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Weather Theory

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Altitude that cloud bases form depends on moisture content & its dewpoint ... Severe WX can occur in early part of occlusion b/c unstable air is forced up (short) ... – PowerPoint PPT presentation

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Title: Weather Theory


1
Weather Theory
  • Aviation 310

2
The Atmosphere
  • Divided into layers based on temperature
  • troposphere
  • stratosphere
  • mesosphere
  • thermosphere
  • Most flying occurs in tropospherejets may cruise
    in stratosphere
  • tropopause is boundary b/w the two regions
  • 78 nitrogen, 21 oxygen, 1 other gases

3
Atmosphere
  • Temp decreases w/ gain in altitude in troposphere
    until reaching the tropopause where the temp is
    constant
  • The rate of change of temp. w/ altitude is called
    temperature lapse rate
  • Dry adiabatic lapse rate 3 deg/1000 ft
  • Wet adiabatic lapse rate 1.5 deg/1000 ft
  • Ambient lapse rate 2 deg/1000 ft

4
Troposphere
  • Most weather occurs in troposphere (I.e. clouds,
    precipitation, wind)
  • Tropopause is about 20,000 over the poles
    60,000 _at_ equator
  • It is higher in each region in summer than winter
  • Average is 36,000
  • Thunderstorm clouds can reach into the
    stratosphere if severe enough

5
Air Masses
  • Maritime is air over an oceanit absorbs moisture
  • Continental is over is air over landvery dry

6
Weather Theory
  • The cause of all weather is unequal heating of
    the Earths surface by the sun
  • Water requires more heat energy than land to
    effect a change in temperature
  • Therefore, oceans will heat and cool more slowly
    than landmasses

7
Atmospheric Circulation
  • The movement of air relative to earths surface
  • Due to unequal heating, warm, less dense air
    rises this air is then replace by colder, more
    dense air
  • Advection vertical movement of air
  • Convection horizontal movement of air

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10
Atmospheric Pressure
  • Standard 29.92 in of Mercury or 1013.2 millibars
    _at_ S.L. decreases 1 inch/ 1000 ft
  • Pressure changes due to unequal heating of earth
  • Pressure systems
  • Highs an area of pressure higher than standard
  • Ridge an extended area of high pressure
  • Lows an area of pressure lower than standard
  • Trough an extended area of lower pressure
  • Air flows from high pressure to low pressure

11
Coriolis Effect
  • Due to the rotation of the earth, winds are
    deflected to the right in the northern hemisphere
  • H L

12
Coriolis Force
  • In the Northern Hemisphere
  • Winds flow out of a high pressure system
    clockwise due to Coriolis force.
  • Winds flowing into a low will be rotating
    counter-clockwise due to Coriolis force.

13
Frictional Effects
  • Below 2,000 feet AGL surface friction will tend
    to reduce wind speed and Coriolis force allowing
    the winds to flow at an angle to the isobars
  • The angle that these winds flow at is
    approximately 30 degrees

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14
  • Therefore winds flowing around a high moves
    outward, downward and clockwise.
  • Winds moving around a low moves inward, upward
    and counter-clockwise

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16
Isobars
  • Isobars connect points of equal pressure
  • Shows horizontal variations in pressure as well
    as identifying high and low pressure

1000
998
L
996
992
17
Isobars
  • The more closely spaced the isobars, the stronger
    the pressure gradient and thus the stronger the
    wind

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18

Isobars
  • Pressure gradient force starts air moving
  • Coriolis effect turns it right
  • These forces keep working until wind is
    blowing parallel to the isobars
  • This is called geostrophic wind.

