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1
Lecture 11

• Meteorology
• For
• Flying
• Chapters 6, Jeppesen Sanderson

2
Meteorology

• Basic weather theory
• The atmosphere
• Atmospheric circulation
• Weather patterns
• Atmospheric stability
• Moisture
• Clouds
• Weather hazards
• Thunderstorm
• Turbulence
• Wind shear

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The Atmosphere

• Weather is an important factor in flying
• Weather is what happens in the atmosphere
• The atmosphere is a very thin protective blanket
  surrounding the earth.
• 99 of it is within 30 kilometers (about 100,000
  feet) from the earths surface.
• When white sunlight passes through the atmosphere
  the light bends. Lights with short wavelength
  like blue, green and violet are bent more towards
  the earth. Since human eyes are more sensitive
  to blue also, thus the sky looks blue.

4
Atmosphere Levels

• The atmosphere are often classified into layers
  called spheres according to their differing
  temperatures.
• From the earths surface to about 36,000 feet is
  the troposphere.
• Above that is a thin level called the tropopause
  which acts a lid to confine most of the water
  vapor and the associated weather, to the
  troposphere.
• The layer above the tropopause which extends to
  about 160,000 feet and is similar in composition
  to the troposphere.
• There are two more layers above that, but they do
  not have much effects on weather.

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Composition of the Atmosphere

• The atmosphere is made up of about 78 nitrogen,
  21 oxygen, and 1 of other gases.
• It can also contain from almost 0 up to 4 of
  water vapor by volume.
• However, this relatively small percentage of
  water vapor is responsible for major changes in
  the weather.

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Atmospheric Circulation (1)

• Since the atmosphere is fixed to the earth by
  gravity and rotates with the earth, in
  equilibrium condition, there would have been no
  relative movement between the atmosphere and the
  earths surface.
• However, due to unequal temperatures at different
  parts of the earths surface this equilibrium is
  disturbed, producing relative movements between
  the air and the earths surface.
• This relative movement is called atmospheric
  circulation.

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Atmospheric Circulation (2) - Temperature

• As the earth rotates around the sun throughout
  the year, the length of time the earths surface
  receiving sunlight each day, and the angle at
  which sunlight shines on the surface varies a lot
  from one place to another. It also varies a lot
  at different times of the year.
• This variation in solar energy at different times
  and places is what causes the seasons.
• In general, however, surfaces around the equator
  receives most direct sunlight and surfaces near
  the poles receives the least.

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Atmospheric Circulation (4) - Convection

• When air is heated, it expands and thus becomes
  lighter (less dense) and rises up.
• Cooler air are more dense and sinks to the bottom
  and replaces the warmer, rising air.
• This circulation of air is called convection.
• If we simplify the convection situation, and
  ignore rotation of the earth, we will get a
  simple one-cell model like this- air near the
  equator would rise and spread while air from the
  poles would replace them, as schematically shown
  in Fig 6-5.

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Convection one-cell model (6-5)
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Atmospheric Circulation (5) - Convection

• A more realistic situation is represented by the
  3-cell model of convection.
• Again, hot air rises near the equator. But
  instead of spreading to the poles, it spread only
  to about 30? latitude, where it cools down enough
  and sink.
• When it spreads to about 60? latitude, it is warm
  when compared to the air there, and rises again.
  Thus a second cell is formed between 30? to 60?
  latitude.
• This rising air is replaced with polar cool air,
  forming the third cell (Fig 6-6)

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Convection 3-cell model (6-6)
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Atmospheric Pressure (1)

• Besides movement due to convection temperature
  differences also cause changes in pressure.
• In pressure maps, points of equal air pressure
  are connected to form isobars, that is, lines of
  equal pressures.
• Isobars are measured in millibars and are usually
  drawn at four-millibar intervals.
• The resulting pressure maps shows the pressure
  gradients, i.e. the change in pressure over
  distance.

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Atmospheric Pressure (2)

• Air moves from a high pressure area (higher
  density, cooler temperature) to a low pressure
  area (with lower density, warmer temperature).
• Where isobars are spread widely apart, pressure
  changes slowly. Air movement will be slow (with
  mild wind).
• Where isobars are closely packed, pressure
  changes quickly with distance. Air movement
  should be fast (strong wind).
• Isobars also help to identify the pressure
  systems, which are classified as highs, lows,
  ridges, troughs, and cols.

