Squall Lines

CHAPTER THREE

AVIATION WEATHER

Precipitation and icing conditions: active cold fronts usually have a relatively narrow belt of

precipitation, especially if the precipitation is showery. Icing may be severe in cumuliform

clouds. Slow-moving cold fronts may have a broader area of precipitation and greater threat of

remaining in icing conditions for a longer period.

Thunderstorms and squall lines: severe weather is implied to exist in areas of reported

thunderstorms. Chapter Four will detail the hazards associated with thunderstorms.

Squall Lines

A squall line is a line of violent thunderstorms. They are indicated on surface charts by a dashed,

double-dotted purple line. They develop 50 to 300 miles ahead of the cold front and roughly

parallel to it. They form when cold air downdrafts flowing ahead of a cold front lift additional

warm, unstable air. The uplifted air develops its own updrafts and downdrafts and starts the

thunderstorm development cycle (Figure 3-13). Sometimes, however, squall lines can be located

nowhere near a cold front, possibly from the convergence of air flows at one location. Squall

lines are usually the most intense during the late afternoon and early evening hours, just after

maximum daytime heating.

Figure 3-13 Squall Line Formation

It is often impossible to fly through squall lines, even with radar, since the storms are extremely

close to one another. Similar to cold fronts, squall lines will also have a 90 wind shift from the

SW to the NW.

309.

WARM FRONTS

A warm front is the boundary of the advancing warm air mass overtaking and replacing a colder

air mass. To do so, the warmer, less dense air must ride up and over the top of the cold air mass.

Figure 3-14 shows the manner in which a warm front is depicted on a surface weather chart. The

warm air mass gradually moves up over the frontal surface creating a broad area of cloudiness.

3-14 Mechanics of Frontal Systems