 0.2−0.5 mm0.2-0.5\ \mathrm{mm}.

    • Rain (RA): diameter 0.5−5 mm0.5-5\ \mathrm{mm}.

    • Shower (SH): droplets with diameter > 5\ \mathrm{mm} (often short-lived).

    • Hail (GR): solid ice balls or hailstones with diameters 5−50 mm5-50\ \mathrm{mm} or more.

    • Snow (SN): ice crystals in aggregation.

    • Sleet (often written as SW in notes): wet snow or ice pellets; Ice Pellets (IP) as small transparent ice particles.

    • Snow grains (SG) and ice crystals (IC) also occur as precipitation forms.

    • Note: 1 kg of hail has been reported; precipitation processes involve growth to sizes capable of overcoming updrafts.

  • Formation theories for precipitation

    • Bergeron Ice Crystal Theory (cold clouds):

    • In clouds with tops well above freezing, supercooled liquid droplets coexist with ice crystals.

    • Water vapor pressure is higher over droplets than over ice; water drops evaporate/sublimate from ice crystals, allowing ice crystals to grow at the expense of water droplets.

    • As ice crystals fall, they encounter supercooled drops which freeze on contact; larger crystals form and fall as snow or rain.

    • Coalescence Theory (warmer clouds):

    • Large drops form by collision and coalescence of smaller droplets; vertical currents promote growth; larger drops then collide with and collect smaller drops on their path, forming very large drops that fall as rain.

    • Giant Nuclei (Giant Hygroscopic Nuclei) Theory (maritime areas):

    • Abundant salt particles from sea spray serve as giant condensation nuclei; facilitate growth of large drops and coalescence, aiding rain formation.

  • Clouds and precipitation (types and associated precipitation)

    • Stratus (ST): generally drizzle and light rain.

    • Altostratus (AS) and Nimbostratus (NS): continuous precipitation (rain or snow).

    • Towering Cumulus (TCU) and Cumulonimbus (CB): heavy precipitation, including rain, showers, hail, and snow.

    • Cumulus (CU) – fair weather; little to no precipitation.

    • Altocumulus (AC) and Cirrus/ Cirrostratus (CI, CC) – typically little precipitation; occasional virga or light precipitation at certain layers.

    • Cirrostratus (CS) – can be associated with precipitation via distant/virga events; exact associations vary with conditions.

  • Snow and sleet

    • Snow forms when the air temperature is sufficiently low; snowflakes form from ice crystals and remain as snow if surface temperature is low enough.

    • Snowfall typically occurs when the mean cloud-base to surface temperature is below freezing; snow grains are tiny; sleet is a mixture of rain and snow that freezes upon reaching the ground.

  • Cloud bursts and flash floods

    • Cloud burst: very heavy showers over a small area in a short period.

    • Flash floods: rapid rise in river/stream levels due to intense rainfall.

  • Rainy day and rainfall distribution

    • Rainy Day: daily rainfall ≥ 2.5 mm2.5\ \mathrm{mm}.

    • Diurnal and seasonal variation:

    • Tropics: convection-driven rainfall, most common in the afternoon.

    • Coastal regions/valleys (land-sea breeze): rainfall often at night or early morning in some locales (e.g., NE India due to topography).

    • Temperate latitudes: rainfall more common in winter due to frontal systems and depressions; western disturbances contribute variability; regional patterns differ by location.

  • Artificial rain making / Cloud seeding

    • Cloud seeding involves introducing nuclei into clouds (e.g., Potassium chloride, silver iodide, common salt, dry ice) to stimulate condensation and coalescence processes (Bergeron process) and enhance rainfall.

    • Potential benefits include firefighting, drought relief, augmented harvests, and fog dispersal, but rainfall may be redistributed rather than created.

  • Fog dispersal

    • Fog can be cleared temporarily by warming the air or dispersing the fog to enable landing and takeoff operations.

  • Classification of rainfall by intensity (Table concept)

    • Light: around 7.5 mm7.5\ \mathrm{mm} or less.

    • Moderate: roughly 7−35.5 mm7-35.5\ \mathrm{mm}.

    • Rather heavy: 35.6−64 mm35.6-64\ \mathrm{mm}.

    • Heavy: 65−84.9 mm65-84.9\ \mathrm{mm}.

    • Very heavy: ≥85 mm\ge 85\ \mathrm{mm}.

    • Exceptionally heavy: up to or above 250 mm250\ \mathrm{mm}.