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Atmospheric Pressure (3)

• A high is a center of high pressure surrounded by
  lower pressure.
• A low is a center of low pressure surrounded by
  higher pressure.
• A ridge is an elongated area of high pressure.
• A trough is an elongated area of low pressure.
• A col can be a neutral area between two highs and
  two lows, or the intersection of a ridge and a
  trough. (Fig 6-7)

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A pressure map (6-7)
16
Moisture (1)

• Although even in tropical rain regions, moisture
  only account for a small percentage of the total
  volume of the atmosphere.
• However this small amount of water vapor is
  responsible for many flight hazards in aviation.
• In general, if the air is very moist, the weather
  can be poor or even severe.

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Moisture (2)

• Water is present in the atmosphere in three
  states- solid, liquid, and gas.
• Changes from one state to another happens
  readily, accompanied with exchange of heat.
• When liquid water evaporates and becomes water
  vapor a lot of heat is absorbed from the nearby
  atmosphere.
• Conversely when gaseous water vapor condenses
  into liquid form, a lot of heat is given out to
  the nearby atmosphere.

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Humidity

• At any fixed temperature there is a certain
  maximum amount of moisture that the air can hold.
  
• The lower the temperature, the less moisture the
  air can hold.
• Relative humidity is the actual amount of
  moisture in the air compared to the maximum
  amount it can hold, with its temperature
  remaining unchanged.
• The air is said to be saturated if it is already
  filled with the maximum amount of moisture it can
  hold (i.e., the relative humidity is 100).

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Dewpoint

• The temperature at which the amount of moisture
  in the air becomes saturated is called the
  dewpoint for that air.
• Further cooling of the air will cause the
  moisture to condense into liquid water droplets
  and clouds would form.
• When rising in altitude, unsaturated air cools
  down at about 5.4? F per 1000 feet.
• Dewpoint temperature drops at about 1? F per 1000
  feet.
• Thus after rising 1000 feet the air is closer to
  its dewpoint by (5.4 1) 4.4? F .

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Dewpoint and cloud forming

• If the air keeps rising, eventually, its
  temperature will equal to its dewpoint. In that
  condition clouds often form.
• Knowing the dewpoint at a certain altitude, one
  can estimate at what altitude clouds are likely
  to start forming.
• For example, on the runway if the temperature is
  80?F and the dewpoint is 62? F, the difference is
  18? F. At 1000 feet higher the difference will
  be 4.4? F less. Thus at (18 / 4.4) 1000 feet
  4090 feet, the difference will be zero
  (temperature will equal dewpoint), and clouds
  will start to form.

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Dew and Frost

• If air is cooled by a surface below its dewpoint
  it will condense on the surface in the form of
  dew.
• If the temperature of the surface is not only
  below the dewpoint but also below freezing point
  (0? C), water vapor will change directly to ice
  on the surface and becomes frost.
• In very cold nights frost might be formed on your
  airplane.

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Clouds (1)

• Clouds are composed of very small droplets of
  water or ice crystals attached onto very tiny
  particles in the air like dust or products of
  combustion.
• When clouds form near the surface of the earth
  they are called fog.
• Clouds and fog usually form when the air become
  saturated with moisture (temperature of air near
  or equal to dewpoint).

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Clouds (2)

• Clouds are of four types
• 1. Low clouds
• near the earths surface below 6500 feet
• Consist almost entirely of water but sometimes
  contain supercooled water which can create icing
  on aircrafts
• Also include fog which are within 50 feet from
  the ground
• 2. Middle clouds
• Form 6500 to 20,000 feet
• Composed of water, ice crystals, or supercooled
  water, and many contains moderate turbulence.

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Clouds (3)

• 3. High clouds
• Above 20,000 feet
• Generally white to light gray in color
• Composed mainly of ice crystals and seldom pose a
  serious turbulence or icing hazard
• 4. Clouds with vertical development
• Long vertical clouds that have the base at
  altitudes of low to middle clouds, and their tops
  extend to the altitudes of high clouds.
• They often contain turbulence and can cause
  thunderstorms

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Precipitation

• Precipitation can be defined as any form of
  particles, whether liquid or solid, that fall
  from the atmosphere.
• For precipitation to occur, water or ice
  particles must grow in size until they cannot be
  supported by the atmosphere.
• Precipitation can affect visibility, affect
  engine performance, increase braking distance,
  freeze on wings and control systems, and cause
  dramatic shift in wind direction and/or velocity.

