Download Lesson 9: Meteorology III

Lesson Nine: Meteorology III

Don’t read:
o St. Elmo’s Fire.
o Lightning Detection Equipment.
o Weather Radar
o The entire section on High Level Weather
o In the Weather Information section, focus only on material related to TAFs and
METARs. Skim over GFAs. Quite frankly, this entire section SHOULD be read
because you’ll need to know about this stuff sooner or later, but for the entrance
exam just focus on TAFs and METARs.

Thunderstorms
o Cumulus cloud on steroids (cumulonimbus)
o About 1800 thunderstorms in progress around the world at any given time
o Requirements: unstable air, some form of lifting action, and a high moisture
content
o If a mass of superheated moist air rises rapidly, an equal amount of cooler air
rushes down to replace it (this leads to huge downdrafts and updrafts in a
thunderstorm)
o Area in which rising and descending currents are active is called a thunderstorm
cell
o Three distinct stages of formation
 Cumulus
 Towering cumulus starts growing upward, driven by release of
latent heat
 Temperature within cell are higher than temperatures at the
same level in the surrounding air, leads to further instability and
convective currents
 No precipitation yet, droplets and crystals are still being drawn
upward
 Mature
 Fully-developed cumulonimbus, tops as high as 60000’ (above
tropopause)
 Updrafts 6000’/min., downdrafts 2000’/min.
o Water droplets falling down drags air down with them
 Lasts for 15-20 min.
 Precip., tornadoes and downbursts can occur during the mature
stage

Dissipation
 Cool precip tends to cool lower region of cloud and cell loses
energy
 Downdraft spreads throughout the whole area of the cell, except
within a small portion near the top where updrafts may still occur
 Rainfall gradually ceases
 Formation of an anvil
o Two main types
 Air mass thunderstorms
 Usually form singly or in clusters on hot summer days in warm
moist air
 Typically scattered, so easily spotted and avoided
 Form as a result of convection or orographic lift
 Frontal thunderstorms
 Associated with advancing cold fronts, but can develop in highly
unstable air in warm fronts
 Forms a line that may extend for hundreds of miles
 Often embedded in cloud decks, so hard to spot
 May be associated with a squall line
o Weather
 Gust front
 Downdrafts hit ground during the mature stage, and the air
spreads out in front of the thunderstorm
 Generates strong, gusty winds near the surface
 Downburst
 Severe, sudden downward rush of air and its outburst of
damaging winds
 Macroburst is a large downburst with a diameter of 2 n.m. or
more when it reaches the earth’s surface, winds lasting 5-20 min.
 Microburst is a smaller downburst with a diameter of less than 2
n.m. and peak winds lasting less than 5 min.
o Can reach speeds of 6000’/min.
o Outflow can reach speeds as high as 80 knots
 Downward flowing column of air often contains precipitation,
which if the precip. evaporates in mid-air due to reaching warmer
air below results in virga
o Evaporation of precip. aloft tends to cool the air, which
can intensify downbursts

Lightning
 Discharge of electricity produced in a thunderstorm
 Charge build up associated with graupel (i.e. ice particles/soft hail)
rubbing off on each other in updrafts/downdrafts, leading to
transfer of electrons (thereby breaking up electrical neutrality)
 Positive charge usually collects in the upper portion of the cloud,
negative charge at cloud base
 Cloud-to-cloud, cloud-to-air, cloud-to-ground can occur
 Hail
 Especially dangerous to aviation
 Usually occurs during the mature stage of cells having updrafts of
more than average intensity
 Icing
 Encountered at or above the freezing level, in areas of heaviest
turbulence during the mature stage of the storm
 Altitudes within a few thousand feet of the fz level (above or
below) are especially dangerous, due to risk of coming into
contact with supercooled droplets
 Pressure
 Falls abruptly ahead of a TS as the storm approaches, rises quickly
when the rain comes, and returns to normal when the storm
subsides
 Can happen in a matter of 10-15 minutes
o Dangers
 Turbulence
 Vertical currents can displace an aircraft up or down vertically as
much as 2000-6000’
 Gust loads can stall an aircraft flying at maneuvering speed, or
cripple the plane flying near VNO
 Turbulence can extend 20 miles from severe storms, and will be
greater downwind than into wind
 Downbursts kill (duh)
 Lightning
 Static can build on airframe, can interfere with operations of the
radio
 “Lightning blindness”
 Probability of lightning strike greatest when temp is between -5oC
and 5oC; can result in structural damage, or fry circuitry of
avionics

