Short notes 08 : Visibility and Fog
Reduced visibility is a major operational hazard in aviation. Fog, mist, and haze are surface-based visibility-reducing phenomena that form under stable atmospheric conditions, but they differ in composition, formation mechanism, persistence, and operational impact.
Fog (FG)
Definition (ICAO):
Fog is the suspension of water droplets in the air that reduces horizontal visibility to less than 1000 metres. Relative humidity is typically close to 100%.
Types of Fog
1. Radiation Fog
- Forms due to radiational cooling of the ground.
- Occurs at night or early morning over land.
- Favourable conditions:
- Clear skies
- High relative humidity
- Light wind: 2–8 kt
- Calm conditions produce dew or frost, not fog.
- Strong winds disperse or lift fog into low stratus.
- Usually burns off after sunrise due to solar heating.
- Common in autumn and winter (long nights).
2. Advection Fog
- Forms when warm, moist air moves horizontally over a cold surface (land or sea).
- Can occur day or night.
- Much more persistent than radiation fog.
- May last 24 hours to several days or even weeks.
- Clears by:
- Air-mass change (wind shift), or
- Wind speed increase >15 kt (lifts into stratus).
- Represents a severe aviation hazard.
3. Frontal (Precipitation) Fog
- Associated mainly with warm fronts and warm occlusions.
- Rain or snow falls into colder air ahead of the front.
- Evaporation raises dew point → saturation → fog.
- Can extend up to 200 NM ahead of the front.
- Often dense and long-lasting, frequently disrupting operations.
Radiation vs Advection Fog
| Feature | Radiation Fog | Advection Fog |
|---|---|---|
| Surface | Land only | Land or Sea |
| Time | Night / Early morning | Any time |
| Wind | 2–8 kt | Light to moderate |
| Cause | Radiational cooling | Horizontal air movement |
| Duration | Short | Long / Persistent |
| Clears by | Sunlight | Wind shift / stronger wind |
Mist (BR)
Definition (ICAO):
Mist is caused by fine water droplets reducing visibility to 1000–5000 metres, with relative humidity greater than 95%.
- Occurs when air is near saturation, but not fully saturated.
- Common in early morning.
- Less dense than fog.
- Improves with slight warming or increased mixing.
Haze (HZ)
Definition (ICAO):
Haze is caused by a suspension of dry particles such as dust, smoke, salt, or pollution, giving the atmosphere a bluish or greyish appearance.
- Visibility usually 5 km or more.
- Relative humidity is well below saturation.
- Not a condensation phenomenon.
- Forms in:
- Stable air
- Temperature inversions
- High-pressure systems
- Light or calm winds
- Often persistent in winter.
- Reduces slant visibility more than horizontal visibility.
- Can transform into mist or fog if relative humidity increases.
Fog, Mist, and Haze – Comparison
| Feature | Fog (FG) | Mist (BR) | Haze (HZ) |
|---|---|---|---|
| Particle type | Water droplets | Fine water droplets | Dry particles |
| Visibility | < 1000 m | 1000–5000 m | ≥ 5 km |
| Relative Humidity | ≈ 100% | > 95% | Lower RH |
| Formation | Saturation | Near saturation | Stable air trapping particles |
| Stability | Stable | Stable | Very stable |
| Typical weather | Radiation, advection, fronts | Early morning | High pressure, inversion |
Runway Visual Range (RVR)
- RVR is reported when visibility is less than 1500 m.
- Instrumented RVR uses three transmissometers:
- Touchdown
- Mid-point
- Stop-end
- Mid-point and stop-end values are omitted if:
- They are 800 m or more, or
- Equal to or greater than touchdown value (unless <400 m).
Aviation Significance
- Fog is the most operationally restrictive visibility phenomenon.
- Advection and frontal fog are most persistent.
- Haze significantly affects approach judgement and visual cues.
- Inversions associated with fog and haze often produce wind shear.
Key Points
- Fog: visibility <1000 m
- Mist: 1000–5000 m
- Haze: ≥5 km
- Radiation fog: night, land, clear sky, 2–8 kt wind
- Advection fog: day or night, very persistent
- Haze is caused by dry particles, not condensation
- Wind >15 kt usually lifts fog into stratus