3.4 Thunderstorms, Microbursts, Lightning, Hail and Tornadoes

19 min · UA.III.B.K1e UA.III.B.K1f UA.III.B.K1h UA.III.B.K1k

A thunderstorm is the most concentrated collection of aviation hazards that exists: severe turbulence, severe icing, hail, lightning, extreme up- and downdrafts, sudden wind shear, heavy precipitation, and tornadoes, all in one place. Airliners avoid them. Your three-pound aircraft has no business anywhere near one, and the FAA tests this area hard. The good news is that storms are also the most predictable hazard here: they need three specific things, and if any one is missing you get no storm.

The three ingredients

This is why afternoon thunderstorms are a summer phenomenon in humid regions and why they are rare over a dry desert in the morning. It is also why the trigger matters: on a day with the moisture and instability already loaded, a passing cold front or a sea-breeze boundary is enough to set off a line of storms in under an hour.

The three stages of a cell

Every thunderstorm cell goes through the same three-act life. The exam wants the defining characteristic of each act, and it wants you to know which one will kill you.

The mechanism is worth understanding as well as memorizing. In the cumulus stage the updraft is strong enough to hold all the water it has condensed, so nothing falls out. As the cell grows, the water load becomes too heavy, and falling precipitation drags cold air down with it. That descending column is the downdraft. Once it reaches the ground, the storm has both an updraft feeding it and a downdraft cutting it apart — the mature stage. Eventually the downdraft spreads out across the whole base, chokes off the inflow, and the storm starves. That is dissipation.

Storm types and the gust front

A single-cell (air mass) thunderstorm is one cell, thirty minutes, usually a local afternoon event. A multicell cluster is several cells at different stages sharing one outflow, which is why it outlives any single cell. A supercell has one persistent rotating updraft, lasts for hours, and produces most significant hail and nearly all strong tornadoes. A squall line, from the previous lesson, can run 50 to 300 miles ahead of a fast-moving cold front.

Whatever the type, the hazard that catches drone pilots is the gust front. Cold downdraft air spreading out from a storm base behaves like water poured on a table, and its leading edge can arrive 15 miles or more ahead of the storm itself. The signs are a sudden wind shift, a sharp temperature drop, a wall of dust or a shelf cloud — with clear sky overhead. Your aircraft is airborne in what looks like fine weather when the wind goes from 6 knots to 35 in under a minute.

Microbursts

A microburst is a small, intense, concentrated downdraft. It is the most dangerous single wind event in aviation and it has brought down airliners. Here is the anatomy, with the numbers the FAA publishes.

Now translate those numbers into your aircraft. Six thousand feet per minute is 100 feet per second straight down. A typical prosumer quadcopter climbs at about 1,000 feet per minute in its best mode — call it 17 feet per second. The downdraft beats your maximum climb rate by a factor of six. From 300 feet AGL, you have roughly three seconds.

You cannot run away sideways either. Forty-five knots is 52 miles per hour. Folding consumer quadcopters are typically rated to resist somewhere around 20 to 24 knots of wind, and their maximum forward speed in sport mode is in the neighborhood of 40 miles per hour — into a 52 mph outflow, your groundspeed is negative. The aircraft goes where the air goes.

This is the point of the lesson. A microburst is not a windy patch you fly through carefully. For a three-pound aircraft it is a total loss, and it begins and ends in less time than it takes to walk back to the truck. The only defense is not being airborne, and that decision has to be made while the storm is still miles away and the sky overhead still looks fine.

Lightning

A thunderstorm separates electrical charge as ice and water particles collide inside it, then discharges within the cloud, between clouds, or to the ground. The stroke heats the air in its channel violently, and that is what produces thunder.

You do not need a direct strike for lightning to end your flight. The electromagnetic pulse from a nearby discharge can corrupt your control link, upset the aircraft's magnetometer so the compass reads wrong, and interfere with GNSS reception. Flying through precipitation also builds static charge on the airframe — precipitation static — which shows up as radio noise and, in extreme cases, corona discharge on the propeller tips.

The bigger risk, honestly, is you. The remote pilot and visual observer stand outdoors in the open holding a controller with a metal antenna, often on a roof or in a field. Lightning can strike more than 10 miles from the parent cell out of apparently clear sky — the bolt from the blue, which travels out through the anvil.

Hail

Hail forms when a strong updraft carries droplets above the freezing level, where they freeze, gather more supercooled water, are lifted again, and grow in layers until the updraft can no longer hold them. Stronger updraft, bigger hail. A stone of 3/4 inch or larger is one of the criteria that makes a thunderstorm officially severe.

The operationally important fact is that hail does not stay under the storm. It can be carried out along the anvil and fall in clear air up to 20 miles from the parent cell. For a small drone hail is not a control problem but a structural one: a 3/4-inch stone at highway speed shatters propellers and destroys a gimbal, and the shaft arrives faster than you can descend and land.

Tornadoes

A tornado is a violently rotating column of air extending from the base of a cumulonimbus down to the ground. Winds in the core can exceed 200 knots, with an extreme pressure drop. When the visible condensation funnel has not reached the surface it is called a funnel cloud; over water the same phenomenon is a waterspout.

Tornadoes are most associated with supercells, but they also form along squall lines and in hurricane rainbands. The visual precursors — a lowered, rotating wall cloud beneath a rain-free storm base, or a dust whirl on the ground under it — are the point at which the aircraft should already be in its case.

Putting it into a decision

For drone work the rule is short. If convective activity is observed or forecast within about 20 miles and is moving toward you, do not launch. If you are airborne, land before the outflow arrives, not when it arrives. Check the Convective SIGMET and the radar mosaic twice: at the office, and again before you open the case. Lesson 3.9 shows you where to find both.