Hail




Typically encountered between 10000-30000’ within a
thunderstorm
Can be encountered outside of a thunderstorm if updrafts throw
them out of the cloud
Icing
Pressure
 Can screw with altimeter setting
 Wind
 Can result in crazy loads
 Wind shear can stall planes/result in structural damage
 Tornadoes are rotating columns of air; localized, highly intense
lows, 200 knots or more
 Rain
 Can cause structural damage to planes flying at high speed
through a storm
 Contamination of airfoils  can lead to higher stalling speeds
 Reduces visibility
o Can cause refraction on windscreen
 Can cause hydroplaning on the ground
o Avoidance
 Don’t fly through a thunderstorm. Just don’t.
 Detour around them as early as possible.
 Avoid landing or T/O at an airport near an approaching thunderstorm or
squall line.
 Do not fly under a TS, since severe turbulence/downbursts may be
present.
 Reduce airspeed to maneuvering speed when in vicinity, or at first
indication of turbulence.
 When flying around a TS, fly to the right side of it (since winds circulate
counter-clockwise around a low, you want a tail-wind to get out of there
as quickly as possible)
 If you HAVE to fly through a thunderstorm
 Go straight through a front, not across it
 Hold as steady of a heading as possible
 Reduce speed to VA before entering the storm
 Turn cockpit lights full bright (to counter lightning blindness)
 Maintain constant attitude and power setting
 Avoid unnecessary maneuvering



Determine freezing level and AVOID it. Also avoid dark areas of
the cell
Don’t use autopilot.
Icing
o While flying at a level where the temp is at or below freezing, an airplane can
encounter supercooled drops which freeze and stick to the plane.
o Can occur in cloud, freezing rain or freezing drizzle
o Most common in stratocumulus in winter, but the heaviest deposits are found in
thunderstorms
o Clouds made of ice crystals (like cirrus) don’t pose a threat, because they’re
already frozen and won’t stick to the wings
o At lower temps, the drops would hit the surface and freeze right away, thus
won’t have much time to spread out.
o At higher temps, the drops spend more time spreading out first before freezing,
thus can affect a greater area.
o Rate of catch is the amount of water intercepted by an aircraft in a given time;
increases as airspeed increases
o Thin wings catch more droplets than thick wings
o Effects
 First structures to accumulate ice are the surfaces with thin leading
edges: antennas, props, horizontal stabilizer, etc. Wings normally last
structural component to collect ice
 Frost on windshield can affect visibility
 Icing on props results in slow loss of power and gradual onset of engine
roughness
 Would accumulate on the wings, leading to higher stalling speeds,
greater drag as well
 Buildup on pitot-static sources can affect flight instruments
 Gyroscopic instruments powered by venture would be affected by ice
building up in the venture
 Can impede communications if ice collects on antennas
o Tail plane stall
 Narrow camber/small radius of curvature of the LE of the horizontal stab
makes the tail plane susceptible to ice accretion
 Has an ice collection efficiency twice/three times that of the wings
 Principal danger encountered during approach and landing when flaps or
extended
 Most horizontal stabs have an AoI to provide negative lift to balance the
lift of the wings

