Fog, Visibility & RVR — Summary Notes

1. Fog — Basic Definition

Fog appears as stratus cloud at the Earth’s surface that reduces horizontal visibility to:

Less than 1,000 m

Composition

Fog consists of:

• Tiny water droplets
• Or ice crystals

Water vapour itself is invisible and does not reduce visibility.

Key values

Important distinction

Fog → water droplets/ice crystals

Haze → solid particles

2. Why Fog Forms

Fog requires air to reach saturation.

The basic process is:

Cooling of moist air → Temperature reaches dew point → Saturation → Condensation → Fog

Fog formation is strongly favoured by stable air, because stability prevents vertical mixing and allows moisture to remain concentrated near the surface.

3. Stability and Fog

A temperature inversion produces an extremely stable atmosphere.

Inversion

Temperature increases with height.

Therefore:

• ELR becomes negative.
• Vertical convection is suppressed.
• Moisture remains trapped near the ground.
• Fog can persist.

The inversion acts like a lid, trapping moisture and condensation nuclei in the surface layer.

Important

Stable atmosphere → favours fog

Strong vertical mixing → dissipates fog

4. Main Types of Fog

The source covers:

1. Radiation fog
2. Advection fog
3. Frontal fog / precipitation fog
4. Subsidence fog

5. Radiation Fog

Radiation fog is also called:

• Ground fog
• Valley fog

It forms when the ground loses heat through terrestrial radiation, especially at night.

Formation sequence

Clear night → Ground loses heat → Ground cools → Adjacent air cools → Air reaches dew point → Condensation → Fog

6. Conditions Required for Radiation Fog

Four important requirements

1. Moist air / high RH
2. Clear sky
3. Radiational cooling
4. Light wind

Wind

Ideal wind:

Approximately 2–8 kt

Light wind provides enough mixing to bring slightly warmer, moist air into contact with the cold surface.

Too little wind

Calm conditions → dew/frost may form instead of fog

Too much wind

Strong wind:

• Increases mixing.
• Disperses the fog.
• May lift fog into low stratus.

7. Radiation Fog — Time of Occurrence

Radiation fog is primarily a night-time / early-morning phenomenon.

It usually:

• Forms during the night.
• Becomes most extensive near dawn.
• Dissipates after sunrise as solar heating increases.

Exam clue

Lowest temperature + early morning + clear sky + light wind → Radiation fog

8. Radiation Fog — Location

Radiation fog occurs primarily over land.

It is not normally formed over the sea because the sea surface has insufficient diurnal temperature variation to cool rapidly enough.

Common locations

• Valleys
• Plains
• Low-lying land
• Sheltered areas

9. Pressure Systems Favouring Radiation Fog

Radiation fog is favoured by:

High pressure / Anticyclone

• Subsidence
• Stable atmosphere
• Inversion
• Light winds

Ridge

• Stable conditions
• Weak pressure gradient
• Light winds

Col

• Very weak pressure gradient
• Light or stationary winds
• Prolonged contact between air and ground

Unfavourable systems

Lows and troughs generally discourage persistent radiation fog because:

• Winds are stronger.
• Rising air promotes mixing.
• Stability is reduced.

10. Slack Pressure Gradient

A slack pressure gradient means:

• Isobars are widely spaced.
• Wind is weak.
• Air remains near the surface for a longer time.

Therefore:

Slack pressure gradient + clear night + high RH → Radiation fog

11. Radiation Fog in North India

Radiation fog is especially important over the plains of North India during winter.

Main period

December to March

January

January is representative of the winter season in the source context.

12. Western Disturbance and North Indian Fog

After a Western Disturbance (WD) passes:

1. Rainfall supplies low-level moisture.
2. Rain stops.
3. Skies often clear.
4. Night-time radiational cooling becomes strong.
5. A surface inversion develops.
6. Radiation fog forms.

Fog can become widespread over the plains of North India.

