The Cumulonimbus: Why a Single Cloud Outweighs an Entire Weather Report

In every briefing I delivered during my service, one principle was constant: if cumulonimbus was present, it became the subject of the briefing regardless of what else the reports contained.

This is not caution for its own sake. The cumulonimbus is the only cloud type which contains, within a single structure, every hazard which aviation meteorology exists to identify. Severe turbulence, severe icing, hail, lightning, windshear, downdraughts, heavy precipitation and rapid visibility reduction are not separate risks which may accompany a thunderstorm. They are components of it.

How the cloud develops

A cumulonimbus develops in three recognisable stages, and understanding the sequence assists considerably in interpreting what a report or radar image is describing.

The developing stage

Warm, moist air rises and condenses, releasing latent heat which drives further ascent. At this stage the cloud is dominated by updraught. It appears as a growing cumulus with sharp, hard-edged cauliflower tops. When these tops become distinctly taller than they are wide, the cloud is reported as towering cumulus, encoded TCU.

Towering cumulus is the stage immediately preceding cumulonimbus. Its appearance in a report during a warm afternoon should be read as an indication of what is likely to follow rather than as a stable condition.

The mature stage

Precipitation begins to fall, and its drag initiates a downdraught alongside the continuing updraught. The cloud now contains adjacent columns of air moving vigorously in opposite directions, which is the source of the severe turbulence associated with these systems.

The top reaches the tropopause and spreads horizontally into the characteristic anvil, composed of ice crystals and frequently extending many tens of kilometres downwind. A particularly vigorous updraught may briefly penetrate above the tropopause, producing an overshooting top — a reliable visual and satellite indication of severe intensity.

This is the stage at which the cloud is most dangerous, and it is encoded simply as CB.

The dissipating stage

The downdraught eventually spreads throughout the cloud, cutting off the supply of warm moist air which sustains it. Precipitation becomes lighter and the structure decays.

The hazards, considered individually

Turbulence

Vertical currents within a mature cumulonimbus may exceed 6,000 feet per minute. Adjacent updraughts and downdraughts create shear of an intensity capable of producing structural loads beyond design limits. Severe turbulence is also encountered in clear air beneath the anvil and for a considerable distance downwind.

Hail

Hail is carried aloft by the updraught and may be ejected from the anvil, which means it can be encountered in apparently clear air several kilometres from the visible cloud. Hail damage to radomes, windscreens and leading edges is well documented, and the encounter is frequently unexpected precisely because the aircraft was not in cloud.

Icing

The abundant supercooled liquid water within a cumulonimbus produces severe clear icing, capable of accumulating at rates which exceed the capability of airframe protection systems.

Lightning

A strike may cause structural damage at the attachment point, damage to composite structures, disruption of avionics and compass systems, and temporary blindness of the crew at night.

Windshear and microburst

This is, in my assessment, the most immediately dangerous of all, because it occurs at the altitudes where the aircraft has least energy and least room to recover. It is discussed in detail in our separate guide on windshear and microbursts.

Avoidance criteria

The standard guidance, which I would endorse without qualification, is as follows.

• Avoid all thunderstorm cells by at least 20 nautical miles laterally, and by a greater margin where the cell is severe or where hail is suspected.
• Do not attempt to overfly a cumulonimbus unless clearing the top by at least 5,000 feet, which in the Indian tropics is frequently beyond the capability of the aircraft, as tops may exceed 50,000 feet.
• Do not attempt to fly beneath a cell. The region below the cloud base contains the downdraught, the heaviest precipitation and the gust front.
• Avoid flight beneath the anvil, where hail ejection and severe turbulence occur in clear air.
• Treat a line of cells as a single obstacle. Gaps close rapidly, and a gap which appears adequate on radar may be considerably narrower by the time it is reached.

Recognition in the reports

In a METAR or TAF, the indications are unambiguous:

• CB or TCU appended to any cloud group
• TS in the present weather group, alone or in combination such as TSRA or +TSRA
• VCTS — thunderstorm in the vicinity, meaning within approximately 8 to 16 kilometres of the aerodrome
• GR for hail, or GS for small hail and snow pellets
• SQ for squall

On radar, reflectivity values above 50 dBZ, rapid vertical development, and the tight rotational couplets visible in velocity imagery all indicate severe convection. Our guide to reading Doppler radar covers this in detail.

A closing observation

Modern aircraft are remarkably capable, and modern weather radar is excellent. Neither alters the fundamental position, which is that the energy contained within a mature cumulonimbus substantially exceeds what any airframe is designed to withstand.

The correct response to a thunderstorm has not changed in the history of aviation, and I do not expect it to change: go around it, with generous margin, and accept the additional fuel and time as the cost of doing so.