Chapter 18, Weather and Obstructions to Visibility 18-1
18 Weather and Obstructions to
Visibility
18.1 Introduction
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.
Chapter 18, Weather and Obstructions to Visibility 18-2
18.1.1 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).
18.1.1.1 Fog Types
Fog types are named according to their formation mechanism.
18.1.1.1.1 Radiation Fog
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.
Figure 18-1. Radiation Fog Formation
Chapter 18, Weather and Obstructions to Visibility 18-3
Figure 18-2. Radiation Fog
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.
18.1.1.1.1.1 Mountain/Valley Fog
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 lowe st. See Figure
18-3 and Figure 18-4.
Chapter 18, Weather and Obstructions to Visibility 18-4
Figure 18-3. Mountain/Valley Fog Formation
Figure 18-4. Mountain/Valley Fog
18.1.1.1.2 Advection Fog
Advection fog (see Figure 18-5 and Figure 18-6) forms when moist air moves over a colder surface and the
subsequent cooling of that air to below its dewpoint. It is most common along coastal areas but often moves
deep into continental areas. At sea, it is called sea fog. Advection fog deepens as wind speed increases up
to about 15 kt. Wind that is generally stronger than 15 kt lifts the fog into a layer of low stratus or
stratocumulus clouds.
Chapter 18, Weather and Obstructions to Visibility 18-5
Figure 18-5. Advection Fog Formation
Figure 18-6. Advection Fog
The West Coast of the United States is quite vulnerable to advection fog. This fog frequently forms offshore
as a result of cold water and then is carried inland by the wind. It can remain over the water for weeks,
advancing over the land during night and retreating back over the water the next morning.
During the winter, advection fog over the central and eastern United States results when moist air from the
Gulf of America spreads northward over cold ground. The fog may extend as far north as the Great Lakes.
Chapter 18, Weather and Obstructions to Visibility 18-6
Water areas in northern latitudes have frequent dense sea fog in summer as a result of warm, moist, tropical
air flowing northward over colder Arctic waters.
A pilot will notice little difference between flying over advection fog and over radiation fog. Also, advection
fog is usually more extensive and much more persistent than radiation fog. Advection fog ca n move in
rapidly regardless of the time of day or night.
18.1.1.1.3 Upslope Fog
Upslope fog forms as a result of moist, stable air being adiabatically cooled to or below its dewpoint as it
moves up sloping terrain (see Figure 18-7). Winds speeds of 5 –15 kt are most favorable since stronger
winds tend to lift the fog into a layer of low stratus clouds. Unlike radiation fog, it can form under cloudy
skies. Upslope fog is common along the eastern slopes of the Rocky Mountains and somewhat less frequent
east of the Appalachian Mountains. Upslope fog is often quite dense and extends to high altitudes.
Figure 18-7. Upslope Fog Formation
18.1.1.1.4 Frontal Fog
When warm, moist air is lifted over a front, clouds and precipitation may form. If the cold air below is near
its dewpoint, evaporation (or sublimation) from the precipitation may saturate the cold air and form fog
(see Figure 18-8). A fog formed in this manner is called frontal (or precipitation-induced) fog. The result is
a more or less continuous zone of condensed water droplets reaching from the ground up through the clouds.
Frontal fog can become quite dense and continue for an extended period of time. This fog may extend over
large areas, completely suspending air operations. It is most commonly associated with warm fronts but
can occur with other fronts as well.