JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
carried great distances at high altitudes. Aircrews flying at these altitudes may encounter dense
smoke, although the lower altitudes are clear.
Rain and Drizzle – Precipitation in its liquid form can, on its own, reduce visibility. Precipitation
also reduces visibility as it streams across a windshield or canopy. Drizzle is a feature of stable
air, and fog or smog are also likely to be present. 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 —Snow affects visibility much more than rain or drizzle and can easily reduce visibility to
less than 1 mile. It is often difficult to see snow falling ahead of you; you may enter the snow
unexpectedly.
Blowing Snow – Fine, dry snow can be 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 ½ 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.
SKY COVERAGE AND CEILINGS
For determining the amount of sky covered by clouds, the celestial dome is divided into 8 ths . The
terms contained in Table 4-4 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 ft., BKN at
5000 ft., OVC at 10,000 ft., where the altitudes refer to the bases of the cloud layers in feet AGL.
Version 3.2/Dec 08 4-19