Sky Coverage and Ceilings

CHAPTER FIVE

AVIATION WEATHER

with a stable atmosphere. The top of a haze layer, which is usually confined by a low level

inversion, has the appearance of a horizon when viewed from above the layer. In this case, the

haze may completely obscure the ground in all directions except the vertical. Dense haze may

reduce visibility to less than three miles, with slant range visibility generally less than surface

visibility. Visibility in haze is lower when looking toward the Sun than away from it.

Smoke causes the sunrise and sunset to appear very red. Smoke reduces visibility in a manner

similar to haze. Smoke from forest fires is often concentrated in layers aloft with good visibility

beneath. Smoke may be a major concern near industrial areas. Smoke from forest fires has been

carried great distances at high altitudes. Aircrews flying at these altitudes may encounter dense

smoke, although the lower altitudes are clear.

Rain and drizzle are precipitation in liquid form that can reduce visibility. Precipitation also

reduces visibility as it streams across a windshield or canopy. Drizzle is a feature of stable air

with the likely presence of fog or smog. Therefore, drizzle may result in extremely poor

visibility. Approaches and the ensuing transition to visual flight can be very hazardous since

moderate to heavy rain conditions can seriously affect the recognition of visual cues. Night

approaches in these conditions can be even more critical as you may be distracted by the

aircraft's flashing strobes or sequenced flashing runway lights.

Snow affects visibility much more than rain or drizzle and can easily reduce visibility to less than

one mile. It is often difficult to see snow falling ahead of you; you may enter the snow

unexpectedly.

Blowing snow is fine dry snow easily lifted by the wind up to 300 feet AGL, depending on wind

strength and air stability. During or after a fresh snowfall with brisk winds, surface visibility

may be reduced to less than 1/2 mile. Blowing snow is accompanied by many of the same

hazards as rain, such as turbulence (creating difficulties in reading flight instruments) and

obscured visual cues (a lack of visual cues for runway identification during the visual portion of

the approach). The approach and runway lights will provide some identification of the runway

environment; however, runway markings may be lost in the whiteness. Therefore, depth

perception will be difficult, requiring more emphasis on instruments.

Dust and sand form when strong winds combined with unstable air and loose dry soil can blow

dust or sand into the air. Dust is finer than sand and strong winds may lift the dust to

considerable heights. Sand will usually be limited in altitude to 50 or 100 feet. In severe

conditions, visibility can be near zero. Blowing dust is common behind cold fronts moving

rapidly across prairies in early spring before a cover of vegetation has appeared. This effect may

cause blowing dust conditions and reduced visibilities over a wide area.

510.

SKY COVERAGE AND CEILINGS

For determining the amount of sky covered by clouds, the celestial dome is divided into eighths.

The terms contained in Figure 5-16 are used to report the percentage of sky coverage as well as

any obstructions to visibility. These coverages apply to a given altitude; therefore, more than

one is normally reported. For example, the sky may be reported as follows: SCT at 2000 feet,

5-22

Weather Hazards of Turbulence, Icing, Ceilings, Visibility, and Ash Clouds