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 June.

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.