JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
Surface vs Flight Visibility
RVR and prevailing visibility are horizontal visibilities near the Earth’s surface. They may be
quite different from the vertical visibility when looking down at the ground from an aircraft in
flight. For example, surface visibility may be seriously reduced by fog or blowing snow, yet only
a slight reduction in visibility is apparent when viewed from above the field. In Figure 4-12, the
airfield may be seen relatively clearly from above the fog. When descending to the level of the
fog, however, the airfield may disappear from sight. In another situation, flying into the setting
sun on a hazy day may reduce flight visibility to values less than the surface visibility. When
given the surface visibility, learn to anticipate what your flight visibility is likely to be. It may
vary, depending on other weather conditions present.
Figure 4-12 — Surface vs. Flight Visibility
Obscuring Phenomena
Obscuring phenomena are any collection of particles that reduce horizontal visibility to less than
six miles. They may be either surface based or aloft. Examples include fog, haze, smoke,
volcanic ash, and blowing spray, to name a few.
Haze produces a bluish color when viewed against the ground. Although haze may occur at any
level in the troposphere, it is more common in the lower few thousand feet. Haze is associated
with a stable atmosphere. The top of a haze layer, which is usually confined by a low-level
inversion, has the appearance of a horizon when viewed from above the layer. In this case, the
haze may completely obscure the ground in all directions except the vertical. Dense haze may
reduce visibility to less than 3 miles, with slant range visibility generally less than surface
visibility. Visibility in haze is lower when looking toward the Sun than away from it.
Smoke causes the sunrise and sunset to appear very red. Smoke reduces visibility in a manner
similar to haze. Smoke from forest fires is often concentrated in layers aloft with good visibility
beneath. Smoke may be a major concern near industrial areas. Smoke from forest fires has been
4-18 Version 3.2/Dec 08