Flying a Drone in Fog: UK Visibility Rules

VLOS: The Fundamental Rule

As of May 2026, the vast majority of UK drone operations — including those under the Open Category (A1, A2 and A3 subcategories) — require the remote pilot to maintain visual line of sight (VLOS) with the drone at all times. This means you must be able to see the drone unaided (or with corrective lenses if normally worn) well enough to monitor its flight path and avoid collisions.

Fog, mist, haze and low cloud all reduce visibility. When atmospheric conditions prevent you from seeing your drone clearly, the VLOS requirement cannot be met and you should not fly.

How Fog Affects Drone Operations

Visibility Distance

The Met Office defines fog as visibility below 1,000 metres and thick fog as visibility below 200 metres. For a drone pilot, even 1,000 metres of visibility may be insufficient — VLOS requires not just seeing the drone but being able to determine its orientation and flight path. A small drone at 500 metres in misty conditions may be visible as a dot but impossible to orient, which does not satisfy VLOS.

Moisture and Electronics

Fog is composed of suspended water droplets. Whilst individual droplets are tiny, prolonged flight through fog coats the drone's surfaces, camera lens and sensors with moisture. This creates the same risks as light rain — potential electrical damage and sensor interference — without the obvious visual cue of rainfall.

Obstacle Avoidance Degradation

Infrared and visual obstacle avoidance sensors can behave unpredictably in fog. The suspended water droplets scatter infrared signals and reduce the contrast that visual sensors rely upon, potentially creating false obstacle readings or failing to detect real obstacles.

CAP722 and Meteorological Assessment

CAP722 Chapter 4 addresses meteorological conditions for unmanned aircraft operations. The guidance is clear that weather assessment is an ongoing responsibility — conditions can change during a flight. Fog can roll in rapidly, particularly near coasts, rivers, valleys and elevated terrain.

The pilot-in-command is responsible for continuously monitoring visibility and landing if conditions deteriorate to the point where VLOS can no longer be maintained.

Types of Fog in the UK

Understanding fog formation helps pilots anticipate when it might appear:

• Radiation fog: Forms overnight in clear, calm conditions as the ground cools. Common in autumn and winter, typically in valleys and low-lying areas. Often clears by mid-morning as the sun warms the ground.
• Advection fog: Forms when warm, moist air moves over a cooler surface. Common on coastlines when warm maritime air meets cool land or sea. Can persist throughout the day.
• Upslope fog: Forms when air is forced up a slope, cooling and condensing. Common in hilly and mountainous areas of Scotland, Wales and northern England.
• Freezing fog: Fog in sub-zero temperatures. Droplets freeze on contact with surfaces, including drone propellers, creating ice. Particularly hazardous as ice changes the aerodynamic profile of propeller blades.

Practical Tips for Foggy Conditions

• Check the Met Office visibility forecast before every flight — not just wind and rain
• If visibility is below 1,000 metres, seriously reconsider whether VLOS can be maintained
• Be especially cautious near water, in valleys and on elevated terrain where fog forms and lingers
• If fog begins to form during a flight, land immediately — visibility can deteriorate within minutes
• In autumn and winter, plan flights for mid-morning to early afternoon when radiation fog has typically cleared
• Dry the drone and camera lens after any flight in misty conditions, even if the moisture seems minimal

BVLOS and Fog

Beyond visual line of sight (BVLOS) operations — which as of May 2026 require specific CAA authorisation under the Specific Category — have their own meteorological requirements defined in the operational authorisation. Even BVLOS operators must comply with visibility minimums specified in their risk assessment (typically a SORA). Fog remains a significant challenge even for authorised BVLOS operations.