12. ICE ACCRETION

  • Ice accretion (icing) on aircraft is a serious aviation hazard affecting aerodynamics and performance; de-icing devices mitigate but knowledge of icing types, formation, and avoidance is essential.

  • Types of icing

    • Airframe icing

    • Engine icing

  • Airframe icing (leading to performance degradation)

    • Hoar frost: feathery ice crystals that form on an airframe from sublimation; occurs when the airframe is below the frost point; may form on the ground or during a cold descent; dissipates with warming or vibration.

    • Opaque (Rime) ice: white, porous ice formed by freezing of supercooled droplets on the airframe in clouds above freezing; tends to accumulate on leading edges; traps air and is relatively light but can alter aerodynamics; can be wiped off relatively easily.

    • Translucent (Glaze) ice: clear ice formed by freezing of large supercooled droplets; forms a smooth, glassy layer that adheres strongly and is heavy; difficult to remove; poses serious hazard due to added weight and altered aerodynamics.

  • Temperature ranges for airframe icing (approximate ranges from the transcript)

    • Severe icing: 0∘C to −7∘C0^{\circ}\mathrm{C} \text{ to } -7^{\circ}\mathrm{C}

    • Moderate icing: −7∘C to −12∘C-7^{\circ}\mathrm{C} \text{ to } -12^{\circ}\mathrm{C}

    • Light icing: −12∘C to −20∘C-12^{\circ}\mathrm{C} \text{ to } -20^{\circ}\mathrm{C}

    • Very light icing: −20∘C to −40∘C-20^{\circ}\mathrm{C} \text{ to } -40^{\circ}\mathrm{C}

  • Icing dependence on cloud type (relative hazard)

    • Cirrus (CI), Cirrostratus (CS), Cirrocumulus (CC): mostly ice crystals; icing hazard is negligible.

    • Altostratus (AS), Nimbostratus (NS): contain supercooled water droplets; icing can be light to moderate.

    • Altocumulus (AC): light to moderate icing; severe icing possible in mountainous regions.

    • Towering Cumulus (TCU) and Cumulonimbus (CB): icing may range from light to severe; can extend up to -20°C level.

    • Cumulus (CU) – fair weather clouds: minimal icing risk.

  • Effects of airframe icing on aircraft

    • Changes in aerodynamics; increased weight; reduced lift.

    • Stall speed increases; control surface icing can cause loss of effectiveness and induce vibrations.

    • Unequal ice accumulation on propeller blades can cause imbalanced loads and power loss.

    • Drag increases; instrument readings (e.g., airspeed indicator) may be biased due to pitot/static tube icing.

    • Ice on antennas and sensors can degrade communications and navigation.

  • Engine icing

    • Two main types described conceptually: ingestion/airflow icing and carburetor icing.

    • Inlet icing/engine icing (seasonal impact): icing reduces air inflow, lowering engine power.

    • Carburetor icing: pressure drop in the carburetor during throttle operation with adiabatic cooling can drop temperature to very low values; humidity and fuel evaporation can foster icing; can occur even in warm ambient air (e.g., around +30°C) but is unlikely below -10°C unless liquid water is present; relative humidity below ~60% reduces icing risk.

  • Height of freezing level in India (regional seasonality)

    • North:

    • Pre-monsoon: around 700 hPa

    • Southwest (SW) monsoon: around 650 hPa

    • Post-monsoon: around 550 hPa

    • Central:

    • Pre-monsoon: around 600 hPa

    • SW monsoon: around 600 hPa

    • Post-monsoon: around 500 hPa

    • South:

    • Pre-monsoon: around 600 hPa

    • SW monsoon: around 550 hPa

    • Post-monsoon: around 500 hPa; other entries mention 650 hPa in some cases

  • Tables and figures referenced in the notes (for quick reference)

    • Normand Point and LCL relation formatted via DALR, SALR, and isohydric lines; the first Normand Point from the surface corresponds to LCL.

    • Radius and angle values in optical phenomena: rainbow ~42∘42^{\circ}, secondary rainbow ~52∘52^{\circ}, halo ~22∘22^{\circ}, etc.

    • Temperature-lapse relationships in icing tables use ranges above.

  • Practical takeaways for aviation

    • Understanding stability and lapse-rate interactions helps in predicting atmospheric turbulence, cloud formation, and icing potential.

    • Optical phenomena serve as practical cues about cloud type, moisture, and icing risk for flight planning.

    • Cloud seeding and fog dispersal are considered instruments for weather modification, with benefits and potential drawbacks in rainfall distribution.