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Learning hub · Meteorology · Lesson 4

Thunderstorms, icing and turbulence

Most hazardous weather has a warning sign in the briefing, and most of it is simply avoided. This lesson covers the four hazards that student pilots hear about most: thunderstorms, ice, turbulence and wind shear.

About 13 minutes, plus the quiz. Part of Meteorology. It builds on Clouds, stability and fronts.

1. How a thunderstorm lives and dies

A thunderstorm needs three ingredients: moist air, an unstable atmosphere, and something to start the air rising (heating from the ground, a front, or hills). It has three stages.

  1. Cumulus stage: a growing cloud with updraughts only. The cloud builds upward fast, but little rain falls yet.
  2. Mature stage: rain and hail form, heavy enough to drag air down, so updraughts and downdraughts sit side by side. Lightning, heavy rain, hail, strong turbulence and the worst gusts occur now. The top spreads out into the typical anvil shape of a cumulonimbus (CB).
  3. Dissipating stage: downdraughts take over, cut off the supply of warm air, and the storm weakens. Showers may continue for some time.

A single cell typically lasts about 30 minutes to an hour. Larger storms, including squall lines, are a chain of cells that replace each other and can last much longer.

2. The hazards, and how to avoid them

  • Severe turbulence in and around the cloud, strong enough to damage an aircraft or put it out of control.
  • Hail, which can fall well away from the cloud, particularly from under the anvil.
  • Icing in the cloud above the freezing level.
  • Lightning, which can damage the aircraft, upset instruments and blind the pilot.
  • Heavy rain and very poor visibility.
  • Wind shear from the downdraught. A strong downdraught that spreads out along the ground, a microburst, is especially dangerous close to the ground on take-off and landing. A gust front can also run ahead of the storm for many miles.

There is no safe way to fly through a thunderstorm in a light aeroplane and no trick for finding a gap. The rule is to keep well away. How far is a judgement, and training texts quote different figures: the American Pilot's Handbook of Aeronautical Knowledge suggests at least 20 nautical miles from a severe storm, so treat anything shorter with respect. Embedded storms hidden in layer cloud and storms at night are the cases you cannot see, which is why the forecast and a briefing matter. If the storm cannot be avoided, the best plan is to turn back or land and wait until it passes.

3. Airframe icing

Structural icing needs two things: visible moisture (cloud or rain) and an air temperature near or below freezing. Clouds below 0°C often still contain liquid droplets. These supercooled droplets freeze when they hit the aircraft.

  • Rime ice: small droplets freeze at once, trapping air. White, rough and brittle. Common at colder temperatures in layer cloud.
  • Clear ice: larger droplets spread before they freeze, forming a smooth, hard, glassy layer that is hard to see and hard to remove. It is typical in freezing rain, and in cloud at temperatures not far below freezing.
  • Mixed ice: a combination of the two.

Ice adds mass, adds drag, and spoils the shape of the wing, so the stall speed goes up and the angle of attack at which it stalls goes down. It can block pitot tubes, static ports and the propeller, and reduce control movement. Frost on the ground does the same to the wing, so a wing must be cleaned before take-off.

Most light training aeroplanes are not certified for flight in known icing conditions. The way out is to leave the conditions: turn back, descend into warmer air if it is safe to do so, or climb out of the cloud. Freezing rain below a warm layer is the worst case, because there may be no escape downward. Carburettor icing is a different problem and can happen well above freezing, even in clear air: see Piston engine, ignition and carburettor icing.

4. Turbulence and wind shear

Convective and mechanical turbulence. Thermals under cumulus, and wind flowing over buildings, trees and hills, cause bumps in the lower layers. It is worst on hot afternoons and on windy days, and it is more intense with a strong wind over a rough surface.