During approach and landing, tail is already at high AoA; extension of the
flaps causes a major change in the downwash on the tail and a further
increase in the angle of attack on the tail plane
 If ice accumulates on the tail plane, it’s likely the tail plane would stall
 Buffeting, reduction in elevator effectiveness, all indicate a tail plane
stall; recommend using lower flap settings, and be wary of power settings
which may increase the downwash on the tail
o Types of icing
 Clear ice
 Heavy coating of glassy ice which forms when flying in dense
clouds or freezing rain
 Spreads unevenly over wing/tail surfaces
 Forms when only a small part of the supercooled droplet freezes
on impact; lots of time for it to spread out before finally freezing
 Solid sheet of ice forms, with no embedded ice bubbles, hence it
is strong and durable
 Builds up into a single or double horn shape that projects ahead
of the surface on which it is collecting
 Can increase drag as much as 300-500%
 Results in loss of lift, increase in drag, increase in weight, vibration
caused by unequal loading on the wings and on the prop blades
 Often occurs in 0oC to -10oC temp. range and often found in
cumulus clouds and unstable conditions
 Rime ice
 Opaque, milky white deposit of ice is known as rime
 Accumulates on leading edges of surfaces; formed by the
supercooled drops freezing instantly upon impact
 Associated with a low rate of catch of small supercooled droplets,
encountered when flying through filmy stratiform clouds
 -10oC to -20oC
 Deposit has no great weight, and is typically brittle
 Can still disrupt lift, and increase drag
 Can form concurrently with clear ice to form mixed icing
 Frost
 White, semi-crystalline (feathery) structure which covers the
surface of an airplane in clear air by deposition
 Little or no effect on flying, but can obscure vision and interfere
with radio equipment

Frost forming on wings/tails/control surfaces must be removed
prior to T/O, because it can still contribute to poorer aerodynamic
characteristics
 Frozen dew formed on aircraft parked outside on a night when
temps are just below freezing
o Condensation on cold airplane surface before freezing
o Usually clear and somewhat crystalline
o Intensity of icing
 Can be described as light, moderate and severe/heavy
 Dependent on type of aircraft and de-icing and anti-icing equipment
available
 Clear ice > rime ice > frost
o Icing in clouds and in precipitation
 Cumulus
 Found in supercooled region
 Typically associated with thunderstorms, although horizontal
extent is narrow, so exposure is limited
 Stratus
 Less severe than in cumuliform clouds, but can be serious if cloud
has high water content
 Extensive horizontal, so long periods of exposure
 More severe if cumulus is embedded
 Freezing rain
 Common ahead of warm fronts in winter
 High rate of catch, since raindrops are bigger than cloud droplets
 Freezing drizzle
 Fall from stratus clouds with high water content, decrease in size
as they fall due to evaporation
 Usually maximum just below the cloud base where drops are
largest
 Clear ice
 Snow and ice crystals
 Typically little risk of icing, unless the airplane’s surface is warm;
in which case the crystals can melt, spread out and refreeze
o Protection
 Anti-icers prevent formation of ice, while de-icers remove ice after it has
formed
 Fluids released through porous leading edge members flow over props
and wings, used as an anti-icing device since it makes it difficult for ice to
form

Rubber boots are attached to leading edges, can be made to pulsate so
that ice is cracked after formation. De-icing.
 Heating devices
o Avoidance
 Most single-engine planes have very little ice prevention devices
 When ice formation is observed in flight, make sure to get out of there as
quickly as possible
 Easily done by performing a 180o turn to fly back out
 If accumulation is serious, may be necessary to do a precautionary
landing
 Decision must be made quickly, because once ice starts to form the
condition because critical within 6 minutes

Turbulence
o Irregular motion of air resulting from eddies and vertical currents
o Classified as light, moderate, severe or extreme
o Causes
 Mechanical Turbulence
 Friction between the air and the ground.
 Can be expected on the windward side and over the crests of
mountains and hills if the air is unstable
 Thermal Turbulence
 Sun heats the Earth’s surface unevenly
 Barren ground, rock and sandy areas are heated more rapidly
than grass-covered fields and water
 Isolated convective currents can cause turbulence (look at
diagram in book; really important!)
 Frontal Turbulence
 Lifting of warm air by the sloping frontal surface and friction
between the two opposing air masses produce turbulence in the
frontal zone
 Turbulence most marked when warm air is moist and unstable,
and will be extremely severe if TS develop
 Most commonly associated with cold fronts
 Wind Shear
 Any marked changes in wind with height produce local areas of
turbulence

