To and From the Airport - Light Aircraft Flying

Thunderstorms

AIM: To understand the formation and hazards of thunderstorms.

Formation:

Thunderstorms can occur in any geographical location, however certain areas (eg Florida in Summer) are particularly susceptible. They can form very rapidly and unpredictably and can persist for long periods. In order for a thunderstorm to develop 3 conditions must be met:

1) A deep instability in the atmosphere

This allows the rising air always to be warmer than the ambient environmental air around it, allowing it to continue to rise. As the water vapour condenses to form droplets, latent heat of vaporisation is released, and it is this heat which fuels the thunderstorm by encouraging further rising and thus more heat release.

It is this runaway heat generating process that causes such rapid build up of storms, and leads to such strong updraughts within.

2) A high moisture content

Since it is the latent heat of vaporisation which fuels the process, we must have sufficiently moist air present to begin with.

3) A trigger action to cause initial lifting
All that is now needed is some trigger to begin the lifting process. Common triggers are:

Orographic uplift of an air mass on reaching a line of hills is enough to force the air upwards. This is common where moist tropical air masses are blown from a sea towards a mountainous or hilly coast, where a line of thunderstorms will often form.
Insolation: Strong heating of a land mass by the sun will cause the air to rise and lead to thunderstorm formation. This often happens in a Col in the summer months.
Frontal Activity: As a cold front advances, it pushes underneath the moist, warm air in the warm sector, causing it to rise. This is common in the UK.
Air mass movement: As Polar maritime air approaches moves south, it warms from beneath by conduction from the warmer land (or sea) below. This causes lifting and CB or thunderstorm activity. Due to the low moisture content of Polar maritime air, the storm created in this way is likely to be less intense.

However if the Polar Maritime air routes further south over the Atlantic before being caught up in a jet-stream heading towards the UK, more moisture is present and more severe thunderstorms may result. This is known as Returning Polar Maritime Air. Once the initial trigger is supplied, the storm becomes self-sustaining.

Stages of a Thunderstorm

Cumulus Stage

The thunderstorm begins life as a cumulus cloud, but due to the deep instability of the atmosphere, much heating is generated as water vapour condenses, and this leads to strong warm updraughts. This stage usually lasts 10-20 mins and is characterized by strong updraughts and no significant downdraughts or precipitation. The rate of building is often faster than the rate of climb of an aeroplane. A thunderstorm will tend to move in accordance with the wind at 10 000’.

Cumulus clouds Towering Cumulus

Mature Stage Once the water droplets become too large to be held up by the strong updraughts (currents as high as 10 000fpm have been recorded), they fall as rain (often very heavy rain or hail) and they pull cold air down with them leading to strong downdraughts. This leads to strong Windshear and possible microbursts underneath the storm. The Cumulonimbus cloud that forms may extend over 30 000’ or even 60 000’ in the tropics. The cloud may have an anvil shape at the top due to the strength of the upper wind. Thunder and lightning occur at this stage.

Mature CB Mature CB with anvil

Dissipating Stage

Eventually the updraghts lessen and cease, and the downdraughts dominate. This leads to the collapse of the storm into stratiform cloud. Embedded Thunderstorms:Sometimes a thunderstorm is embedded inside stratiform cloud, and may not be visible to the pilot. Forecasts, satellite and radar imagery and pilot reports can indicate where this is occurring.

Thunderstorm on Weather Radar Line of thunderstorms

Squall Lines:

Sometimes, thunderstorms form a line, such as at a cold front. It may be difficult or impossible to find a gap through which to fly. In this weather chart, there are several points of interest:

A Cold front over Eastern Poland and another over Northern Russia, which have led to an associated line of thunderstorms.
The Inter-tropical Convergence Zone (ITCZ) over central Africa with associated embedded thunderstorms.

Hazards of Thunderstorms

Thunderstorms should always be avoided either visually or using electronic means since they represent a serious hazard to flight safety. At night, in the absence of any lightning strikes, or when embedded, thunderstorms may be impossible to see. The main dangers are: Severe Turbulence & Windshear This can occur anywhere in or around the thunderstorm including above, sometimes over 20 Nm away. The appearance of the storm is often a poor guide to the level of turbulence to be expected. The effects of a thunderstorm close to the ground may be felt 40 miles away as the cold air descends and spreads out. The Windshear associated with the storm may have a serious negative impact on aircraft performance.

