Recognizing Warm Fronts During Flight
The most common cloud found along a warm front is the stratiform cloud. If one were to
approach the front from the east, the sequence of clouds would be cirrus, cirrostratus, altostratus,
nimbostratus, and stratus, rain and fog (Figure 3-15). Steady precipitation gradually increases
with the approach of this type of warm front and usually continues until the front passes.
Warm Front Flight Problems
Wind Shift: warm front wind shifts are not as sudden as those of a cold front, and therefore,
turbulence isn't likely. The wind generally shifts from SE to SW.
Ceiling and Visibility: the widespread precipitation ahead of a warm front is often accompanied
by low stratus and fog. In this case, the precipitation raises the moisture content of the cold air
until saturation is reached. This produces low ceilings and poor visibility covering thousands of
square miles. Ceilings are often in the 300 to 900 foot range during steady, warm frontal rain
situations. Just before the warm front passes the station, ceilings and visibilities can drop to zero
with drizzle and fog. The worst conditions often occur in the winter when the ground is cold and
the air is warm; the best scenario for dense fog and low ceilings.
Turbulence and Thunderstorms: if the advancing warm air is moist and unstable, altocumulus
and cumulonimbus clouds can be embedded in the cloud masses normally accompanying the
warm front. These embedded thunderstorms are quite dangerous, because their presence is often
unknown to aircrews until encountered. Even with airborne radar, it can be difficult to
distinguish between the widespread areas of precipitation normally found with a warm front and
the severe showers from the embedded thunderstorms. The only turbulence along a warm front
would be found in such embedded thunderstorms. Otherwise, little to no turbulence exists in
warm front systems.
Precipitation and Icing: approaching an active warm front from the cold air side (from the east),
precipitation will begin where the middle cloud deck is from 8000 to 12,000 feet AGL. Often,
this precipitation will not reach the ground; a phenomenon called virga. As you near the front,
precipitation gradually increases in intensity and becomes steadier. Occasional heavy showers in
the cold air beneath the frontal surface indicate thunderstorms exist in the warm air aloft.
Drizzle, freezing drizzle, rain, freezing rain, ice pellets (sleet), and snow are all possible in a
warm front, depending on the temperature. The shallow slope and widespread thick stratiform
clouds lead to large areas of icing. It may take a long time to climb out of the icing area and you
may need to descend into warmer air to avoid the icing.
Sometimes the frontal border between the air masses shows little or no movement. Since neither
air mass is replacing the other, the front is called a stationary front (Figure 3-16). Stationary
fronts are indicated on surface charts by an alternating warm and cold front symbols, retaining
their original red and blue colors, but pointing in opposite directions. Even though the front may
not be moving, winds can still be blowing. Surface winds tend to blow parallel on both sides of
3-16 Mechanics of Frontal Systems