Visibility in weather refers to the measure of the horizontal opacity of the atmosphere at the point of observation and is expressed in terms of the horizontal distance at which an object or light can be clearly discerned. It is a measure of how far one can see in the atmosphere and depends on the transparency of the surrounding air. In meteorology, visibility is measured by sensors that provide a measurement of the meteorological optical range, which is defined as the length of atmosphere over which a beam of light travels before its luminous flux is reduced to 5% of its original value.

Visibility perception depends on several physical and visual factors, including the sensitivity of the human visual system to spatial frequencies and the Fourier transform and contrast sensitivity function of the HVS to assess visibility. Fog, mist, haze, and freezing drizzle can reduce visibility, and air pollution and high humidity can also reduce visibility somewhat. Smoke, sandstorms, and forest fires can reduce visibility to near zero, making driving extremely dangerous.

In aviation weather reports, visibility is defined as a measure of the horizontal opacity of the atmosphere at the point of observation and is expressed in terms of the horizontal distance at which a person should be able to see and identify a prominent dark object against the sky at the horizon during the day or a known, preferably unfocused, moderately intense light source at night. The prevailing visibility should be reported for aviation purposes, and if the visibility varies from one direction to another, the lowest value should be reported.