White Paper - Introduction to atmospheric visibility estimation

Visibiity is usually referred to as the maximum horizontal distance through the atmosphere that objects can be seen by the unaided eye. Visibility will depend upon several factors, for example:

• The size and colour of the object to be observed against the sky in the horizon
• The brightness of the sky itself (background luminance)
• Whether the object is emitting light (particularly important a night!)
• The skill of the observer at resolving the object against its background.  This is referred to as the minimum contrast between the object and sky. During the night, the illumination of the object relative to the background light levels is most appropriate.