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Weather theory. Kyle Black Carol Cushman. Environment and Flying. As pilots, do you need to have a basic understanding of weather? Do you need to know how different types of weather can affect you, be it rain, snow, sleet, or ice?. The Atmosphere.

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Weather theory

Kyle Black Carol Cushman


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Environment and Flying

  • As pilots, do you need to have a basic understanding of weather?

  • Do you need to know how different types of weather can affect you, be it rain, snow, sleet, or ice?


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The Atmosphere

  • 99% located within 100,000 feet of the earth’s surface


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Levels of the Atmosphere

  • What are the different layers of the atmosphere?

    • Troposphere (Surface to ~36,000’)

      • Tropopause (Layer just above Troposphere which helps to hold in water vapor and weather)

    • Stratosphere (to 100,000’)

    • Mesosphere

    • Thermosphere


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Atmosphere Composition

  • How are the gases of the atmosphere proportioned? What percentage?

    • Nitrogen?

      • 78%

    • Oxygen?

      • 21%

    • Other Gases

      • 1%


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Atmospheric Circulation

  • Temperature

    • Circulation due to temperature variations created by unequal heating of the earth’s surface.

  • Convection

  • Three-Cell Circulation Pattern

    • 0-30 degrees latitude – Hadley Cell (Rises from equator, sinks by 30 degrees lat. etc)

    • 30-60 degrees latitude – Ferrel Cell

    • 60 degrees to poles – Polar Cell

  • Atmospheric Pressure (Pressure Gradient Force)

    • Air generally flows from cool, dense air associated with highs to less dense air of lows.


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Atmospheric Circulation

  • Coriolis Force

    • Earth’s rotation

    • Effects objects such as air (or aircraft) travelling large distances

  • Frictional Force

    • Wind is result of Pressure and Coriolis Force

    • Friction causes wind to shift directions when near the surface of the earth (within 2000’ of ground)

    • Lessens effects of Coriolis Force


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Atmospheric Circulation

  • Global Wind Patterns

  • Local Wind Patterns

    • Terrain Variations

      • Mountains

      • Valleys

      • Water


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Atmospheric Circulation

  • Sea Breeze

    • Heating differential between land and water surfaces…

    • Cool air moves to warm air (cool air over water to warm air over land)

  • Land Breeze

    • Land cools faster than water in evening

    • Cool air moves to warm air (cool air over land to warm air over water)


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Atmospheric Circulation

  • Valley Breeze

    • Air flows up valley and up the slopes during the day

  • Mountain Breeze

    • As ground cools air flows down slope and away from higher terrain


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Atmospheric Circulation – Katabatic Winds

  • Generally , any downslope wind

  • Usually refers to downwind flows which are stronger than mountain breezes

    • Usually given special names


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Katabatic Winds

  • Cold Downslope Winds

    • Large ice and snow fields accumulate in mountainous terrain

    • Overlying air becomes extremely cold and high pressure forms

    • Pressure gradient pushes air through gaps in mountains

    • What happens when constriction is reached? (Canyon, Valley, etc.)

  • Examples?

    • Bora in Croatia

    • Mistral in Rhone Valley of France

    • Columbia Gorge wind in northwestern U.S.


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Katabatic Winds

  • Warm Downslope Winds

    • Warm airmass moves across mountain range at high levels

    • Forms a trough of low pressure on downwind (lee) side – downslope wind develops

    • As air descends it is compressed – resulting in temp increase

    • Warmer wind can raise temperatures over 20 degrees in an hour

    • 20-50 knot winds, extreme cases can reach 100 knots

  • Examples?

    • Chinook winds along eastern slopes of the Rocky Mountains

    • Foehnin the Alps

    • Santa Ana in Southern California



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Atmospheric Stability

  • How is the stability of an airmass decreased?

    • Warming from below

  • What happens to temperature as pressure changes?

    • As an airmass moves downward it is compressed – raising the temperature

    • As an airmass moves upward it expands – lowering the temperature

    • Known as adiabatic heating or adiabatic cooling (change of temperature in dry air)


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Stability

  • Rate at which temperature decreases with an increase in altitude is referred to as what?

