Visual line of sight (VLOS) is a foundational requirement in nearly every country's drone regulations, and visibility distance is what determines whether it's actually achievable.
What "visibility" means for flying
Meteorological visibility is the distance at which a large object can still be clearly distinguished from the background. Haze, fog, smoke, heavy rain, and even bright low-sun glare can all reduce it well below what a forecast's "clear" label implies.
Typical regulatory expectations
Many national frameworks (including the FAA's Part 107 in the U.S.) require at least 3 statute miles (about 4.8 km) of visibility for the operating area, alongside minimum cloud-clearance distances. Even where a specific number isn't mandated, the underlying principle — you and any required observer must be able to see the drone and surrounding air traffic — is close to universal.
Why this matters beyond compliance
Reduced visibility doesn't just risk a violation; it directly increases collision risk with manned aircraft, especially near airports, and makes it easy to lose orientation of a small drone against a hazy sky.
Practical guidance
Check current visibility, not just cloud cover, before flying — a day can look clear from indoors while haze near the horizon limits practical VLOS. If visibility drops noticeably during a flight (common at dusk or as fog rolls in), bring the drone back well before you'd actually lose sight of it.