JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
warm air mass ahead of the front begin to increase in speed. Meanwhile, the barometric pressure
decreases, and altocumulus clouds appear on the horizon. Next, the cloud bases lower, and rain
or snow showers begin as the cumulonimbus clouds move into the area. The precipitation
increases in intensity and may persist as the front nears the station. As the front passes, the
pressure rises sharply and the wind shifts approximately 90° from SW to NW. The postfrontal
weather includes rapidly clearing skies, fair weather cumulus clouds, and decreasing temperature
and dew point. The extent of postfrontal cloudiness depends on the degree of stability and
moisture content of the cold air mass. In some cases, the sequence of events described here may
be considerably different, depending on the specific atmospheric conditions (Figure 3-9).
Figure 3-9 — Cold Front Cloud Formation
Weather with fast-moving cold fronts occurs in a narrow band, is usually severe, and clears
rapidly behind the front. Cumuliform clouds, showers, or thunderstorms may form near the front
position. Lines of fast-moving thunderstorms, or squall lines, can form well ahead of the front.
Weather with slow-moving cold fronts (usually from late fall through early spring) occurs over a
large area, is less severe, but may persist for hours, even after the front is past.
Recognizing Cold Fronts During Flights
During a flight over flat terrain, you may see a long line of cumuliform clouds on the horizon.
These clouds may indicate you are flying toward an approaching active cold front. When flying
above an altocumulus layer extending ahead of the front, the lower frontal clouds are often
hidden. Stratus or stratocumulus decks extending many miles ahead of a front may conceal the
main clouds from a low flying aircraft.
Cold Fronts Flight Problems
Wind shifts — Expect an abrupt wind shift when passing through a frontal zone, especially when
flying at lower altitudes. Turbulence is often associated with the wind shift. The wind generally
shifts from SW to NW with greater speeds behind the front.
Ceiling and visibility — If an active cold front moves at a moderate or rapid speed (15-30 knots),
its weather zone is generally less than 50 miles wide. If the front moves slower, its weather zone
may be broad enough to seriously affect flight operations for many hours. Ceilings and
3-10 Version 3.2/Dec 08