Mountain wave and rotor. When a strong, stable airflow crosses a ridge, it can set up waves downwind. The upwind side can have smooth lift, while the lee side has strong downdraughts, sometimes stronger than the climb rate of a light aeroplane. Under the wave crests, a rotor can form: a turning mass of violent turbulence. Lens-shaped lenticular clouds and rotor clouds suggest it is present, but wave can exist in clear air. When you cross a ridge, do it at a high enough altitude, and at an angle so that you can turn away if the air sinks.

Clear-air turbulence (CAT). Turbulence in cloudless air, often near the jet stream where the wind changes quickly over a short distance. It is mostly an issue at high levels, but it is why you keep your seat belt fastened. In turbulence, slow down to the manoeuvring speed in your flight manual.

Wind shear. A rapid change of wind speed or direction over a short distance. Near the ground it can come from microbursts, fronts, or the wind flowing around obstacles, and it can cut your airspeed just as you are close to the runway. If the approach looks unstable in gusty conditions, go around early.

This lesson is a basic overview for training. Follow the limits in your flight manual and your school’s rules, and use official forecasts, SIGMETs and a full briefing before a flight.

5. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

  1. A METAR shows -2/-3 with OVC008 and light snow, and your flight would go through the cloud layer. Which icing type would you expect, and what are your options?
    Show the answer

    The temperature is below freezing and there is visible moisture (cloud and snow), so icing is possible. Rime, clear or mixed ice are all possible: at a temperature only just below freezing, clear or mixed ice is quite likely if the droplets are large. The snow itself is frozen and does not usually stick, but the supercooled droplets in the cloud do. The type cannot be told from the METAR alone.

    Options: do not go, because a light aeroplane without ice protection should not fly in these conditions. If you are already airborne, leave the icing layer by turning back, climbing out of the cloud top, or descending to a layer with a temperature above 0°C, whichever is safe given terrain and the rules.

  2. Put the stages of a thunderstorm in order and say which has both updraughts and downdraughts: dissipating, cumulus, mature.
    Show the answer

    Cumulus, then mature, then dissipating. The mature stage has both updraughts and downdraughts side by side, which is why turbulence and hail are at their worst there. The cumulus stage has only updraughts. In the dissipating stage, downdraughts take over.

  3. You are 20 minutes from landing. The ATIS wind is 250° at 12 kt, and a thunderstorm is reported 8 nautical miles west of the aerodrome, moving east. What should you worry about, and what would you do?
    Show the answer

    The storm is close and moving toward you. Dangers: a gust front and microburst ahead of the storm, wind shear on approach, heavy rain, hail and turbulence. The calm ATIS wind may not last.

    A sensible plan: do not try to land ahead of it as a gamble. Divert to an aerodrome away from the storm, or hold well clear on the other side, and land when the storm has passed. Tell ATC what you intend and keep enough fuel to divert.

6. Watch

The three stages of a thunderstorm, from the cumulus to the dissipating stage. Check what you learn against your course material.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

More to watch

Types of airframe ice and how pilots deal with it. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.

Open on YouTube if the player does not load. Not made by VectorEFB.

7. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

1. In which stage of a thunderstorm are updraughts and downdraughts both present?

The mature stage has rain dragging air down beside the updraughts. This is when turbulence, hail and lightning are at their worst.

2. What are the two conditions that icing needs on an aeroplane in flight?

Supercooled droplets in cloud or rain freeze on hitting the aircraft. Without visible moisture, structural icing does not form.

3. What does ice on the wing do to the stall speed?

A rough wing with a bad shape produces less lift at a given angle of attack, so the stall comes sooner and at a higher speed.

4. What is a rotor?

Rotors form in the lee of a ridge under the wave crests. They can be severe, and lenticular or rotor clouds may mark them.

5. What is the safest plan when a thunderstorm lies on your route?

There is no safe way through or under a storm. Hail can fall far from the cloud, and cumulonimbus tops are far higher than a light aeroplane can climb.

Next: Wind, fog and visibility. Back to the learning hub.

For training and situational awareness only. Not certified for navigation. Always use official charts, the AIP, NOTAMs and a proper weather briefing for real flights.