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19
Moisture
  • As air is heated and cooled, water changes state
    between vapor, liquid, and solid
  • This change of state results in an exchange of
    heat (latent heat)

20
Moisture
  • Latent heat absorbed
  • Sublimation solid to gas
  • Melting solid to liquid
  • Evaporation liquid to gas
  • Latent heat released
  • Deposition gas to solid
  • Condensation gas to liquid
  • Freezing liquid to solid

21
Moisture
  • Latent heat absorbed
  • Latent heat released

22
Saturation
  • Air is limited in ability to contain moisture as
    water vapor
  • Ability to hold moisture in vapor form is
    dependent on temperature of the air
  • Warm air can hold more water vapor than cold air
  • When air reaches 100 capacity it is saturated

23
Saturation
  • When saturation is reached, any additional
    moisture MAY condense to visible moisture, i.e..
    water
  • Water is continuously changing states from liquid
    to gas
  • When the air is warmer, there is a greater number
    of molecules evaporating than condensing we have
    a net evaporation
  • As air is cooled, the evaporation rate decreases
    and condensation exceeds evaporation. These
    sub-microscopic drops clump together into a cloud
    drop.

24
Relative Humidity
  • Amount of water vapor present, compared to the
    actual amount that air can hold
  • Expressed in percentage
  • Cold air at 0 degrees C that is 100 saturated
    may only be 80 saturated if it is warmed to 15
    degrees.

25
Relative Humidity
  • For this reason, the air becomes saturated more
    quickly in winter while the conditions may still
    be relatively dry compared to the summer.
  • When it is muggy, there is a lot of moisture in
    the air making it feel wet.
  • Drier conditions tend to prevail in winter than
    in summer.

26
Dewpoint
  • The temperature to which air must be cooled to
    become saturated
  • When the dewpoint is reached, if condensation
    nuclei are present, visible moisture may condense
    out of the air into liquid drops which we see as
    clouds or fog

27
Moisture, Temperature, and Stability
  • Stability- The tendency for a body to return to
    its original condition following displacement.
  • With respect to the atmosphere, stability is the
    tendency for a parcel of air to continue lifting
    if it is provided an initial lifting force.
  • Lifting may be by mechanical, convective, or
    frontal means.

28
Unstable Airmass
  • Cumuliform clouds
  • Showery precipitation
  • Rough air (turbulence)
  • Good visibility (except in blowing
    obscuration)

29
Stable Airmass
  • Stratiform clouds fog
  • Continuous precipitation
  • Smooth air
  • Fair to poor visibility (in haze smoke)

30

Cloud Formation
  • Criteria for cloud characteristics
  • Moisture
  • Lifting action
  • Condensation nuclei
  • Type dependant upon stability

31
Methods Of Lifting
  • Convective
  • Surface heating
  • Mechanical
  • Frontal
  • Orographic
  • Low pressure

32
Convective Lifting
  • Cold air moving over or lying over a warm surface
    will be warmed from below causing it to become
    less stable
  • To find the cloud base, use the following
    equation
  • air temperature - dewpoint
  • ---------------------------------
  • 2.5C or 4.4F

33
Orographic Lifting
  • Air flowing over mountains rises is cooled
  • When cooled below dewpoint, clouds form
  • Altitude that cloud bases form depends on
    moisture content its dewpoint
  • Clouds may be below mountain tops or above them

34
Orographic Lifting Cont.
  • A lens-shaped cloud that forms as a cap over a
    mountain is a lenticular cloud
  • It stays in place as air moves thru it even up to
    50 kts or more
  • When an airstream flows over a mountain range a
    stable layer is above, waves occur
  • Clouds may form in the crest of the waves while
    roll clouds may form _at_ a lower altitude
  • These indicate strong turbulence exists

35
Clouds Formed by Turbulence Mixing
  • As air flows over surface, friction causes
    variations in wind strength direction
  • Stronger the wind the rougher the surface, the
    larger the eddies the stronger the mixing
  • Air in rising currents will cool if turbulence
    extends high enough, it may cool to dewpoint
    clouds could form
  • W/ turbulent mixing, stratiform clouds may form
    over a large area

36
Cloud Families
  • Low
  • Middle
  • High
  • Clouds with extensive vertical development

37
Ten Basic Cloud Types
  • Stratus (ST)
  • Stratocumulus (SC)
  • Nimbostratus (NS)
  • Cumulonimbus (CB)
  • Cumulus (CU)
  • Altocumulus (AC)
  • Altostratus (AS)
  • Cirrocumulus (CC)
  • Cirrostratus (CS)
  • Cirrus (CI)

38
Broad Cloud Categories
  • Cumulus--heaped clouds associated with
    instability at that level
  • Stratus--flat clouds associated with stability at
    that level
  • Nimbus--dark clouds associated with rain

39
Cloud Types
  • Moist air that is unstable will continue rising,
    forming cumulus-type clouds w/ vertical
    development turbulence
  • Moist air that is stable will stop rising
    stratus-type clouds w/ little vertical
    development no turbulence
  • The type of cloud depends on the stability of the
    air!