    • Lapse rate

  • What is the average lapse rate?

    • 2°C per 1000’


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Stability

  • Dry Adiabatic Lapse Rate

    • 3°C per 1000’

  • Moist Adiabatic Lapse Rate

    • 1.1°C to 2.8°C

  • What is the average lapse rate?

    • 2°C per 1000’


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Stability

  • Temperature Inversions

    • Temperature increase with altitude

    • Below the inversion visibility is restricted by pollutants and weather – fog, haze, smoke, low clouds


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Stability

  • Moisture

    • Change of State

    • Humidity

      • Amount of moisture air can hold depends on air temperature

    • Dewpoint

      • Air contains 100% of moisture possible at that temperature

    • Dew and Frost

      • Objects cooling below dewpoint of surrounding air

      • Water vapor to ice on surface below freezing


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Stability - Clouds

  • At saturation point, invisible water vapor changes to a visible state:

    • Fog

    • Clouds

  • Small temperature/dewpoint spread indicates favorable conditions for formation of fog


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Stability - Clouds

  • Types of Clouds

    • Low Clouds (Surface to 6500’ AGL)

    • Middle Clouds (6500’ to 20,000’ AGL)

    • High Clouds (Above 20,000’ AGL)


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Stability - Clouds

  • Low Clouds

    • Stratus

    • Nimbostratus

    • Stratocumulus



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Stability - Clouds

  • Low Clouds (Surface to 6500’ AGL)

    • Fog

      • Ground

        • Fog less than 20 feet deep

      • Radiation

        • Forms over low-lying flat surfaces

        • Clear, calm, humid nights

      • Advection

        • Warm moist air moves over cooler surfaces

        • Up to 15 knots intensifies development of fog

      • Upslope

        • Moist stable air forced up a sloping land mass

      • Steam

        • Cold, dry air moves over warmer water


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Stability - Clouds

  • Middle Clouds (6500’ to 20,000’ AGL)

    • Altostratus

    • Altocumulus


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Stability - Clouds

  • High Clouds (Above 20,000’ AGL)

    • Cirrus

    • Cirrostratus

    • Cirrocumulus


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Stability - Clouds

  • Vertical Development

    • Cumulus

    • Towering Cumulus

    • Cumulonimbus


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Precipitation

  • Water / ice particles must grow in size until no longer supportable by atmosphere

  • Types?

    • Drizzle and Rain

      • Virga – Evaporating Precipitation

      • Precipitation-induced fog – warm rain/drizzle falls through cooler air near surface – evaporation may saturate cool air

    • Ice Pellets and Hail

      • Rain freezes passing through colder air

      • Water droplets freeze in clouds before becoming to heavy to fall


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Precipitation

  • Types?

    • Snow

      • Precipitation composed of ice crystals

      • Snow grains are the solid equivalent of drizzle

      • Ice crystals that descend from cirrus clouds are called “fallstreaks”, or mare’s tails.


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AIRMASSES

  • What is an airmass?

    • A large body of air with fairly uniform temperature and moisture content.

    • May be several hundreds miles across and usually forms where air is stationary, or nearly so, for several days.

    • Source Region:

      • The area where an airmass acquires the properties of temperature and moisture that determine its stability.


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AIRMASSES

  • Classifications

    • Airmasses are classified according the the regions where they originate.


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Modification

  • As an airmass moves out of it’s source region, it is modified by the temperature and moisture of the area over which it moves.

  • The degree to which an airmass is changes depends on several factors:

    • Speed

    • The nature of the region it moves over

    • Depth of the airmass

    • Temperature difference of surface and new airmass


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Modification

  • Warming From Below

    • As an airmass moves over a warmer surface, its lower layers are heated and vertical movement of the air develops.

    • Depending on temperature and moisture levels, this can result in extreme instability.


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Modification

  • Cooling From Below

    • When an airmass moves over a cooler surface, its lower layers are cooled and vertical movement is inhibited.

    • Stability of the airmass is increased.

    • If the air is cooled to its dewpoint, low clouds or fog may form.

