Thunderstorm Types and Formation

• Part 7 of the Meteorology in Aeronautics lecture discusses hazardous weather phenomena, including thunderstorms and aircraft icing

Thunderstorms

• Thunderstorms are weather phenomena typically associated with cumulonimbus clouds
• They are necessarily associated with lightning and thunder

Development of Thunderstorms

• Thunderstorm development requires high moisture content in the lower troposphere
• An unstably stratified troposphere is needed for thunderstorm development
• A lifting process to raise air parcels for cooling to saturation is required

Lifting Processes for Thunderstorms

• Heating caused by the sun over land (thermals) or cooling aloft can cause lifting
• Orography (uplift on the mountains) can lift air parcels
• Frontal processes can cause lifting

Basic Forms of Thunderstorms: Heat Thunderstorms

• Heat thunderstorms occur frequently and strongly during summer months over land due to heating from the ground
• First maximum intensity occurs in the afternoon (4-7 pm) due to strongest warming of the ground
• Second maximum intensity occurs in the first half of the night (9 pm - midnight) due to labilization from radiative cooling of the top of the cumulonimbus cloud
• Heat thunderstorms occur especially in spring to autumn over the mainland
• During autumn and summer, heat thunderstorms can occur at night over seas (e.g., North Sea, Baltic Sea) due to warm water causing labilization from below
• Strong overheating of cities can lead to new formation or intensification, known as the Heat Island Effect
• Ideal conditions for heat thunderstorm formation include high temperatures, high humidity/moisture, low pressure gradient (little wind), and cyclonically curved isobars or isohypses (small-scale heat low)

Basic Forms of Thunderstorms: Orographic Thunderstorms

• Orographic thunderstorms are caused by forced uplift of a conditionally unstable air mass at orographic obstacles and linked to orographic uplift areas
• Even the smallest mountain ranges can be enough for orographic thunderstorm formation
• Orographic thunderstorms are relatively independent of the diurnal temperature cycle but intensify during summer months due to warming of the ground

Basic Forms of Thunderstorms: Frontal Thunderstorms

• Frontal thunderstorms on a cold front are mostly linearly arranged with high-reaching cumulonimbus clouds
• They shift quickly with the fast-moving cold front and can occur at all times of the year
• Cold air often flows ahead into the warm sector, forming strong thunderstorms with squall lines before the actual cold front

Frontal Thunderstorms on a Warm Front

• On a warm front, frontal thunderstorms occur
• In summer, warm air from southern subtropical latitudes contains water vapor
• Condensation during uplift releases heat, which can locally labilise the stratified atmosphere
• Thunderstorms are embedded in the compact warm frontal cloud layer making them not visible and dangerous for aviation

Stages Of Thunderstorm Development: Developing Stage

• Initial stage begins with a strongly swelling "cumulus sky"
• Up-drafts of approximately 10 to 25 knots exist, with an increasing tendency
• Horizontal extension is about 2 to 8 km
• No precipitation, hail, or lightning occurs yet
• Air flows in at the sides of the developing cumulonimbus cloud (entrainment)
• Light to moderate icing (clear ice) can occur
• Tops reach approximately 20,000 to 25,000 feet in summer
• This phase lasts approximately 15 to 25 minutes

Stages Of Thunderstorm Development: Matura Stage

• Extreme updrafts (up to 70 kt) and downdrafts occur in the mature stage
• This close proximity creates strong wind shear and strong turbulence
• Heavy precipitation falls in a narrow, localized area
• Cold air suddenly falls from the cloud, spreading out upon ground impact, leading to dangerous microbursts
• An outflow boundary (Böenwalze) extends up to 20 km (and up to 80 km in extreme cases) before the thunderstorm
• Sleet/hail is occurring from ground to cloud tops, and outside the cloud (up to 20 km)
• There is a strong risk of icing within the cloud
• Violent lightning activity is at its max
• Horizontal extension reaches up to 15 km
• Cloud tops may reach the tropopause, forming a small anvil
• This phase lasts approximately 25 to 40 minutes

Stages Of Thunderstorm Development: Dissipating Stage

• Downdrafts occur, ranging from 10 to 20 kt, decreasing in intensity
• Heavy precipitation turns into continuous rain
• Hail ceases
• Icing risk decreases (still light to moderate, resulting in rough ice)
• Lightning occurs sporadically
• The thunderstorm cell's horizontal extension is approximately 20 km
• A strong and large anvil has developed
• This phase lasts approximately 30 to 40 minutes

Hazards of Thunderstorms in Aviation

• Icing is strongest in the zone of mixed precipitation, even at temperatures below -10°C
• Hail is most frequent at altitudes between 4.5 and 7.5 km, and from overhanging cloud parts (anvil)

Recommended Distance Cloud Edge

• A recommended distance cloud edge from the aircraft during flights is: below zero-degree line 5 NM or above zero-degree line 10 NM

Turbulence

• Risk of turbulence is present in transition zones between upwind and downwind
• Turbulence eddies range from 15 to 150 m
• Fast aircraft flying through these areas quickly experience strong acceleration and material load
• Turbulence requires reduced speed (turbulence penetration speed)
• Turbulence can also occur outside the Cumulonimbus cloud (squall line, microburst)