40
Low Clouds
  • Between the surface and 5,000 feet
  • Named with the prefix Strato, or nimbo
  • Stratus, Nimbostratus, Stratocumulus

41
Middle Clouds
  • Named with the prefix alto
  • Found between 5,000 and 18,000 feet
  • Examples
  • Altostratus
  • Altocumulus

42
High Clouds
  • Named with the prefix cirro
  • Found above 18,000 feet
  • Examples
  • Cirrostratus
  • Cirrocumulus

43
Clouds w/Extensive Vertical Development
  • Extend from below 5,000 feet to above 35,000 feet
  • Examples
  • Cumulonimbus
  • Towering cumulus

44
Definitions-Ceiling
  • Ceiling- The lowest layer of clouds or obscuring
    phenomena classified as broken or overcast, but
    not thin or partial

45
Fronts
  • Boundary between two different air-masses
  • When flying thru a front, there is a change in
    wind direction, temperature, and pressure
  • When two masses of air of different temperatures
    meet, the warmer/less dense air will flow over
    the cooler air
  • Warmer air mass cools as it is forced upward
  • When reaches dewpoint, clouds form

46
Frontal Symbols

Cold Front
Warm Front
Occluded Front
Stationary Front
47
Cold-Dry Air
Cool-Moist
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Warm-Moist Air
48
Fronts
  • The name given to a front describes the air
    behind it
  • Since fronts will always involve a temperature
    difference, there will always be warm air rising
    in the proximity of a front

49
Warm Front
  • Warm air overriding cold or cool air
  • Stratiform cloud development
  • Steady precipitation
  • Little turbulence

Cool air
Warm air
50
Warm Front
  • The frontal air _at_ altitude is actually well ahead
    of the frontal line
  • Rain could be falling up to 200 miles ahead of
    the front
  • Rain falling into cooler air beneath the warm
    front may cause precipitation induced fog or
    freezing rain (depending on how cold the air is)

51
Temperature Inversion
  • Can occur near ground, on cold, clear nights when
    surface loses heat making air adjacent to surface
    warmer
  • Visibility can be a problem because there is no
    upward convective movement to carry particles away

52
Temperature Inversion
  • Windshear - sudden, drastic shift in wind speed,
    direction or both
  • This may exist _at_ top of the inversion if there
    are strong winds aloft
  • T.I. may exist in warm fronts which can cause
    freezing rain serious danger to the pilot

53
Signs of a Warm Front
  • As front passes, might see high cirrus clouds
    followed by a lowering base of stratus type
    clouds
  • Rain may be falling evaporating before it
    reaches the ground (virga)
  • Rain may be continuous until front passes along
    with fog

54
Signs of Warm Front Cont.
  • Pressure will fall continuously as front
    approaches as it passes, either stop falling or
    fall _at_ a lower rate
  • Temp. will rise as front moves over surface
  • Relative humidity will decrease b/c warm air
    holds more moisture fog will dissipate
  • Wind direction will veer(clockwise change of
    direction) as front passes

55
Cold Front
  • Cold air underrunning warm air
  • Cumuliform clouds
  • Showery precipitation
  • Turbulence
  • Thunderstorms common

56
Cold Front
Warm air
Cold air
57
Cold Front
  • Air that is forced to rise as front passes is
    unstable you get cumuliform type clouds
  • TS, Squall lines, severe turbulence windshear
  • Windshear turbulence are possible _at_ airport _at_
    or just after front passes

