Thunderstorms
TS Facts
• It's estimated at any given moment nearly 2,000
thunderstorms are in progress over the Earth's surface.
• Lightning strikes the earth 100 times each second.
• There are about 45,000 thunderstorms daily and 16
million annually around the world.
• There are at least 100,000 thunderstorms annually across
the United States.
TS Facts
• In mid-latitudes, thunderstorms are;
– most frequent over land in summer.
– most frequently of frontal type in winter.
• In general movement is in the direction of the 10,000 ft.
(700mb) wind.
Conditions for Thunderstorm Development
• Conditional instability through a deep layer well
above the freezing level.
• Adequate supply of moist air from below.
Conditions for Thunderstorm Development
• Trigger Action:
–
–
–
–
–
Surface heating - convection.
Orographic lifting.
Frontal lifting
Lifting due to convergence in a trough or low.
Differential advection - moist warm air undercutting
colder air aloft.
Types of Thunderstorm
• Airmass Thunderstorm.
– Usually triggered by surface heating such as;
• warm sea surface in winter or
• hot land surface in summer.
• Frontal or Line Type Thunderstorm
– Usually occur along the line of the cold front
embedded in layer cloud associated with the front.
Types of Thunderstorm
• Orographic
– forced ascent of air by the high ground rapidly
destabilises the air generating convective cells.
– Cells are frequently obscured by layer cloud.
Cellular Structure
(Plan view)
Multi-Cell Thunderstorm
Cumulus or Building Phase.
• Begins when one or
more CU clouds
begin to grow into a
towering CU.
• Typical vertical
velocities are
frequently in excess
of 3000 fpm and can
exceed 5000 fpm in
severe cases.
Cumulus or Building Phase.
• Average life span is
15 to 20 minutes.
• No precipitation at
this stage due to cell
consisting mainly of
updraughts.
Mature Stage
• Mature stage begins with onset of first downdraughts indicated by rain shafts.
• Cloud now extends well above the freezing level.
• Tops of the cloud are now glaciated (contain mostly
ice crystals).
• Down draughts associated with increasing volume
of precipitation accelerate and produce gust fronts
spreading out from the base of the storm.
Mature Stage
• Mature stage last about 15 - 20 mins and maximum
vertical development is reached.
Dissipating Phase
• This is reached when
the local supply of
moist air is used up.
• Anvil top begins to
spread out.
Dissipating Phase
• Core consists mostly
of down-draughts
which kill off the updraughts and rain
spreads right across the
base of the storm
towards the end.
• The cloud structure
now tends to stratify
and spread out.
Dissipating Phase
• An individual storm
cell may last for less
than 1 hour but a
system may persist for
several hours.
Thunderstorm Development
CB with Anvil - Dissipating Phase
Mature & Dissipating Phases
Flight Hazards
•
•
•
•
•
•
•
•
•
Turbulence
Icing
Heavy Rain
Hail
Low Level Windshear
Tornadoes
Lightning
Static Electricity
Instrument Errors
Turbulence
• Up/down draughts can vary from 1000 fpm to in
excess of 5000 fpm.
• Aircraft may experience high rates of roll and pitch
and can experience vertical displacements of as
much as 5000 feet.
• Severe turbulence may be found 15 to 20 miles
downwind of severe storm core.
• Turbulence by be found up to 5000ft above the tops
of CB cells.
Thunderstorm Turbulence
Super-cell Thunderstorm
Main cause of supercell thunderstorms is
pronounced wind
shear with height.
Airframe Icing
• Icing range is from 0º C to as low as -45º C
• Clear icing may be encountered down to -30ºC.
• Inlet anti-ice should always be on prior to any
icing encounter, igniters may be needed as well.
• Ice ingestion can severely damage axial flow gas
turbines.
• Piston engines can suffer from induction system
icing.
• Ice accretion leads to a serious loss of
aerodynamic efficiency.
Heavy rain
• Ingestion of large quantities of water may cause
flameout and possible damage in gas-turbine
engines.
• Severe updraughts as well as water may induce
compressor stalling.
• Indicated by loud bangs from the engines.
• Igniters should always be on.
• Thrust changes should be avoided.
• Use approved turbulent air penetration procedures.
• Liquid water may exist as low as -30ºC.
Hail
• Hail outside of cloud may not show up on
weather radar.
• No reliable method to predict hail formation.
• Severe hail (4") can be encountered up to
10,000 ft.
• Damaging hail up to 45,000 ft.
• Hail may fall from overhang of anvil into clear
air so this zone should be avoided.
Hail
• Severe structural damage can result from hail
encounters.
• No known aircraft accident has yet been
attributed to hail.
Typical Hail
Damage
‘CHIRP’ Feedback Reports
• EVENT ONE
• Prior to departure Cb's were observed north and
west of the field. The SID from RWY 29 calls for
a left turn at 2 DME to intercept the XXX VOR
(on the field) 166°R until 12 DME, then turn left
onto a north-westerly track.