    • Cooling from below creates a temperature inversion and may result in low ceilings and visibility for long periods of time.


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Modification

  • Cooling From Below


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FRONTS

  • What is a Front?

    • The boundary between airmasses is called a front.

    • Weather along fronts often presents a serious hazard to flying, it is important to have a thorough understanding of the associated weather.

  • What are some types of Fronts?

    • Cold Front

    • Warm Front

    • Stationary Front

    • Occluded Front


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Frontal Discontinuities

  • Temperature

    • One of the most easily recognized discontinuity across a front.

    • Can be observed in the cockpit by looking at the OAT gauge.

  • Wind

    • The most reliable indication that you are crossing a front are changing wind directions and velocities.

    • In the northern hemisphere, the wind always shifts to the right when crossing a front.

  • Pressure

    • Pressure changes across fronts as well.

    • Must update altimeter settings to maintain the proper altitude.


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Frontal Weather

  • Cold Front

    • Separates an advancing mass of cold, dense, and stable air from an area of warm, lighter, and unstable air.

    • Because of it’s greater density, the cold front moves along the surface and forces the less dense, warm air upward



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Frontal Weather

  • Fast-Moving Cold Front

    • Cold fronts that are pushed along by an intense high pressure systems located will behind the front.

    • Surface friction acts to slow down the movement of the front, causing the leading edge of the front to bulge out and steepen the front’s slope.

    • Hazardous because of the wide differences in moisture and temperature between fronts.

  • Slow-Moving Cold Fronts

    • Produces clouds that move far behind the leading edge of the front. Broad area of stratus clouds.


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Frontal Weather

  • Warm Fronts

    • Occur when warm air overtakes and replaces cooler air.

    • Usually move much slower than cold fronts.

    • The slope of a warm front is very gradual, and the warm air may extend over the cooler air for hundreds of miles.



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Frontal Weather

  • Stationary Fronts

    • When the opposing forces of two airmasses are relatively balanced, the front that separates them may remain stationary and influence local flying conditions for several days.

    • The weather is typically a mixture of weather found in both warm and cold fronts.


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Frontal Weather

  • Occluded Fronts

    • Occurs when a fast-moving cold front catches up to a slow-moving warm front.

    • The difference in temperature within each frontal system is a major factor that influences which type of front and weather are created.


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Frontal Weather

  • Occluded Fronts

    • Cold Front Occlusion

      • Develops when the fast-moving cold front is colder than the air ahead of the slow-moving warm front.

      • The cold air replaces the cool air at the surface and forces the warm front aloft.

    • Warm Front Occlusion

      • Takes place when the air ahead of the slow-moving warm front is colder than the air within the fast-moving cold front.



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Weather Hazards

  • What types of hazards do we as pilots have to worry about?

    • Thunderstorms

    • Turbulence

    • Lightning

    • Hail

    • Tornadoes

    • Windshear

    • Icing

    • And even Volcanic Ash


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Thunderstorms

  • Thunderstorms are arguably the single greatest threat to aircraft operations.

  • Contain strong wind gusts, icing, hail, driving rain, lightning, and sometime tornadoes.

  • In order for thunderstorms to form, three conditions must be met:

    1. Air that has a tendency toward instability

    2. Some lifting action

    3. Relatively high moisture content


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Thunderstorms

  • Types of Thunderstorms

    • Airmass Thunderstorms

      • Scattered thunderstorms which are common during summer afternoons, or coastal areas at night. Short lived.

    • Severe Thunderstorms

      • Violent t-storms with wind gust +50 kt., hail ¾” diameter, and/or tornadoes.

    • Single-Cell

      • Thunderstorm that last less than one hour

    • Super-Cell

      • Thunderstorm that lasts up to two hours or more

    • Multicell

      • Compact cluster of thunderstorms


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Thunderstorms

  • Types of Thunderstorms

    • Squall Line

      • Thunderstorms that form a line

      • Often forms 50 to 300 miles ahead of a front, but the existence of a front is not necessary.

