1. Thunderstorms and Icing
ATC Chapters 3 & 4

2. Aim
To learn about Thunderstorms and Icing and the hazards associated with them

3. Objectives State how conditions necessary for thunderstorm formation
Describe the lifecycle of a thunderstorm
State hazards associated with thunderstorms
Explain the conditions necessary for formation of ice
State different types of ice

4. Revision The standard atmosphere assumes sea level to be:
A pressure of hPa
A temperature of 15°C
A density of kg/m³
Diurnal variation is the heating and cooling of the earth over a:
24 hour period.
The 3 states of water are:
Gas (vapour)
Liquid (water)
Solid (ice)
Cloud height datum in an area forecast is given above:
above mean sea level (AMSL)

5. Revision The different layers of the atmosphere are: The troposphere
The stratosphere
The mesophere
The thermosphere
Most flying occurs in the troposphere, although most jets cruise in the stratosphere

6. 1. Thunderstorm Formation
Background
A thunderstorm is one where there are one or more sudden electrical discharges
They are shown by a flash of lightning and a rumble of thunder
The speed of sound is a lot slower than the speed of light therefore the thunder comes after the lightning
Thunderstorms are often accompanied by heavy rain, hail, squalls, microbursts and tornadoes.

7. 1. Thunderstorm Formation
3 conditions necessary
There are 3 conditions necessary for the formation of thunderstorms.
Deep instability
Significant moisture
Trigger mechanism/lifting mechanisim

8. 1. Thunderstorm Formation
Instability
The atmosphere must be conditionally stable or unstable from the surface to high levels
A high environmental lapse rate will cause the atmosphere toward instability as the air will keep rising
Either heating of the bottom layers or cooling of top layers increases the ELR and reduces atmospheric stability.

9. 1. Thunderstorm Formation
Moisture
Sufficient water vapour must be present in the atmosphere to produce a cumulonimbus cloud,
The latent heat released during the formation increases their buoyancy and enhances their development.

10. 1. Thunderstorm Formation
Trigger mechanism
This trigger mechanism must be present to produce the initial lifting of the surface air.
Once it has been lifted, the latent heat released during cloud formation and the atmospheric instability will ensure the continued ascent
Some thunderstorms are classified according to the trigger mechanism that caused the initial lifting of air, ie frontal thunderstorms.

11. 2. Lifecycle of thunderstorms
Lifecycle of a thunderstorm
A thunderstorm makes its way through three distinct stages before dissipating.
Cumulus or building stage
Mature stage
Dissipating stage

12. 2. Lifecycle of thunderstorms
Cumulus stage
The cumulus stage is where the thunderstorm begins with the start of the lifting action of air
If sufficient moisture and instability are present, the clouds continue to increase in vertical height
Continuous, strong updrafts prevent moisture from falling
The updraft region grows larger then the individual thermals feeding the storm

13. 2. Lifecycle of thunderstorms
Mature stage
Within approximately 15 minutes, the thunderstorm reaches the mature stage
This is the most violent part of a thunderstorms lifecycle.
An anvil at the top usually develops composed of cirrus cloud and ice crystals
The wispy cirrus anvil is blown flat by the wind on top, usually a good indicator of storm movement.

14. 2. Lifecycle of thunderstorms
Mature stage
At this point, drops of moisture, whether rain or ice, are too heavy for the cloud to support them
This creates a downward motion air
Strong updrafts are still present resulting in windshear and severe turbulence.
Lightning is most frequent in the mature stage

15. 2. Lifecycle of thunderstorms
Dissipating stage
Updrafts then die out leaving only downdrafts
Rain gradually decreases and the cloud breaks up
The downdrafts spread out and replace the updrafts needed to sustain the storm
The whole lifecycle of the thunderstorm usually lasts around 60 minutes, but could be longer.