Timing

The source notes that fog commonly develops:

1–2 days after rainfall ceases

and particularly in the rear sector of the Western Disturbance.

13. Advection Fog

Advection fog forms when:

Warm, moist air moves horizontally over a colder surface.

The cold surface cools the air from below until its temperature reaches the dew point.

Formation

Warm moist air → Horizontal movement → Cold surface → Cooling from below → Dew point reached → Saturation → Condensation → Fog

14. Conditions for Advection Fog

Essential conditions

• Warm air
• Moist air
• Horizontal movement
• Cold underlying surface
• Surface temperature sufficiently below the air’s dew point

Wind

Unlike radiation fog, advection fog requires air movement.

The source gives approximately:

Up to 15 kt over land

and potentially stronger winds over the sea.

15. Advection Fog — Time

Advection fog is not restricted to night.

It can occur:

• During the day
• During the night
• At any time when the required air/surface temperature relationship exists.

16. Advection Fog — Persistence

Advection fog is generally:

More extensive and persistent than radiation fog.

It can last:

• 24 hours or more
• Several days
• In some situations, much longer

Dissipation

It may clear when:

• The air mass changes.
• Wind direction changes.
• Wind speed increases sufficiently.
• Fog is lifted into low stratus.

The source identifies approximately 15 kt or more as a useful dispersal clue.

17. Advection Fog — Where It Occurs

Over land

Common when:

Warm, moist maritime air → cold land

Especially:

• Winter
• Early spring

Over sea — Sea Fog

Common when:

Warm air → relatively cold ocean

Especially:

• Late spring
• Early summer

18. Advection Fog vs Radiation Fog

19. Very Important Wind-Speed Comparison

Radiation fog

2–8 kt → favourable

Calm → dew/frost

Strong wind → fog dissipates/lifts

Advection fog

Wind is necessary

Up to ~15 kt over land can support it

Stronger wind can lift/disperse it

Memory

Radiation = light wind

Advection = moving air

20. Advection Fog and Low Stratus

When advection fog is subjected to moderate turbulence/wind:

Fog can be lifted from the surface → Low stratus / stratocumulus

The source notes that wind stronger than about 8 kt and up to around 15 kt can lift the fog into a low cloud layer.

21. Frontal Fog

Frontal fog is also called:

Precipitation-induced fog

It is primarily associated with:

• Warm fronts
• Warm occlusions

Formation

1. A shallow layer of cold air exists ahead of the warm front.
2. Rain/snow falls from warmer air above.
3. Precipitation evaporates into the cold air below.
4. Moisture increases.
5. Dew point rises.
6. Cold air reaches saturation.
7. Fog forms.

22. Frontal Fog — Important Details

Location

Cold air mass ahead of the warm front

Cloud producing precipitation

Typically:

Nimbostratus (NS)

Extent

The source states that frontal fog can form in a belt:

Up to approximately 200 NM wide

and can travel with the front.

Other fronts

It can occasionally occur with:

• Slow-moving cold fronts
• Stationary fronts

Fast-moving vigorous cold fronts are generally unfavourable for widespread persistent fog because of strong lifting, showers and convection.

23. Subsidence Fog

Subsidence fog is associated with sinking air, commonly under anticyclonic conditions.

Mechanism

Subsidence → warming aloft → inversion → stable layer → cool moist air trapped below → fog

The subsidence inversion acts as a cap over the low-level moist air.

24. Fog and Stratus

Fog is essentially a surface-based cloud.

The cloud type it most closely resembles is:

Stratus (ST)

When fog lifts from the ground, it can become:

Low stratus

Memory

Fog = Stratus at the surface

25. Fog vs Mist vs Haze

26. RVR — Runway Visual Range

RVR is the distance over which a pilot can see:

• Runway markings
• Runway lights

along the runway.

The source describes RVR as particularly important for takeoff and landing, including precision approaches.

27. When Is RVR Reported?

The source gives the key threshold as:

1,500 m

RVR assessment/reporting is initiated when meteorological visibility or RVR falls to approximately 1,500 m or below, or when shallow fog is reported/forecast.