Severe icing
Both airframe and carburettor icing is a distinct possibility in a thunderstorm. Large amounts of super-cooled water are present in a thunderstorm, particularly in the central updraughts. This can lead to very rapid deposits of ice on all aircraft surfaces. This will lead to an increase in aircraft mass, and to the loss of lift from a contaminated wing.

Heavy rain and hail
Hailstones of up to five and a half inches in diameter have been recorded, but much smaller hailstones can damage or destroy an aircraft. Hail occurs more often at the edge of thunderstorms, and may fall from an overhanging lobe.

Tornados
These are small and localised and can move unpredictably at high speeds. These occur frequently in the central US in summer, and occasionally in Europe, including the UK. They can destroy an aircraft in flight or on the ground, with wind speeds sometimes as high as 200kts.

Microbursts
Localised, low level, very intense down draughts from the base of a thunderstorm, that have been responsible for several air accidents.

Poor visibility
Very poor visibility may result from heavy precipitation, and mist of fog may form after the thunderstorm, due to evaporation of the heavy rainfall.

Underneath an active CB

Lightning strikes
Lightning can occur not only in the storm, but below and at some distance to the side. Lightning usually occurs in areas of the storm between 10°C and -10°C. Although not usually dangerous, lightning strikes can be alarming and may cause temporary blindness or deafness. After a strike, magnetic compasses should be treated with caution as they may now have large errors. The compass should be checked by an engineer after landing.

Radio communications interference
The principal effect of this to the Private pilot is to the ADF. All bearings should be treated with caution. The ADF needle may wander or point directly towards the thunderstorm. HF and possibly VHF communications may be affected.

Instrument Errors
Local pressure variations may give rise to errors in the altimeter and VSI. Altitude errors of up to 1000’ are believed to occur. Be prepared for ASI errors also in very heavy rain. If airspeed is in question, fly power and attitude.

St Elmo’s fire
Although not hazardous, this display of static electricity on the windscreen, wings or engines may cause concern to passengers. Some of these effects may be felt many miles away from the storm.

Flight in Thunderstorms

The most important rule concerning flight in thunderstorms is AVOID THUNDERSTORMS AT ALL COSTS

Do not take off if a thunderstorm is overhead or approaching.
Do not land if a thunderstorm is at or approaching your destination. Hold or divert.
Carry extra fuel for holding or diversion when thunderstorm activity is expected.
Strong vertical currents and precipitation may damage or destroy an aircraft. If, however avoidance is impossible, the following procedures should be adopted. --- FLY THE AIRCRAFT ---
Seat Belts: Ensure all crew and passengers have seat belts firmly tightened, and that any loose articles are stowed.
Crew Workload: If more than one pilot is on board, then one should fly the aeroplane, whilst the other monitors the instruments and performs ancillary tasks.
Altitude: Vertical draughts in a thunderstorm can be severe, and can exceed the performance of most aircraft at times. Choose a cruising altitude which allows separation from the ground, making allowance for strong downdraughts. Concentrate on holding a constant pitch attitude rather than chasing altitude. Avoid abrupt control movements. Avoid excessive roll. Do not try to out-climb the storm.
Speed: Fly at the manufacturer’s turbulence penetration speed and trim the aircraft for level flight. Do not chase power settings or speeds. Concentrate on holding a constant pitch attitude and accept small deviations in airspeed.
De-icing Equipment: Make use of any anti or de-icing equipment that may be fitted to the aircraft to minimise the effects of ice build up. Use of Carburettor heat as necessary. Ensure Pitot heat is on to reduce the chance of icing blocking the tubes.
NDB Navigation: Treat all ADF information with extreme caution near and in thunderstorms.
Cabin Lighting: Turn fully bright to minimise the effects of temporary blindness following a lightning strike. Use of glare shield or sunglasses to reduce blindness. Close one eye.
Autopilot: If the manufacturer recommends the use of the autopilot in turbulence, then its use may reduce structural loads on the aircraft structure.
After Landing: If excessive loads were encountered, have the aircraft checked by an engineer.