Hazards of Thunderstorms Lightning and Electrical Charge

• Most lightning strikes happen between +3°C and -5°C
• Thermal impacts can damage the aircraft sensors due to lightning
• Passengers are protected by a Faraday cage
• Electric fields can magnetize ferromagnetic materials
• Flashes can cause strong dazzling effect (especially at night)
• Electrical charging of the aircraft can cause radio interference

Hazards of Thunderstorms Precipitation and Sensor Displays

• Heavy precipitation causes rapid and strong fluctuations in visibility
• Rapid alterations in ground pressure (pressure mass) and turbulence can lead to incorrect altitude readings
• Erroneous True Air Speed (TAS) readings occur when pitot tubes are blocked by ice

Aircraft Icing

• Aircraft icing is deposition of ice, frost, or snow on aircraft
• Occurs both airborne or parked

Aircraft Icing: Intensity

• Intensity is dependent on number, size and temperature of supercooled droplets as well as the aircraft's own speed plus duration of flight in an icing zone
• The pilot finds information about icing intensities in the Sign. WX - Charts
• The pilot finds information about icing in the Sign. WX - Charts

Aircraft Icing: Intensities Explained

• Trace Icing: Barely perceptible, no significant accumulation
• Light Icing: Significant accumulations for prolonged flight (more than 1 hour)
• Moderate Icing: Significant accumulations for shorter periods of flight
• Severe Icing: Rapid, dangerous accumulations

Types of Aircraft Icing: Frost

• Frost is the sublimation of water vapor on the aircraft when its temperature is below the air's frost point
• Frost appears as a white crystalline coating leading to a slight icing

Types of Aircraft Icing: Rime Ice

• Forms when small supercooled water droplets hit a surface and the density is less than 0.917 g/cm³
• Water droplets immediately freeze causing ice to adhere to parts of aircraft directly exposed to airflow
• Air inclusions give the rough ice a milky, opaque, easy to remove character

Types of Aircraft Icing: Rime Ice in Stratus Clouds

• Rime ice in stratus clouds only has slight vertical movements
• Formation: St and As and in the lowlands in the Ns with light to moderate intensity
• Temperature range is between -1°C and -15°C

Rime Ice in Cumulus Clouds

• Above the -15°C isotherm, strong updrafts ensure a large number of small drops even at these altitudes
• Moderate to strong intensity icing in Cb, Cu and Ac
• Light icing in the anvil of a Cb

Types of Aircraft Icing: Clear Ice

• Clear ice is caused by very large, slightly supercooled droplets that spread before freezing
• Icing also occurs in places on the aircraft that are not directly hit by the water droplets (gaps)
• Denser and more homogeneous than rime ice
• Adheres firmly and is glassy and hard
• Can occur in temperature ranges between 0°C and -6°C
• Formation disrupts airflow and causes significant drag (300 to 500%)

Types of Aircraft Icing: Clear Ice Stratus vs Cumulus Clouds

• Clear Ice Formation: Only if there are strong vertical movements in Stratus clouds

• Clear Ice Formation: Ns in mountain congestion or in area of fronts with moderate to strong intensity

• In Cumulus clouds: An extreme risk of icing exists

• IStrong updrafts carry large droplets up to high altitudes to trigger condensation quickly

• Exceptional cases occur at temperatures colder than -25°C with moderate to strong intensity

• Strongest intensity in the Cumulonimbus cloud in temperature range 0°C to -15°C

Aircraft Icing; Freezing Rain

• If snow falls from a layer with air temperatures below 0°C into a layer warmer than 0°C, melts into rain, and then falls into air with negative temperatures, freezing rain conditions occur

• Moderate to heavy icing often occurs with freezing rain to be similar to clear ice

• Freezing rain is particularly frequent on warm winter fronts and warm front occlusions

Aircraft Icing and Aerodynamic Heating

• Frictional heat is generated when air moves across the surface of an aircraft in flight, leading to temperature increase
• Icing zone shifts upwards into an area of lower temperatures
• Temperature increases are not the same across the aircraft but greatest on the surfaces exposed to the airflow
• Result varies in start of ice build-up
• Aircraft in cloud uses heat to evaporate cloud droplets, the aerodynamic heating in cloud air is less than in dry air

Aircraft icing on Fronts

• Frontal clouds are more prone to icing vs other clouds
• Strong lifting processes in frontal zones lead to the formation of dense clouds with an increased number of supercooled water droplets
• Convective rearrangements in fronts show high-reaching icing zones
• Approx 85% of observed icing occurs in the vicinity of frontal zones
• Icing zone on stable/unstable front

Effects Of Aircraft Icing On The Aircraft

• Icing causes changes in profile contour, weight increase, asymmetry and vibrations
• Blockage of flight control surfaces
• Loss of power to engines
• Failures of displays and radio equipment
• Loss of visibility (freezing of aircraft windows)

Effects of Aircraft Icing

• Buoyancy, thrust, rudder effect decrease
• Air resistance, weight, and stall speed increase

Technical Options: Aircraft Icing

• Technical options include pneumatic de-icing (rubber mats inflated on the aircraft), liquid de-icing (on the ground or in the air), electrical de-icing (heating), the use of hot turbine air

Explore the formation and behavior of different types of thunderstorms, including frontal, orographic, and heat thunderstorms. Understand how atmospheric conditions and geographical features influence their development and intensity. Learn about the unique characteristics of each type.