Visibility

It depends on the transparency of the surrounding air and as such, it is unchanging no matter the ambient light level or time of day.

It is reported within surface weather observations and METAR code either in meters or statute miles, depending upon the country.

The geometric range of vision is limited by the curvature of the Earth and depends on the eye level and the height of the object being viewed.

ICAO Annex 3 Meteorological Service for International Air Navigation [1] contains the following definitions and note: Annex 3 [1] also defines Runway Visual Range (RVR) as: In extremely clean air in Arctic or mountainous areas, the visibility can be up to 240 km (150 miles) where there are large markers such as mountains or high ridges.

Heavy rain (such as from a thunderstorm) not only causes low visibility, but the inability to brake quickly due to hydroplaning.

The scattering of background light into the observer's line of sight can increase F over the distance dx.

Fog and mist are generally assumed to be composed principally of water droplets, haze and smoke can be of smaller particle size.

These have been put in place in certain areas that are subject to repeatedly low visibility, particularly after traffic collisions or pile-ups involving multiple vehicles.

Absorption of electromagnetic radiation by gases and particles is sometimes the cause of discolorations in the atmosphere but usually does not contribute very significantly to visibility degradation.

Particles that are the most effective at reducing visibility (per unit aerosol mass) have diameters in the range of 0.1-1.0 μm.

The visibility from an elevated observation point down to the surface of the sea can be calculated using the Pythagorean theorem, since the line of sight and the radius of the Earth form the two legs of a right triangle.

The tangent touching the surface of the Earth or water consists of the two short legs of the two right triangles, which are added together to calculate the geometric range of vision.

In geodesy the atmospheric refraction is always taken into account in the calculation, which increases the range of vision, so that even objects behind the horizon can still be seen.

Airplane flying into clouds on descent for landing
Foggy morning road
On clear days, Tel Aviv's skyline is visible from the Carmel mountains, 80 km north
From the summit of Mount Summano (1296 m Vicenza Italy), it is possible to see (dark blue sky line) the Tuscan-Emilian Apennines and Mount Cimone (2165 m Modena Italy) in good visibility. The distance is about 180 km. Note the fog present throughout the Po Valley in the lower layers.