Volcanic Ash Clouds

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