16. 3. Hazards associated with thunderstorms
There are many hazards associated with thunderstorms, and for most aircraft should be avoided at all costs.
Hazards from thunderstorms are but not limited to:
Severe turbulence
Icing
Lightning
Hail
Squalls and gusts
Wind shear
Microbursts
Tornadoes
Dust storms

17. 3. Hazards associated with thunderstorms
Turbulence
Turbulence can be classified according to how the aircraft behaves and occupant comfort.
Light turbulence - briefly causes slight, erratic changes in altitude and/or attitude.
Light chop - slight, rapid and somewhat rhythmic bumpiness without noticeable changes in altitude or attitude.
Moderate turbulence - similar to light turbulence, but greater intensity. Changes in altitude/attitude occur. Aircraft remains in control at all times. Variations in indicated air speed.
Moderate chop - similar to light chop, but greater intensity. Rapid bumps or jolts without obvious changes in altitude or attitude.
Severe turbulence - large, abrupt changes in altitude/attitude. Large variation in indicated airspeed. Aircraft may be temporarily out of control.
Extreme turbulence - aircraft is violently tossed about and is impossible to control. May cause structural damage.

18. 3. Hazards associated with thunderstorms
Turbulence
Within or near a thunderstorm an occupant can expect to encounter severe turbulence.
It is sometimes so severe that accurate control of a light aircraft could be near impossible.
Heavy structural loads are placed on the airframe and in extreme cases airframe damage and injury to occupants can occur.

19. 3. Hazards associated with thunderstorms
Lightning
Lightning is simply a gigantic electrical spark which flows along the pathway of ionised air.
Sometimes the airframe can act as a pathway to the lightning
The lightning enters at one point, flows through and exits at another point
Despite being a gigantic electrical spark, lightning poses no immediate risk to the occupants since they do not form any part of the pathway
Lightning can pose a fire hazard and cause damage to the aircraft electrical system.

20. 3. Hazards associated with thunderstorms
Hail
Hail can be encountered in clear air, both beneath and beside the thunderstorm cell.
Even though hail encounters are short lived, they can pose a real risk of damage to the aircraft.
Hail is most commonly encountered between and 20000ft near the middle of the storm.

21. 3. Hazards associated with thunderstorms
Squalls and gusts
A squall or gust is a sudden, sharp increase in wind speed.
Local and short lived bursts of strong wind can produce severe low level turbulence
The sudden onset of gusting wind can make approach and landing difficult or impossible, especially when it is a gusting crosswind.

22. 3. Hazards associated with thunderstorms
Windshear
Windshear is the sudden change of wind speed at different heights
Windshear can cause a sudden loss of lift and IAS for aircraft
At low level on approach to landing windshear can be disastrous if not detected.

23. 3. Hazards associated with thunderstorms
Microbursts
Apart from the main downdrafts within a cell, many storms feature areas of localised intense downdraft activity
These areas are known as microbursts
Microbursts are known to be fatal and have been the cause of many air disasters involving large passenger jets.

24. 3. Hazards associated with thunderstorms
Tornadoes
Tornadoes always result from very unstable atmospheric conditions
Tornadoes first appear as a funnel shaped cloud which extends from the base of a cumulonimbus cloud
Once it reaches the ground it generates rotating winds of up to 500kmph over a very narrow path
This extreme low pressure can cause the air inside buildings to explode damaging walls ceilings and anything else in its path

25. 3. Hazards associated with thunderstorms
Duststorms
When the air is unstable but too dry to form extensive cloud, a dust storm might develop
As convection occurs in the lower atmosphere, dust from the dry surface is lifted with it.
Unlike a thunderstorm, a duststorm can have a very widespread area, severely reducing visibility.

26. 4. Ice formation
Ice
When ice cubes are made, the liquid water freezes when the temperature drops to below 0.
To allow water to freeze, latent heat must be released from the water to the environment.
When water exists as tiny droplets in the atmosphere, the exchange of latent heat is much less efficient and freezing does not occur until the temperature is well below zero.
Depending on their size, water droplets suspended in a cloud can remain liquid at temperatures as low as -40.
The smaller the droplet, the lower the temperature required to cause it to freeze.
Water that is in its liquid form, but below freezing is called supercooled water.