      • Can be a mixture of single-cell, multicell, super-cell

      • Very dangerous storms

    • Frontal Thunderstorms

      • Thunderstorms that are associated with frontal activity

      • Often obscured by stratiform clouds

      • Can form with Warm or Cold Fronts



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Thunderstorm Life Cycle

  • 50 years ago it was dicovered that thunderstorms go through three definite stages.

  • What are these three stages??

    1. Cumulus Stage

    2. Mature Stage

    3. Dissipating Stage



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Cumulus Stage

  • Lifting Action Required

  • Air rises and cools to dewpoint

  • Condensing air releases heat, causing further vertical development

  • Strong updrafts preventing precipitation from falling

  • Particle impact each other and grow in size

  • Becomes a Towering Cumulus Cloud (TCU)

  • Cloud can reach the Mature Stage in as little as 15 minutes.



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Mature Stage

  • Drops in the cloud become too large to be supported by the updrafts

  • Precipitation begins to fall

  • This creates a downward motion in the surrounding air and signals the beginning of the mature stage

  • This circulation of the thunderstorm cell is organized in this, the storm’s most violent stage

  • Warm updrafts and cool downdrafts exist side-by-side creating severe turbulence

  • At the surface, the air rushes downward and spreads outward rapidly

    • This cause a sharp drop in temperature, a rise in pressure, and strong, gusty winds

  • Gust Front/Roll Cloud


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Mature Stage

  • The top of the mature cell can reach higher than 40,000 ft.

  • Because of the drop in temperature, the moisture at the top of the cloud freezes giving the cloud a fuzzy appearance.

  • The vertical lift decreases towards the top and the moisture particle spread out horizontally, creating the well known anvil shape

  • The anvil is an indicator of the upper-level winds and the storm’s direction of movement.



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Dissipating Stage

  • Occurs 15 to 30 minutes after the storm reaches the mature stage

  • As the storm develops, more and more air aloft is disturbed by the falling drops

  • Downdrafts begin to spread out within the cell

  • The thunderstorm beginnings to weaken, predominated filled with downdrafts, it takes on a stratiform appearance, gradually dissipating.



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Thunderstorm Hazards

  • What are some hazards associated with Thunderstorms?

    • Turbulence

    • Lightning

      • In cloud, Cloud-to-cloud, cloud-to-ground, cloud-to-clear air

      • 300,000 Volts per foot, heats air to more than 50,000 deg. F

    • Hail

      • Can occur at any altitude, inside or outside a cloud

      • Large hailstones have been encountered in clear air miles downwind from a thunderstorm

    • Tornadoes

      • Violent spinning columns of air which descend from the base of a cloud.

      • Winds may exceed 200 kt.





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Turbulence

  • Low Level Turbulence (LLT)

    • Turbulence below 15,000 ft.

    • Most originates due to surface heating or friction within a few thousand feet of the ground.

    • LLT includes:

      • Mechanical Turbulence

      • Convective Turbulence

      • Frontal Turbulence

      • Wake Turbulence


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Turbulence

  • Mechanical Turbulence


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Turbulence

  • Mechanical Turbulence


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Turbulence

  • Convective Turbulence


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Turbulence

  • Frontal Turbulence

    • Occurs in the narrow zone just ahead of a fat-moving cold front where updrafts can reach 1,000 f.p.m.

    • When combine with convection and strong winds across the front, these updrafts can produce significant turbulence.

    • Over flat ground, any front moving at a speed of 30 knots or more will generate at least moderate turbulence.

    • A front moving over rough terrain will produce moderate or greater turbulence, regardless of its speed.


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Turbulence

  • Wake Turbulence

    • Whenever an airplane generates lift, air spills over the wingtips from the high pressure areas below the wings to the low pressure areas above the wings.

    • This flow causes rapidly rotating whirlpools of air called wingtip vortices, or Wake Turbulence.

    • Greatest vortex strength occurs when the aircraft is heavy, slow, and in a clean configuration.

    • Tend to sink below the flight path of the aircraft and spread out horizontally.




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The End

  • Any questions??

  • Next week we will be covering Weather Products!

    • Make sure you review this material before Monday…


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