Squall Line
A squall line is an elongated line of active thunderstorms — typically extending for hundreds of kilometres along or ahead of a cold front. It is one of the most dangerous aviation weather phenomena: a solid line of intense convective activity that may be extremely difficult or impossible to traverse safely.
Formation
Squall lines typically develop when individual thunderstorms along a cold front merge and organize. As each cell’s outflow (from downdrafts and precipitation) meets the warm, moist inflow air ahead of it, a new cell initiates. This self-propagating process can sustain a squall line for 12–24 hours or more, tracking hundreds of kilometres.
Aviation Hazards
- All thunderstorm hazards concentrated into a line: severe turbulence, hail, lightning, microbursts, tornadoes, icing
- The solid line of cells may present no safe passage gap — the entire line must be circumnavigated
- Radar may show apparent gaps that close rapidly as new cells develop
- The leading edge of the squall line’s outflow can create sudden severe wind shear ahead of the visible line
Avoidance
A squall line is not to be penetrated. Circumnavigate around the end of the line, with a margin of at least 20 NM from the nearest cell. Be aware that the line may extend beyond radar coverage. If unable to circumnavigate, divert and wait for the line to pass.
On the TC Written Exam
Squall lines are treated in both PPL and CPL meteorology. Know: they form along cold fronts; they cannot safely be penetrated; all thunderstorm hazards apply; circumnavigation is the only safe option.
Related Terms
See also: Thunderstorm, Cold Front, Microburst, SIGMET.