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3. Stratus Clouds

At times when conditions are favorable for fog, a very low cloud layer may form. This is
especially true over flat terrain. These fog-like clouds form in stable air and often exist
together with fog. When this happens, there is no exact point where it can be said that the
fog layer stops and the stratus cloud layer begins. An observer on the surface then reports
the vertical visibility as the distance he can see upward into the fog.
4. Haze and Smoke

Visibility is reduced when a stable layer of air contains large amounts of very small dust or
salt particles. The particles produce the condition called haze. The haze may occasionally
extend from the surface up to 15,000 feet. Haze layers often have definite upper 1imits or
tops, above which air-to-air visibility is good. However, air-to-ground visibility from above a
haze layer is poor, especially on a slant. Smoke restricts visibility in a manner similar to haze.
Smoke sometimes collects in layers aloft, restricting visibility at that altitude, while visibility
under it and over it may be good. Haze and smoke wil1 normally be a more severe restriction
to visibility when a temperature inversion exists.

5. Blowing Snow, Dust, and Sand

ong surface winds and vertical currents in unstable air carry a erials from the surface such as
dust, sand, or snow. These I reduce surface visibility to near zero over large areas. I favorable
conditions dust can be carried aloft to 15,000 feet and restrict slant, flight, and surface
visibility. Sand and snow are seldom carried aloft beyond a few hundred feet.

6. Precipitation

Snow, drizzle, and rain are the most common forms of precipitation that cause restrictions to
visibility. Of these, snow is usually the most effective in reducing visibility. Heavy snow
frequently reduces surface and slant visibility to near zero. Rain rarely reduces surface
visibility to less than one mile and has a tendency to wash dust, smoke, and even fog particles
out of the air. However, drizzle often occurs at the same time fog, haze, and smoke are
present, resulting in visibility being less than it would be if rain had occurred. Precipitation on
the windows of an aircraft greatly reduces a pilot's visibility; and if it freezes, he may have no
forward visibility at all.