Read Weather Signs

Aviation Routine Weather Report (METAR)
o Observations typically taken and disseminated on the hour
o SPECI is a special aviation report, when weather changes of significance to
aviation are observed, released independently of the hourly schedule
o Codes generated electronically.
o See book for codes
o Runway visual range reported if prevailing visibility is 1 mile or less and/or the
runway visual range is 6000’ or less
o Vicinity modifier (VC) used when phenomena observed within 8 km (5 s.m.) of
aerodrome but not at the aerodrome
o Cloud heights are AGL
o PRACTICE PRACTICE PRACTICE
METAR CYVR 060000Z 27003KT 20SM FEW035 SCT240 18/15 A3010 RMK SC1CI3
SLP194 DENSITY ALT 100FT=
METAR at Vancouver airport, issued on the 6th day of the month at 0000Z. Winds from 270 at 3
knots. 20 statute miles of visibility. Few clouds at 3500’, scattered clouds at 24000’.
Temperature is 18 degrees, dewpoint is 15 degrees. Altimeter setting 30.10’’ Hg. Remarks:
stratocumulus 1 okta, cirrus 3 oktas (given in order of altitudes). Sea level pressure of 1019.4
hectopascals. Density altitude 100’.
(for SLP, find the closest to 1013 hPa, and place the decimal in the right place. So SLP992 would
mean 999.2 hPa, whereas SLP172 would mean 1017.2 hPa.)
METAR CYBL 060000Z VRB02KT 20SM SCT027 BKN060 BKN080 BKN240 18/15
A3010 RMK SC4SC1AC1CI1 SLP195 DENSITY ALT 600FT=
METAR at Campbell River airport, issued on the 6th day at 0000Z. Variable 2 knot winds, 20
statute miles of visibility. Scattered clouds at 2700’, broken at 6000’, broken at 8000’, broken at
24000’. Temperature 18, dewpoint 15. Altimeter setting of 30.10’’Hg. Remarks: stratocumulus 4
oktas, stratocumulus 1 okta, altocumulus 1 okta, cirrus 1 okta. Sea level pressure 1019.5 hPa.
Density altitude 600’.

Aerodrome Forecast (TAF)
o Uses same code as METARs
o Wind, LLWS, prevailing visibility and significant weather given for an area within
5 n.m. of the aerodrome
o Issued no later than 20 minutes prior to period of coverage
o Issued at least four times daily at 0440, 1040, 1640 and 2240 UTC
o Normal validity period is 24 hours, beginning at 0500 Z, 1100Z, 1700Z, 2300Z
o 30-hour TAFs issued for some ICAO aerodromes.
o Valid from time of issue until it is amended (AMD) or until the next scheduled
TAF is issued. When a new TAF is issued, it automatically cancels the previous
one.
o Issued only for those aerodromes for which routine hourly and special reports
are available.
o Vis. greater than 6 s.m. is coded as P6SM
o Significant weather
 TEMPO = phenomena occur over a period of <1hr at a time, < 50% of
period indicated, and is not permanent
 FM = permanent change after a defined time
 BECOMG = permanent, but gradual, change
 PROB = listed phenomena are considered significant weather hazards, or
when probabilities are less than 50%
 only elements that are changing are included
TAF CYVR 052338Z 0600/0706 27005KT P6SM SCT040 SCT280
FM060600 VRB03KT P6SM SCT030 TEMPO 0606/0706 BKN030
BECMG 0618/0620 27008KT
RMK NXT FCST BY 060300Z=
Forecast for Vancouver airport, issued on the fifth day at 2338Z. Valid from the 6 th day at 0000Z
to the 7th day at 0600Z (30 hours!). Winds from 270 at 5 knots, plus 6 statute miles visibility.
Scattered at 4000’, scattered at 28000’. From the 6th day at 0600Z, winds become variable at 3
knots. Plus 6 statute miles visibility. Scattered at 3000’. Temporarily between the 6 th day at
0600Z and the 7th day at 0600Z the skies become broken at 3000’. Becoming between the 6th
day at 1800Z and the 6th day at 2000Z, winds changing to 270 at 8 knots. Remarks next forecast
by the 6th day at 0300Z.
Are the winds at CYVR stronger or weaker than predicted at the time of the METAR?