Fog
This page contains information reguarding different types of fog and the
dangers associated with them.
Possibly the most deceptive hazard to safe flight is fog because it can form or dissipate
virtually instantaneously and can form under conditions that appear ideal for flight. The
main weapon to predict fog formation is the temperature/dew point relationship. The closer
the temperature is to the dew point, the more likely it is that an unpleasant supprise may
develop.
Radiation fog is the type of fog which catches most pilots off guard. Calm winds and clear
skies, which would seem perfect for flying, can result in radiation fog if the temperature/dew
point separation is small. This is due to the fact that the clear skies allow the land to radiate
heat freely, cooling air near the surface to the dewpoint and causing fog to form. This
phenomenon occurs mainly at night or in the early morning. Usually, a few hours of sunshine
is enough to "burn the fog off" by raising temperatures above the dew point. The main danger
associated with radiation fog is that, from cruise altitude, a thin layer of ground fog (a type of
radiation fog) may seem harmless since pilots can often see runways and buildings as
through a veil. However, upon the approach to landing, what looked like a thin layer of fog
from altitude becomes a thick layer of fog, obscuring everything because the pilot is now
looking at the fog horizontally.
Advection fog occurs when moist air moves in, typically moving from over water to over land.
The moist air condenses into fog when cooled by the land. The best way to predict this type
of fog is to know the wind direction and surface temperatures along the shores, both over
the water and over the land. Another way to predict it is to obtain the relative humidity (a
measure of how saturated the air is--at 100% relative humidity, fog will begin to form) from
an observation station upwind from you. This is the air mass that will be moving into your
area and if it is reporting near 100% humidity, you can expect fog to form. Advection fog can
be fairly persistent and can form at any time.
Upslope fog forms when moist stable air is forced up to a higher altitude by terrain (such as
a high hill or a mountain). As the moist air is forced upwards, it is cooled to its dewpoint and
condenses as fog. It forms first in valleys, filling them and obscuring as much of the high
ground as the moisture content of the air allows. It lasts as long as the upward force exists.
Another condition to be weary of is rain. It is safe to fly in the rain, however, when rain falls
into cool, almost saturated air, the moisture it adds can result in precipitation-induced fog.
The necessary ingredients for this type of fog are included in occluded fronts, warm fronts, slow
moving cold fronts and stationary fronts. When any of these conditions are forcast in your flight
route, the possibility of fog must be considered.