TAKING A LOOK at VISIBILITY—FOG
In aviation we must be able to SEE. While we can fly and not be able to see anything, we cannot do that near the ground. The FAA has given aviators visibility MINIMUMS in order to taxi, takeoff and land an aircraft (A/C). Poor visibility has caused many accidents near airports (and on the airport surface).
We will begin by looking at some of the common factors causing limited vision (most typically caused by weather conditions).
Fog
There are several types of fog:
RADIATION FOG (two types):
“Ground Fog” is the cooling of air near the ground at night, causing an inversion, which prevents convection (keeping air from rising). Dry air cools to its Dew Point temp and becomes saturated, locked in by the inversion, causing very thick fog. This type of fog usually burns off by mid-morning.
“Inversion Fog” Is a downward extension of a layer of stratus clouds under the base of a temperature inversion causing the fog to build down from the inversion level. We see this type of fog near oceans or large bodies of water (like San Francisco, where we can see the fog roll in over terrain).
ADVECTION FOG
Most prevalent in the winter months, “Advection Fog” is the movement of warm, moist air over a cold surface (such as a lake or river). As the air cools to its saturation point, there is a turbulent mixing, creating a wind that can make this fog extend upward. This type of fog reduces ceilings and visibilities to near zero, and it can (and does) last for many hours.
EVAPORATION FOG
Also called “steam fog,” this occurs when cold air moves over warm water (such as a warm rain falling through a cooler layer of air that is cooled by the surface temp of the water). Most often seen in late fall or early winter months.
UPSLOPE FOG
This fog consists of air moving up a mountain side or sloping plane. Adiabatic cooling occurs due to the ascent, resulting in saturation and fog.
Most fog has common characteristics, such as:
The temperature and dew point within 4 degrees F, seen in the morning and after a cold frontal passage,
A temperature inversion, preventing warming air from rising, usually at about 50 feet altitude or less, keeping cool air near the surface,
A light wind typically below 7 or 8 knots, this allows the mixing of air and moisture, if there is no wind, fog may not form. Too much wind breaks the fog.
Other Visibility Restrictors
Heavy rain will restrict vision that can be caused by a frontal movement and may stay in an area for long periods of time such as (warm, stationary, or occluded fronts). Thunderstorms limit vision but usually are short lived and move through the area quickly, waiting for a short time will allow for an approach or departure.
Snow falling from light to heavy will decrease visibility from minimal to zero. The problem with snow is that it can last for long periods of time and cover a lot of real estate. Snow also effects runway conditions as well as your takeoff alternates. Also airfoil icing is dangerous requiring de-icing, make sure your wings, tail, and fuselage are clear of snow and ice.
Smoke from local factories and forest fires, and tend to linger until a front moves through.
Again, KNOW before you GO! Be aware of the weather:
What is happening at your airport of intended landing weather and (NOTAMS).
Where are your alternates? What is the topography enroute?
How much fuel will you need? NEVER land with less than 30 minutes of fuel.
A well flown instrument approach is very rewarding and fun.
Fly Safely Know the Weather Plan Ahead FUEL
Until next time,
Captain Will Rondeau
References
FAA visibility minimums: https://fsims.faa.gov/wdocs/orders/8260_3b-chg20.htm
FAR AIM Federal Aviation Regulations Aeronautical Manual. FAR Parts: 91, 121, 135. AIM Chapters: 1, 2, 3,5.