Important distinction

Fog threshold = <1,000 m

but:

RVR reporting threshold = 1,500 m

Therefore, RVR may be reported even when the weather is classified as mist, not yet fog.

28. RVR Reporting Scale

According to the source:

The source notes that some regional practices may extend the 25 m range to 400 m.

Exam memory

Lowest RVR → smallest increment

25 → 50 → 100 m

29. Transmissometer

A transmissometer is an instrument used for measuring atmospheric light transmission and determining:

• Visibility
• Particularly RVR

Basic principle

It sends light from a transmitter toward a receiver.

The amount of light transmitted through the atmosphere indicates atmospheric opacity/transmissivity.

Do not confuse

Transmissometer → Visibility/RVR

Ceilometer → Cloud-base height

30. IRVR and Three Measurement Positions

A station equipped with Instrumented RVR (IRVR) may use three transmissometers at:

1. Touchdown zone
2. Mid-point
3. Stop-end

This allows RVR to be determined at different sections of the runway.

31. Why Only One RVR Value May Be Reported

Sometimes a report may show only the touchdown value, for example:

R 1000

The source explains that mid-point and stop-end values can be omitted/suppressed when they meet the relevant criteria, including values of 800 m or more.

Therefore, if only R 1000 is reported, the omitted values may be ≥800 m.

32. Radiation Fog — Exam Scenario

Given:

• Autumn/winter
• Clear sky
• High RH
• Light wind: 2–8 kt
• Land surface

Answer:

Radiation fog

33. Odd-One-Out Type Question

If the situation says:

• Early morning
• No significant wind
• No hill
• Conditions favour ground cooling

The source identifies early morning as the best answer for radiation fog because:

• Advection fog requires air movement.
• Hill/upslope fog requires suitable terrain.
• Radiation fog develops after prolonged night-time cooling.

34. Conditions That Prevent Radiation Fog

Strong wind

Strong wind causes excessive mixing and can:

• Dissipate fog
• Lift fog into stratus

Summer

The source associates radiation fog primarily with autumn/winter because longer nights provide greater cooling time.

Sea

Radiation fog is generally not formed over the sea because of insufficient diurnal temperature variation.

Hill

Radiation fog is not the same as hill/upslope fog; terrain-related fog has a different formation mechanism.

35. Fog Formation — Pressure System Comparison

36. Aviation Hazards of Fog

Fog creates major operational problems because of:

• Reduced visibility
• Reduced RVR
• Difficult runway identification
• Takeoff/landing restrictions
• Approach limitations
• Potential suspension of operations

Frontal fog can become particularly extensive and dense, sometimes severely disrupting or suspending operations.

37. Fog Dissipation

Radiation fog

Mainly dissipates through:

Sunrise → Solar heating → Turbulent mixing → Fog burns off

Advection fog

Mainly dissipates through:

• Change of air mass
• Wind shift
• Increased wind speed
• Lifting into low stratus

38. Master Fog Comparison

39. High-Yield Numbers to Memorize

40. Final Rapid Revision

Radiation Fog

Night + clear sky + moist air + 2–8 kt + land

→ Radiation fog

Advection Fog

Warm moist air + horizontal movement + cold surface

→ Advection fog

Frontal Fog

Warm front + precipitation + cold air below

→ Frontal/precipitation fog

Subsidence Fog

High pressure + sinking air + inversion + trapped moisture

→ Subsidence fog

Visibility

FG < 1,000 m

BR = 1,000–5,000 m

RVR

Reporting threshold ≈ 1,500 m

Instruments

Transmissometer → RVR/visibility

Ceilometer → cloud-base height

Cloud

Fog → surface-based Stratus

Stability

Inversion → extreme stability → moisture trapped → fog favoured

Wind

Radiation fog → light wind

Advection fog → air movement required

North India

Winter + Western Disturbance + post-rain clearing → widespread radiation fog