Visibility and Fog

1. Definition of Visibility

Visibility on the ground and from the air are of great importance to a pilot for landing and take-off. The pilot needs to see various land marks, targets, obstructions, beacon lights, other aircraft, runways etc., while in flight and during approach and landing. Poor visibility is a serious aviation hazard. Even at airfields where ILS is installed, a pilot would prefer better visibility conditions for safe landing and take-offs.

2. Visibility Reports & RVR

Visibility Reports

• The visibility reported by a Met Office is the lowest horizontal visibility at an airfield
• It may be different in different directions due to varying amounts of atmospheric obscurities
• This visibility may also differ from vertical and slant visibilities as estimated by an aircraft while in-flight

Runway Visual Range (RVR)

• RVR equipment installed along various runways, close to the take-off and landing areas
• RVR is reported when the visibility is less than 1500 m
• RVR 2000 and 1500 are included in METAR as supplementary information
• RVR 2000 and 1500 are NOT disseminated internationally

3. Causes of Poor Visibility

Visibility in Precipitation

flowchart TD
    PV["CAUSES OF POOR VISIBILITY"] --> H["Haze ≤5000m\n(smoke/dust/water)"]
    PV --> DH["Dust Haze\n(desert storms, summer)"]
    PV --> SH["Smoke Haze\n(industrial, inversion)"]
    PV --> M["Mist 1000-5000m\nRH~100%"]
    PV --> F["FOG <1000m\nRH~100%"]
    PV --> DS["Dust Storm ≤5000m"]
    PV --> SS["Sea Spray\n(coastal, thin layer)"]
    PV --> VA["Volcanic Ash\n(almost nil, engine damage)"]
    PV --> P["Precipitation\n(1-3km mod rain,\n<1km heavy/drizzle/snow)"]
    F --> RF["Radiation Fog"]
    F --> AF["Advection Fog"]
    F --> STF["Steaming Fog"]
    F --> FF["Frontal Fog"]
    F --> UF["Upslope Fog"]
    F --> IF["Inversion Fog"]

4. Vertical and Slant Visibility

As one goes higher and higher, the transparency of the atmosphere increases considerably, except in cloud and precipitation. Visibility is lower looking towards the sun due to reflection of the moon light (also applies to looking towards the sun during daytime).

5. Instruments for Measuring Visibility

• At some Indian Met Offices: Automatic Visual Range Assessor (AVRA) is used for measuring RVR

6. Types of Fog

6.1 Radiation Fog

Favourable Conditions for Radiation Fog Formation

Characteristics of Radiation Fog

• Forms in anticyclones, ridges, and ground inversions — wind is light, skies are clear, atmosphere stable
• Fog would form even when RH is less than 100% due to hygroscopic condensation nuclei
• Minimum temperature usually occurs just after sunrise
• Most frequent at about sunrise or it thickens at that time
• Radiation fog dissipates about 2–3 hours after sunrise due to heating
• If deep, sun's heat takes longer to penetrate ground; fog may persist to midday
• Usually lifts into low stratus clouds before finally dissipating

Radiation Fog in India

6.2 Advection Fog

• Coastal fogs of Bengal and Orissa in winters are due to a combined effect of advection and radiation
• Advection fog forms any time of day and night
• More persistent than radiation fog and more extensive
• Common over sea in spring when warm air moves over cooler sea water

6.3 Steaming Fog (Sea Smoke)

• Common in Icelandic and Norwegian regions
• Over sea, often referred to as sea smoke

6.4 Frontal Fog

6.5 Upslope Fog

6.6 Inversion Fog

Fog Types — Comparison Table

flowchart LR
    subgraph Formation
    A["Clear night\n+ High RH\n+ 3-7kt wind\n+ Stable air"] --> RF["RADIATION FOG\n(most common in India)"]
    B["Warm moist air\ntransported over\ncold surface"] --> AF["ADVECTION FOG\n(land + sea, day/night)"]
    C["Cold air over\nwarm sea"] --> STF["STEAMING FOG\n(Sea Smoke)"]
    D["Rain ahead of front\nor cloud lowering"] --> FF["FRONTAL FOG"]
    E["Moist air lifted\nalong slope"] --> UF["UPSLOPE FOG\n(Adiabatic cooling)"]
    F["Subsiding cold air\n+ warm water body"] --> IF["INVERSION FOG\n(Anticyclone)"]
    end

7. Fog Dispersal

Thermal Method

When air is heated to temperature above the dew point, fog or clouds will dissipate. During the Second World War, heaters, burners, fires were used to disperse fog but the method involved large expenditures and was discontinued.

Use of Hygroscopic Nuclei

When hygroscopic particles are injected into fog water drops form. This lowers the RH of the air, leading to dissipation of fog. Seeding with dry ice or silver iodide or propane gas produces freezing and precipitation.

Practice Q&A — Chapter 7: Visibility and Fog

Master Reference Tables — Chapter 7: Visibility and Fog

Key Numerical Values

Fog Formation Conditions Summary

Visibility Obscurity Classification

Q&A Answer Key

Quick Revision Summary — Chapter 7