58
Signs of Cold Front
  • Pressure falls then rises after front has past
  • Cumuliform clouds
  • Sudden drop in temp lower dewpoint
  • Veering of wind direction, thus windshear

59
Occluded Front
  • Occurs when a cold front overtakes a warm front -
    3 air masses involved
  • Warm air is completely pushed aloft
  • Weather characteristics of both types of fronts
  • An occlusion marks the diminishing of a low
    pressure center (frontoloysis)

60
Occluded Front
Warm Air
Cool Air
Cold Air
61
Occluded Front
  • Cold front travels faster so overtakes warm front
  • Low pressure systemit is common for cold front
    to occur in SW sector of a low warm front in E

62
Signs of Occluded Front
  • Clouds will be dependent on what the warm/cold
    fronts have
  • Warm front could have stratus type cold front
    could have cumuliform type
  • Stratus can hide the cumuliform clouds hide TS
    (known as embedded TS)
  • Severe WX can occur in early part of occlusion
    b/c unstable air is forced up (short)
  • Flight thru can be dangerous due to severe wx

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65
Categories of Icing
  • 1. Structural
  • 2. Induction

66
Structural Icing
  • Primarily caused by water droplets freezing on
    the skin of the aircraft as it passes through a
    cloud/precipitation.
  • The way the droplets freeze produce three
    different icing types.
  • Clear
  • Rime
  • Mixed

67
Clear Icing
  • Large droplets that freeze slowly forming a clear
    coating
  • Resembles ice cubes in freezer
  • Heaviest and most solid type of icing
  • Most dangerous
  • Found in cumiliform clouds

68
Rime Icing
  • When droplets are small and freeze immediately
    air is trapped in the icing
  • The trapped air makes the icing opaque, or milky
  • Rougher surface, but lighter than clear ice
  • Easier to get rid of
  • Found in stratus clouds

69
Mixed Icing
  • A combination of rime and clear
  • When snow or ice particles become trapped in
    clear ice, very rough accumulations develop.

70
Induction Icing
  • Includes icing on intakes and in carburetors.
    Mainly causes loss of engine power due to loss of
    intake.
  • Induction ice occurs due to the same effects as
    structural icing. Carburetor ice occurs due to
    the decrease in temp from decrease in pressure
    and latent heat of vaporization.

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72
Thunderstorm Hazards
  • Hail
  • Turbulence
  • Windshear
  • Lightning
  • Icing
  • Heavy precipitation
  • Tornadoes

73
Requirements for a Thunderstorm
  • Moisture
  • Instability
  • Lifting Mechanism
  • Fronts
  • Thermals
  • Orographic

74
Classic Thunderstorm Formation
  • Three stages of a thunderstorm
  • Cumulus development of thunderstorm
    characterized by updrafts
  • Mature most sever phase characterized by
    updrafts and downdrafts beginning of
    precipitation at the surface
  • Dissipating thunderstorm dies out characterized
    by downdrafts

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Fog Types
  • Radiation
  • Advection
  • Upslope
  • Precipitation Induced
  • Steam

77
Radiation Fog
  • Calm, clear nights, cool nights
  • Earth radiates heat that was absorbed during day
  • As earth cools, it cools the adjacent air to its
    dew point

78
Fog
  • Advection Fog
  • Moist air over cool ground or water
  • Occurs with winds up to 15K

Air Mass
79
Fog
  • Advection Fog
  • Moist air over cool ground or water
  • Occurs with winds up to 15K

Air Mass
80
Fog
  • Advection Fog
  • Moist air over cool ground or water
  • Occurs with winds up to 15K

Air Mass
81
Fog
  • Upslope Fog
  • Stable, moist air forced upslope

82
Fog
  • Upslope Fog
  • Stable, moist air forced upslope

83
Fog
  • Upslope Fog
  • Stable, moist air forced upslope

84
Fog
  • Precipitation induced fog
  • Rain saturates lower levels of the atmosphere and
    cools air

85
Fog
  • Steam fog or arctic sea smoke
  • Forms when cool air moves over warm water
  • Air/ water temperature difference of gt10 degrees

30 degrees
25 degrees
40 degrees
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