The crew believed that this SID would keep them
clear of the Cb's and a suitable northbound track
could then be requested.
‘CHIRP’ Feedback Reports, cont’d
During the left turn after the 2 DME point, ATC
gave a radar heading of 250°. The flight then
passed through a Cb with intense hail. The noise
was so loud they couldn't hear and an ATC
instruction to maintain FL 120 on reaching was
missed.
Both radio altimeters decreased to zero
(presumably due to the intensity of the hail) and
the GPWS 'PULL UP' terrain warning activated.
‘CHIRP’ Feedback Reports, cont’d
The crew responded correctly by increasing
attitude and climb rate rapidly and turning left
onto heading 210° away from high ground. At FL
120 the aircraft was struck by lightning.
During turns, the radar picture may be difficult to
interpret. Intense hail doesn't always show red on
the radar, which prefers water.
It would be helpful if ATC did not vector aircraft
into Cb's!
Gust Front Turbulence
Low Level Windshear
• Rapid changes in wind speed and direction.
• Microbursts or strong downdraughts can produce
catasrophic loss of height during take-off and
landing. Microbursts may be wet or dry.
• Typical microbursts are less than 5km across lasting
1 - 5 minutes with vertical speeds as high as 60 kt.
Low Level Windshear
• Gust front may extend in excess of 20 km from
storm centre and up to 6000 ft above the surface.
• Cold outflow from down draught may produce wind
speed changes as much as 80 kt and direction
changes of 90 or more.
Microburst/Downburst
Microburst Wind Shear
Lightning
Lightning
• Can occur within and in clear air adjacent to
a thunderstorm.
• Most recorded lightning strikes occur
between +10ºC and -10ºC (i.e. within 5000
ft above or below the freezing level.)
• Strikes may cause serious errors in
magnetic compasses and remote magnetic
sensors.
• They should not be relied upon after a strike
and should be checked as soon as possible.
Static Electricity
• Seriously affects reception on VLF, MF and
HF frequencies.
• ADF tends to point to nearest thunderstorm.
• Static electricity frequently indicated by a
discharge of glowing ionised air particles
called St Elmo’s fire.
• No effect on VHF and UHF frequencies.
Global Electrical Current
Electrical Charge Distribution
Instrument Errors
• Local pressure variations occur in or very close to
TS.
• Altimeters may be in error by as much as 1000 ft.
• Heavy rain may also give false indication on air
speed indicators even if pitot heat is on.
Instrument Errors
• Air speed indicator may be subjected to violent
short term oscillations.
• Always maintain a level attitude.
• Use turbulent air penetration speed.
• A/P Altitude hold should be disengaged and
attitude hold engaged.
Tornado
Tornadoes
• Threat of tornadoes is always present with severe
thunderstorms.
• Occur frequently in USA east of Rocky Mts, and in
the central plains Mississippi, UK and other parts of
the world.
• Average ocurrence in UK is about 30 days a year
although not as violent as USA.
• Vortex may extend right from surface to upper
reaches of cloud.
• Speed in vortex may exceed 200 kt.
Tornado
Thunderstorm Avoidance.
• Radar returns are attenuated by intervening heavy
rain.
• Wx radar detects steep rainfall gradients which are
the most likely zones for hail.
• Areas giving strong radar returns should be avoided.
• Larger cells may be concealed behind nearest
returns.
• Overflying building cells can lead to unexpected
encounters due to rapid vertical growth.
Weather Radar Guidance
• If visual avoid any cells that by visual
inspection are tall, growing rapidly or have
anvil tops by at least 10 miles.
• In IMC
– Up to 25,000 ft avoid echoes by 10 miles.
– 25,000 to 30,000 ft avoid echoes by 15 miles.
– Above 30,000 ft avoid echoes by 20 miles.
Stormscope
Stormscope with Nav Display
Squall or instability lines
• Continuous or broken line of non-frontal TS
• Can be 100 + miles long
• Can be a mix of multi-cell or supercell
storms
• Frequently develops ahead of a cold front
Squall or instability lines
• Moves in direction of 500 hPa wind
• Associated with roll cloud + severe
turbulence
• In the USA a ‘dry line’ can produce severe
supercell TS with tornadoes
Summary of Thunderstorms
• Thunderstorms are composed of several cells.
• They move with the general tropospheric flow at
700 mb but develop small violent circulations
around themselves.
Summary of Thunderstorms, cont’d
• The cells go through three stages:
• The Building Stage
– up draught throughout(avr +3000fpm max
+5000fpm).
Summary of Thunderstorms, cont’d
• The Mature Stage
– rain starts,
– down draughts develop(in severe eases they are
a called micro-bursts)
– hail can occur,
– lightning occurs,
– gust fronts develop from the down draught,
– tornadoes can occur,
Summary of Thunderstorms,cont’d
• The Dissipating Stage
– down draughts throughout.
Thunderstorm Hazards