Quantcast Volcanic Ash Clouds

 

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CHAPTER FIVE
AVIATION WEATHER
The west coast of the United States is quite vulnerable to sea fog (Figure 5-20). 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 resulting stable temperature also prevents any solar heating from causing the
dissipation of sea fog. Only a change in wind direction moving the air from a colder surface to a
warmer surface, reversing the saturation process, can cause advection fog to dissipate.
514.
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.
Avoid flight into an area of known volcanic activity. Avoiding volcanic ash clouds is
particularly difficult during hours of darkness or in daytime instrument meteorological
conditions when the flight crew may not detect the volcanic ash cloud. 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
ash cloud. Expect minor eye irritation if odors become noticeable (i.e., eyes watering). Remove
contact lenses if this occurs. Consider using oxygen when odors or eye irritation occurs.
If volcanic activity is reported, the planned flight should remain at least 20 NM from the area
and, if possible, stay on the upwind side of the volcano even when flying outside of the 20 NM
limitation. Volcanic ash clouds may extend downwind for several hundred miles and thousands
of feet in altitude. Volcanic ash can cause rapid erosion and damage to the internal components
of engines with loss of thrust within 50 seconds.
Since airborne radar cannot detect volcanic ash clouds, weather forecasts are occasionally wrong,
and other clouds may hide ash clouds, inadvertent flight through an ash cloud may occur. It may
be difficult to determine if you are in an ash cloud when flying through other clouds or at night.
The following conditions may indicate you have inadvertently flown into an ash cloud:
5-28
Weather Hazards of Turbulence, Icing, Ceilings, Visibility, and Ash Clouds


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