Low visibility in the local area has different causes
depending on the season. Liquid precipitation accounts for most low visibility in the summer and 20% of low visibility in the winter. Most low visibility in the winter is associated with with
Fog also differs seasonally in terms of its effects on visibility. In winter, radiation and advection fog are responsible for 75% of low visibility conditions. In summer, fog does not often affect flight operations for long periods. Summer fog is radiation or ground
fog that forms inland and is advected over the field by the land breeze.
Summer fog rarely reduces visibility to less than 3 miles at KNPA.
During the summer, visibility is usually unrestricted.
Visibility is restricted to below VFR minimums only 6% of the time.
The number of hours of restricted visibility increases during September,
reaches a maximum in January, and gradually trails off to a minimum by
Smoke restricts visibility to less than 3 miles at KNPA
on the average of 10 times a year. Sources of smoke include several area
paper mills and inland brush fires. Inland brush fires are most common
in fall during periods of drought. Smoke is most detrimental to flight
operations when it is in a layer aloft. During July and August, flight-level
visibility may be reduced to 1 mile or less. Pilots reports (PIREPs)
have reported tops of this haze layer as high as 18,000 feet, while surface
visibility may only be slightly affected. Haze usually forms in combination
with the smoke layer aloft. These layers are sometimes as high as
5,000-8,000 feet (the average base of the subsidence inversion).
Reduced visibility at these levels will continue until there is a change
of wind flow or air mass.
Heavy haze can restrict visibility in the local flying
area during late November to late January. Haze develops under stable conditions
from an upper-level confluence of high pressure cells, which cause subsidence
and a marked surface inversion. Visibility will remain unrestricted if
the wind has a light northerly flow. If the wind veers to a northeasterly
direction, visibility may be reduced to 3-5 miles. Visibility may improve
if the wind veers southeasterly, or with instability, high winds, and the
movement of the high pressure cell.
During July and August, if there is a stationary ridge
at 500 mb between Georgia and the Mississippi River, resultant surface
inversions reduce the surface visibility to 2-4 miles. The worst
visibility will be in the early to mid-morning hours.