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Weather that Affects Aviation

• Six types of weather can seriously affect
  aviation
• 1. Thunderstorms
• 2. Turbulence
• 3. Wind Shear
• 4. Icing
• 5. Restriction to visibility
• 6. Volcanic ash

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Thunderstorms (1)

• Thunderstorms are probably the single greatest
  threat to aircraft operations.
• They may contain strong wind gusts, icing, hail,
  driving rain, lightning, and sometimes tornadoes.
• Before a thunderstorm can develop, three
  conditions must be present
• Air that tends to be unstable
• Some type of lifting action
• Relatively high moisture content

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Thunderstorms (2)

• Formation of a thunderstorm can be divided into
  three stages.
• In the cumulus stage, a lifting action initiate
  the vertical movement of the air upwards.
• As the air rises and cools to its dewpoint vapor
  condenses to small water droplets or ice
  crystals.
• Because of this strong updraft (3000 feet per
  minute) these droplets or crystals do not fall,
  just keep rising and falling within the cloud,
  growing larger with each cycle.
• The cloud grows to 20,000 feet in height and 3-5
  miles in diameter. It reaches the mature stage
  in about 15 minutes.

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Thunderstorms (3)

• As the cloud becomes too large for the updraft to
  support, precipitation begins to fall, creating a
  downward motion in the surrounding air.
• The updraft of warmer air and the downdraft of
  precipitation creates a violent circulation and
  severe turbulence.
• Near the ground, the down rushing air and
  precipitation spreads outwards, producing a sharp
  drop in temperature, a rise in pressure and
  strong, gusty surface wind. This is the mature
  stage of the thunderstorm formation. (Fig 6-41)

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Mature stage of the thunderstorms (6-41)
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Thunderstorms (4)

• Fifteen to thirty minutes after the mature stage
  is reached, the thunderstorm reaches the
  dissipating stage.
• More and more precipitation drops with downdraft,
  eventually spreading out within the cell, taking
  the place of the weakening updrafts.
• When the updraft slows down the process of
  uprising and condensation also slows. The entire
  thunderstorm begins to weaken. Fig 6-42 shows
  the schematic diagram of the three stages.

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Schematics of thunderstorm formation (6-42)
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Thunderstorms (5)

• A thunderstorm may exist as a single-cell, a
  super-cell, or a multicell.
• A single-cell thunderstorm lasts less than one
  hour.
• A super-cell severe thunderstorm may last for two
  hours.
• A multicell storm is a cluster of thunderstorms
  at different stages of development, and thus can
  last much longer than a single or super-cell.

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Thunderstorms (6)

• In some cases, thunderstorm may form in a line in
  the atmosphere, called a squall line.
• This continuous line of thunderstorms can range
  in distance from about one hundred to several
  hundred miles in length.
• The thunderstorms along a squall line may consist
  of super-cell or multicell thunderstorms
• The most severe weather conditions are usually
  associated with squall lines. (Fig 6-37)

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Squall line (6-37)
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Turbulence (1)

• Turbulence are very strong winds that have
  changing directions and/or speed within a short
  distance.
• There are many causes for turbulence
• Thunderstorm
• Strong winds going against obstructions
• Cold wind heated by hot earth surface
• Vortices generated at wingtips of large, fast
  airplanes (Fig 6-49)
• Strong jet engines (Fig 6-50)
• Interface between two layers of wind with
  different wind speeds

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Turbulence due to wing vortex (6-49)
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Jet engine blast (6-50)
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Turbulence (2)

• The effects of turbulence can vary from light
  bumps to severe shocks which can cause personal
  injury and/or structural damage to the airplane.
• If you get into a turbulence during flight, slow
  down the plane, maintain a level flight attitude,
  and accept variations in altitude.
• If you encounter a turbulence during approach for
  landing, consider using a higher than normal
  landing speed.

