8.26 Visibility and Fog

8.26.2 Define prevailing visibility.

Visibility refers to the ability to see through air.

Meteorological visibility relates to the horizontal plane and is defined as:

'How far an observer on the ground can correctly identify distant objects of a given size against the horizon sky'.

8.26.4 Explain why illumination from the sun or moon has no effect on prevailing visibility.

Visibility is a measure of how transparent the atmosphere is to the human eye.

Transparency of air relates to the absence of particles within the air.

These can be in both solid and liquid form.

• Rain, drizzle, fog, haze and smoke reduce visibility,
• while dry, clean and cold air increase visibility.

During the day, the sun is the light that lets us see the transparency.

By night the same criteria apply, except that lights of known power are used to assess the visibility.

8.26.6 Describe the operational characteristics of the visibility sensor used in Automatic Weather Stations (AWS), and reported in METAR AUTO reports.

Handar visibility sensor

This sensor measures the scattering of light by particles in the atmosphere and expresses the result as a voltage which is proportional to the amount of scattering.