Visibility is the greatest horizontal distance at which a person can identify a known object. Airport stations measure it with a forward-scatter sensor: a beam of light fired across a small gap, with a detector reading how much scatters off whatever is floating in the air. More scatter, less visibility.
Why 10 miles is the ceiling
US airport reports cap visibility at 10 statute miles. It isn't a claim that you can see exactly ten miles and no further. It means conditions are clear at least as far as the station bothers to measure. Perfectly clear days and impossibly clear days both read 10SM.
Below that, the number becomes genuinely informative:
What takes visibility away
- Fog and mist: water droplets suspended at the surface. The most common cause, and the most abrupt.
- Precipitation: heavy rain and snow both scatter light aggressively.
- Haze and smoke: fine dry particles. Wildfire smoke can drop visibility hundreds of miles from the fire.
- Blowing dust or sand: regional, sudden, and severe.
Predicting the drop
Watch the gap between temperature and dew point. When they close to within about 4°F, the air is nearly saturated and fog becomes a live possibility, especially overnight as temperatures fall toward the dew point. Visibility is usually the first measured number to react, often while every other reading still looks ordinary.