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Wind Shear (1)

• Wind shear is a sudden, drastic shift in wind
  speed and/or direction that may occur at any
  altitude in a vertical or horizontal plane.
• It can cause your airplane to sudden updrafts,
  downdrafts, or extreme horizontal wind. These
  results in loss of lift or violent changes in
  vertical speeds or altitudes.
• Wind shear can be caused by precipitation, or a
  jet stream, or a moving cold air front. It can
  also occur in low-level temperature inversion
  when cold, still surface air is covered above by
  warmer wind.

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Wind Shear (2)

• One of the most dangerous wind shear (called a
  microburst) which normally occur within a
  vertical distance of less than 1000 feet, and
  usually lasts only 15 minutes or less.
• The downdraft in a microburst can be as strong as
  6,000 feet per minute.
• The average headwind increase in a microburst is
  45 knots, although increase of over 100 knots are
  possible

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Wind Shear (3)

• It is very dangerous for a plane at low altitude
  (e.g., landing) to encounter a microburst since
  the plane will experience a strong headwind that
  tends to increase its lift.
• Suddenly the plane would experience a down draft,
  followed by a strong tail wind that tends to
  decrease lift.
• This may result in uncontrollable descent and
  possible impact with the ground. (Study Fig 6-54
  for details)

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Microburst Wind Shear (6-54)
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Wind Shear (4)

• To try to protect planes from encountering wind
  shear during arrival or departure, airports often
  install wind shear monitoring systems (e.g., the
  low- level wind shear alert system) .
• If the possibility of a wind shear occur around
  the airport, air traffic controllers will inform
  arriving and departing planes.

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Icing (1)

• Ice can build up on any surface of the aircraft
  during flights in areas with moisture when the
  surface of the aircraft is 0? or colder.
• Ice on the surface of the plane can affect the
  plane in several ways
• Reduced thrust
• Increased drag
• Increased weight
• Decreased lift
• These effects combine to increase stall speed

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Icing (2)

• In severe cases, it takes as little as 5 minutes
  for 2 to 3 inches of ice to accumulate on the
  leading edges of the airfoil.
• Some aircraft may experience a 50 decrease of
  lift after only ½ inch of accumulated ice.

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Weather Restrictions to Visibility (1)

• Particles that can absorb, scatter, and reflect
  light are always present in the atmosphere,
  resulting in reductions in visibility.
• The most common conditions that reduce visibility
  are
• Haze
• Smoke
• Smog
• dust

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Weather Restrictions to Visibility (2)

• Haze is caused by a concentration of very tiny
  dry particles (e.g., salt, fine dust).
  Individually they cannot be seen by the naked
  eye, but can restrict your visibility when
  present in large amount.
• A layer of haze is usually just a few thousand
  feet thick. Visibility above the haze layer is
  usually good.

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Weather Restrictions to Visibility (3)

• Smoke is the combustion particles suspended in
  the air. Its restrictive effects on visibility
  depends on its concentration. The sky looks red
  or orange, but when smoke travels over a distance
  of 25 miles or more, the large particles drops
  leaving only the fine ones, and the sky would
  look gray or blue.
• Smog, which is a combination of smoke and fog (a
  low cloud with water attaching onto particles),
  can cause poor visibility that spread over a
  large area.

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Weather Restrictions to Visibility (4)

• Dust refers to fine particles of soil suspended
  in the air.
• When the soil is loose, the winds are strong,
  dust may be blown for hundreds of miles.

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Volcanic Ash (1)

• The ash cloud from an erupted volcano can spread
  over a very large area over the world and remains
  in the atmosphere for months or longer.
• Volcanic ash consists of gases, dust, and ash.
• Due to its extremely abrasive characteristics,
  volcanic ash can damage aircraft windscreens.

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Volcanic Ash (2)

• Under severe conditions the ash can clog the air
  ventilation systems and damage aircraft control
  surfaces.
• Power loss can happen, especially for jet
  aircraft, because the air taken in with the ash
  can damage the engine.
• If you entered an ash cloud you should not
  attempt to fly straight through or climb out as
  ash clouds can be hundreds of miles wide and
  extend to great heights.
• You should reduce power if possible and turns
  back to escape the cloud.