Weather Theory

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Weather Theory

• Aviation 310

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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

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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

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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

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Air Masses

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

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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

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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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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

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Coriolis Effect

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

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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.

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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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• 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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Isobars

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

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998
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Isobars

• The more closely spaced the isobars, the stronger
  the pressure gradient and thus the stronger the
  wind

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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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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)

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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

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Moisture

• Latent heat absorbed
• Latent heat released

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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

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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.

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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.

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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.

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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

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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.

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Unstable Airmass

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

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Stable Airmass

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

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Cloud Formation

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

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Methods Of Lifting

• Convective
• Surface heating
• Mechanical
• Frontal
• Orographic
• Low pressure

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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

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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

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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

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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

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Cloud Families

• Low
• Middle
• High
• Clouds with extensive vertical development

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Ten Basic Cloud Types

• Stratus (ST)
• Stratocumulus (SC)
• Nimbostratus (NS)
• Cumulonimbus (CB)
• Cumulus (CU)

• Altocumulus (AC)
• Altostratus (AS)
• Cirrocumulus (CC)
• Cirrostratus (CS)
• Cirrus (CI)

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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

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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!

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Low Clouds

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

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Middle Clouds

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

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High Clouds

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

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Clouds w/Extensive Vertical Development

• Extend from below 5,000 feet to above 35,000 feet
• Examples
• Cumulonimbus
• Towering cumulus

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Definitions-Ceiling

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

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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

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Frontal Symbols

Cold Front
Warm Front
Occluded Front
Stationary Front
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Cold-Dry Air
Cool-Moist
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Warm-Moist Air
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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

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Warm Front

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

Cool air
Warm air
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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)

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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

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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

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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

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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

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Cold Front

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

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Cold Front
Warm air
Cold air
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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

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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

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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)

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Occluded Front
Warm Air
Cool Air
Cold Air
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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

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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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Categories of Icing

• 1. Structural
• 2. Induction

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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

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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

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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

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Mixed Icing

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

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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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Thunderstorm Hazards

• Hail
• Turbulence
• Windshear
• Lightning
• Icing
• Heavy precipitation
• Tornadoes

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Requirements for a Thunderstorm

• Moisture
• Instability
• Lifting Mechanism
• Fronts
• Thermals
• Orographic

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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

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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

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Fog

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

Air Mass
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Fog

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

Air Mass
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Fog

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

Air Mass
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Fog

• Upslope Fog
• Stable, moist air forced upslope

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Fog

• Upslope Fog
• Stable, moist air forced upslope

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Fog

• Upslope Fog
• Stable, moist air forced upslope

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Fog

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

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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