Fog looks calmer than rain or wind, which is exactly why it catches pilots off guard. It removes the one thing almost every drone rule depends on: being able to see your aircraft.
Visual line of sight is the real limit
Most drone regulations require the pilot to maintain visual line of sight (VLOS) with the aircraft at all times, and in many frameworks — including the FAA's Part 107 — a minimum flight visibility (commonly 3 statute miles in controlled operations) is also required as observed from the control station. Fog can push horizontal visibility to a few hundred meters or less, which fails VLOS long before the drone is even out of theoretical range.
Fog forms faster than most forecasts update
Radiation fog often forms in the hour around sunrise, especially over water, valleys, or open fields, as the ground cools overnight and moisture condenses in the air near the surface. It can appear well after a forecast was issued and can also burn off within an hour once the sun warms the ground. A forecast checked the night before is not a substitute for looking at current conditions right before launch.
Moisture reaches the drone even without rain
Fog is airborne water, and prolonged exposure lets moisture settle on the lens, gimbal, and connector points in the same way a light drizzle would — often without the pilot noticing until footage looks hazy or a component starts acting up. A camera lens fogging over mid-flight is one of the more common ways a fog-weather flight goes wrong.
Obstacle avoidance sensors are less reliable in fog
Vision-based and infrared obstacle avoidance systems rely on contrast and reflected light, both of which degrade in fog. Don't treat these sensors as a safety net when visibility is already reduced; assume they may miss obstacles they'd normally catch.
What to do instead
Check current visibility for your exact location right before flying, not just a general forecast. If fog is present or visibly rolling in, wait it out — fog conditions frequently clear within an hour or two once the sun rises higher. If fog develops mid-flight, land as soon as it's safe rather than continuing on instruments or memory of the area.