JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
Figure 4-16 — Advection Fog
The west coast of the United States is quite vulnerable to sea fog (Figure 4-16). This frequently
occurring fog forms offshore–largely as a result of very cold water from the ocean depths rising
to the surface, cooling the moist air above it–and is carried inland by the wind. Advection fog
over the southeastern United States and along the Gulf Coast results from moist tropical air
moving over cold ground. It is, therefore, more frequent in winter than in summer.
Advection fog dissipates only with a wind shift, blowing the fog away, usually back out over the
sea. Incoming solar radiation will seldom cause the dissipation of advection fog because its
thickness generally prevents enough radiation to warm the Earth sufficiently. The high specific
heat of water and the resulting stable temperature also prevents any solar heating from causing
the dissipation of sea fog. Only a change in wind direction that moves the air from a colder
surface to a warmer surface, reversing the saturation process, can cause advection fog to
dissipate.
VOLCANIC ASH CLOUDS
Volcanic eruptions are rare, but the severe effects ash clouds have on an aircraft make it
important to understand the hazards in order to minimize or avoid them.
Volcanic ash clouds create an extreme hazard to aircraft operating near (especially downwind) of
active volcanoes. Aircraft flying through volcanic ash clouds have experienced a significant loss
of engine thrust and/or multiple engine flameouts along with wing leading edges and windshields
being sandblasted.
Flight into an area of known volcanic activity must be avoided. Avoiding volcanic ash clouds is
particularly difficult during hours of darkness or in daytime instrument meteorological
conditions when the volcanic ash cloud may not be detected by the flight crew. Volcanic ash
clouds are not displayed on airborne or Air Traffic Control (ATC) radar, as the radar reflectivity
of volcanic ash is roughly a million times less than that of a cumuliform cloud.
A volcanic ash cloud is not necessarily visible, either. Aircrews have reported smelling an acrid
odor similar to electrical smoke and smoke or dust appearing in the aircraft, but not seeing the
4-24 Version 3.2/Dec 08