27. 4. Ice formation
Ice
Any disturbance to the supercooled water will cause it to lose its latent heat and freeze.
Any aircraft flying above the freezing layer in cloud will cause the necessary disturbance, and since the airframe is also below zero, the supercooled water will freeze on impact, causing ice.

28. 5. Icing types
Ice
There are different types of airframe icing according to the temperatures and appearance of the ice. They are:
Clear ice
Rime ice
Hoar frost

29. 5. Icing types
Rime Ice
When very cold, supercooled water droplets collide with leading edges, they freeze so quickly they do not have time to splash or spread across the surface.
This type of icing is known as Rime ice
The ice maintains its spherical shape after freezing producing a granular layer of ice with a lot of air trapped between the grains.
The ice formed is white and brittle.

30. 5. Icing types
Clear Ice
Large supercooled water droplets are found at warmer temperatures
These temperatures are still sub-zero
Because the larger supercooled water droplets are not so cold, they tend to have more latent heat to lose and take longer to freeze.
The supercooled water droplets tend to splash and spread further along the leading edge before freezing.
This process can continue and can form several layers of ice
There is not as much air trapped between the layers so therefore tends to be heavier and stronger than rime ice.

31. 5. Icing types Conditions necessary for formation
Sub-Zero temperatures
If the airframe of the aircraft is above zero degrees, the ice will simply melt.
Supercooled droplets
Liquid water must be present in the sub zero temperature range. If the water was in ice form already, it would not stick to the aircraft
Droplet size
If the supercooled droplets are very small, rime ice is likely
If the supercooled droplets are large, clear ice is likely

32. 5. Icing types Conditions necessary for formation
The temperature ranges favourable for the formation of ice are shown below. It is important to know that it is not only the temperature of the droplets that determine what type of ice is likely, it is also the size of the droplet.
From 0°C to -15°C, in the presence of large supercooled water droplets, clear ice is likely.
From -10°C to -30°C, in the presence of small supercooled water droplets, rime ice is likely

33. 5. Icing types Freezing Rain
It is not always necessary to be in cloud for ice to form
If you are flying above the freezing layer but below a cloud base in supercooled rain drops, a rapid build up of severe clear ice could be likely.

34. 5. Icing types Pilot actions if icing is encountered
If you are flying VFR and encounter icing, the easiest thing to do is descend below the freezing layer.
But if you are flying VFR, it would pose the question what are you doing flying in visible moisture above the freezing layer?
For IFR aircraft, flying in icing conditions can be a routine occurrence.
If you are unequipped with de-icing or anti icing equipment it would be necessary to fly below the freezing layer (if you are in visible moisture)
Sometimes IFR aircraft cannot descend below the freezing layer therefore it would be necessary to operate de-icing or anti icing equipment.

35. 5. Icing types Frost Frost is simply frozen dew
It forms on the top surfaces of aircraft left out overnight.
If it is not removed it can disrupt the airflow over the wings and cause a significant decrease in aerodynamic performance.
In extreme cases it might make it impossible for the aircraft to leave the ground at all.
Frost may go un-noticed on some high wing aircraft if the pilot does not physically look above the wing
If sub-zero temperatures have occurred overnight, special attention needs to be given for looking for frost.
If found, frost can be removed by washing with water.
It would be necessary to remove frost before flight.

36. 5. Icing types
Frost

37. 5. Icing types
Hoar Frost
When an aircraft has been cruising at high altitude for a period of time, the airframe will cool to the same temperature as the surroundings
This condition is known as “cold soaking” and may take a while for the airframe to warm up again.
If a cold-soaked aircraft on descent encounters a layer of warmer humid air, water vapour may skip the liquid state and turn straight to ice.
This process is called deposition, and the resulting layer of ice is called hoar frost.
It is a white crystalline deposit

38. 5. Icing types
Hoar Frost
Hoar frost has little effect on aircraft performance.
The frost will disappear once the aircraft warms, but if the subzero temperatures persisted down to ground level, the heater/demister may be required to see through the windscreen.

39. Questions?