Weather and obstructions to visibility include fog, mist, haze, smoke, precipitation, blowing snow, dust storm, sandstorm, and volcanic ash. This chapter will discuss each in detail.

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).

Fog types are named according to their formation mechanism.

Radiation fog (see Figure 18-1 and Figure 18-2) 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, and 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 when wind is calm. Winds up to about 5 kt mix the air slightly and tend to deepen the fog by spreading the cooling through a deeper layer. Stronger winds disperse the fog or mix the air through a still deeper layer, with stratus clouds forming at the top of the mixing layer.

Ground fog usually burns off rather rapidly after sunrise. Other radiation fog generally clears before noon unless clouds move in over the fog. It can be difficult at times to differentiate between this and other types of fog, especially since nighttime cooling intensifies all fogs.

Mountaintops jutting skyward out of the fog can be a beautiful sight, but it can also be dangerous. There are two ingredients that add to the formation of mountain/valley fog in areas of variable terrain.

First, overnight, the ground cools as the heat that was gathered from the Sun’s rays during the day is released back into the air near the ground level. The denser, cooler air on mountaintops sinks into valleys and collects there.

Second, over the course of the night, the valley begins to fill from the bottom with cold layers of air. This phenomenon is known as “cold air drainage.” This cooler air lowers the surrounding air temperatures closer to the dewpoint and subsequently saturation occurs. If there is sufficient moisture in the air, fog will begin to form in these valleys as the night progresses. This type of fog is most commonly observed in the autumn and spring months and is densest around sunrise when surface temperatures are often lowest. See Figure 18-3 and Figure 18-4.