Thunderstorms

Meteorology · Chapter 13 · DGCA CPL and ATPL

How a thunderstorm is born, lives and dies, everything it can do to an aircraft, and India's own spring storms.

Remember

Needs: steep lapse rate (unstable air through 5-6 km), plenty of moisture, and a trigger: convection (most common), orographic lifting, convergence, frontal lifting.
Heat storms: summer afternoons over land, single cells, weaker. Frontal storms: cold fronts, any time, land or sea, in lines (squall lines), more violent.
Cumulus stage: updraughts only, no rain. Mature: starts with precipitation, up and down draughts together, the most dangerous stage (20-40 min). Dissipating: downdraughts only, anvil spreads.
Turbulence and icing are the most serious hazards; lightning most likely between +10 and −10 °C; hail can fall under the anvil; the altimeter over-reads under a CB.
Microburst under 4 km, 1-5 min: headwind, then downdraught, then tailwind. Virga is the warning sign.
Nor'westers (Kal Baisakhi): Mar-May, afternoons, from the Chota Nagpur and Khasi hills over Bengal, Bihar, Odisha and Assam. Andhi: dust storms over the north-west.

What does it take to make a thunderstorm?

Ingredients and triggers

The teacher's recipe: three ingredients, then four ways to start it. Tap Next.

It lives in a CB A thunderstorm is born in a well-developed cumulonimbus, with lightning and thunder, squalls, heavy showers and sometimes hail. It is reported when thunder is heard. Single CB cells are 1 to 10 km across; for aviation the thunderstorm is one of the most dangerous things in the sky.
Unstable, moist, lifted Unstable air: a lapse rate steeper than the SALR through at least 5-6 km, so the cloud can grow above the 0 °C level. Moisture: plenty from below and humid air aloft, or the rising cloud dries out and stops. A trigger: something to lift the air to saturation.
Convection The sun heats the ground, the ground heats the air, and bubbles of warm air rise. The most common trigger: over land, in summer, in the afternoon. These are heat-type (air-mass) storms.
Orographic lifting Air blowing against hills or mountains is forced up the slope. It works by day or night, summer or winter. The Nor'westers are helped by the Chota Nagpur and Khasi hills.
Convergence Around a surface low, air flows in from all sides. With nowhere else to go, it rises, and spreads out again (diverges) high up.
Frontal lifting Where a cold air mass undercuts warm air, the warm air is shoved up. Cold fronts make the most storms: more violent than heat storms, by day or night, over land or sea, often in a line just ahead of the front, the squall line.

Memory hook: Unstable air + moisture + a trigger. Triggers: convection, orographic, convergence, frontal.

Takeaway: A thunderstorm needs a steep lapse rate (unstable air through 5-6 km), plenty of moisture, and a trigger that lifts the air: heating from below, a mountain, converging winds or a front (night-time cooling of cloud tops can also do it). Not every CB gives a thunderstorm, but every thunderstorm comes from a CB.

What exactly must the air be like?

Three ingredients

A thunderstorm is an instability phenomenon.

A steep lapse rate Steeper than the SALR through a deep layer, at least 5-6 km, so the cloud can grow well above the freezing level.
High humidity A good supply of moisture from below and humid air aloft. In dry surroundings the rising cloud is eaten away by evaporation.
A trigger Something must lift the air until it saturates in the unstable layer: strong daytime heating, frontal lifting, convergence, hills (orographic lifting), or night-time radiational or katabatic cooling.

Memory hook: Steep, wet, lifted.

Takeaway: Three things: a lapse rate steeper than the SALR through a layer at least 5-6 km deep (so cloud grows above the 0 °C level), plenty of moisture below and high humidity aloft (dry air would evaporate the rising cloud), and a trigger to lift the air to saturation.