top of page

What causes obstructions to visibility in flight?

Updated: May 23

What can cause an obstruction to visibility in flight? Weather and obstructions to visibility include: fog, mist, haze, smoke, precipitation, blowing snow, dust storm, sandstorm, and volcanic ash. We will discuss each one in detail.

1. Fog

Fog is a visible aggregate of minute water droplets that are based at the Earth’s surface, and it reduces horizontal visibility to less than 5/8 sm (1 km); unlike drizzle, it does not fall to the ground. Fog differs from a cloud only in that its base must be at the Earth’s surface, while clouds are above the surface. Cloud droplets can remain liquid even when the air temperature is below freezing. Fog composed of water droplets and occurring with temperatures at or below freezing is termed freezing fog. When fog is composed of ice crystals, it is termed ice fog. If fog is so shallow that it is not an obstruction to vision at a height of 6 ft (2 m) above the surface, it is called shallow (ground) fog. Fog forms when the temperature and dewpoint of the air become identical (or nearly so). This may occur through cooling of the air to its dewpoint (producing radiation fog, advection fog, or upslope fog), or by adding moisture and thereby elevating the dewpoint (producing frontal fog or steam fog). Fog seldom forms when the temperature-dewpoint spread is greater than 2 °C (4 °F).

Different Types of Fog

Fog types are named according to their formation mechanism.

Radiation Fog

Radiation fog is a common type of fog, produced over a land area when radiational cooling reduces the air temperature to or below its dewpoint. Thus, radiation fog is generally a nighttime occurrence and often does not dissipate until after sunrise.

Radiation fog is relatively shallow fog. It may be dense enough to hide the entire sky, or it may conceal only part of the sky. Ground fog is a form of radiation fog that is confined to near ground level.

Factors favoring the formation of radiation fog are:

1) a shallow surface layer of relatively moist air beneath a dry layer

2) clear skies

3) light surface winds

Terrestrial radiation cools the ground; in turn, the ground cools the air in contact with it. When the air is cooled to its dewpoint, fog forms. When rain soaks the ground, followed by clearing skies, radiation fog is not uncommon the following morning.

Radiation fog is restricted to land because water surfaces cool little from nighttime radiation. It is shallow whe