Thunderstorms, microbursts and convective avoidance

Convective weather is the single largest source of operational disruption in most route networks, and the exam treats it accordingly. The physical picture is worth learning properly because the avoidance rules follow directly from it.

The three ingredients

1. Sufficient water vapour, to supply the moisture that condenses and releases latent heat.
2. An unstable lapse rate, so that a lifted parcel keeps rising rather than returning to its original level.
3. A lifting action, such as frontal lift, orographic lift, convergence or surface heating, to start the parcel rising.

Remove any one of the three and a thunderstorm cannot develop. This is why a forecast can carry high moisture and marked instability and still produce nothing until a trigger arrives.

The three stages

The hazards

• Severe or extreme turbulence, both inside the cell and in the clear air surrounding it.
• Hail, which can be thrown well outside the visible cell and encountered in clear air beneath or downwind of the anvil, up to about 20 miles from the storm.
• Lightning, which can damage structure and avionics and produce temporary flash blindness.
• Low-level wind shear and microbursts on approach and departure.
• Severe icing in the updraft region above the freezing level.
• Tornadoes in the most severe cells.

Microbursts

A microburst is a concentrated, intense downdraft. Typical horizontal extent is less than about 2.5 miles, downdrafts can reach roughly 6,000 feet per minute, and the lifespan is short, commonly around 5 to 15 minutes. The headwind to tailwind shear across one can approach 90 knots.

The hazard on approach is the sequence. The aircraft first encounters an increasing headwind, which raises indicated airspeed and the aircraft tends above the glidepath. Reducing power to correct sets up the second half: the downdraft, followed by a rapid shift to a tailwind, with airspeed and performance falling at low altitude and low energy.

Avoidance

• Avoid a thunderstorm identified as severe, or one producing an intense radar echo, by at least 20 nautical miles.
• Do not attempt to fly under a thunderstorm even when visibility beneath it appears adequate. Turbulence and wind shear beneath a cell can exceed the aircraft's capability.
• Clear the top of a known severe cell by a wide margin if overflight is contemplated at all; updrafts can carry well above the visible top.
• Treat the anvil as part of the storm for hail purposes.
• A squall line is a non-frontal narrow band of active thunderstorms, often forming ahead of a cold front, and it frequently contains the most intense convective activity. It can be too high to overfly and too long to circumnavigate, so the plan must be built before departure.