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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

Weather theory

Kyle Black Carol Cushman

environment and flying
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
The Atmosphere
  • 99% located within 100,000 feet of the earth’s surface
levels of the atmosphere
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
atmosphere composition
Atmosphere Composition
  • How are the gases of the atmosphere proportioned? What percentage?
    • Nitrogen?
      • 78%
    • Oxygen?
      • 21%
    • Other Gases
      • 1%
atmospheric circulation
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.
atmospheric circulation9
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
atmospheric circulation10
Atmospheric Circulation
  • Global Wind Patterns
  • Local Wind Patterns
    • Terrain Variations
      • Mountains
      • Valleys
      • Water
atmospheric circulation11
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)
atmospheric circulation12
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
atmospheric circulation katabatic winds
Atmospheric Circulation – Katabatic Winds
  • Generally , any downslope wind
  • Usually refers to downwind flows which are stronger than mountain breezes
    • Usually given special names
katabatic winds
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.
katabatic winds15
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
atmospheric stability
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)
stability
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’
stability19
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’
stability20
Stability
  • Temperature Inversions
    • Temperature increase with altitude
    • Below the inversion visibility is restricted by pollutants and weather – fog, haze, smoke, low clouds
stability21
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
stability clouds
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
stability clouds24
Stability - Clouds
  • Types of Clouds
    • Low Clouds (Surface to 6500’ AGL)
    • Middle Clouds (6500’ to 20,000’ AGL)
    • High Clouds (Above 20,000’ AGL)
stability clouds25
Stability - Clouds
  • Low Clouds
    • Stratus
    • Nimbostratus
    • Stratocumulus
stability clouds27
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
stability clouds28
Stability - Clouds
  • Middle Clouds (6500’ to 20,000’ AGL)
    • Altostratus
    • Altocumulus
stability clouds29
Stability - Clouds
  • High Clouds (Above 20,000’ AGL)
    • Cirrus
    • Cirrostratus
    • Cirrocumulus
stability clouds30
Stability - Clouds
  • Vertical Development
    • Cumulus
    • Towering Cumulus
    • Cumulonimbus
precipitation
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
precipitation32
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.
airmasses
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.
airmasses34
AIRMASSES
  • Classifications
    • Airmasses are classified according the the regions where they originate.
modification
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
modification36
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.
modification37
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.
modification38
Modification
  • Cooling From Below
fronts
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
frontal discontinuities
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.
frontal weather
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
frontal weather43
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.
frontal weather44
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.
frontal weather46
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.
frontal weather47
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.
frontal weather48
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.
weather hazards
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
thunderstorms
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

thunderstorms52
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
thunderstorms53
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
thunderstorm life cycle
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

cumulus stage
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.
mature stage
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
mature stage60
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.
dissipating stage
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.
thunderstorm hazards
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.
turbulence
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
turbulence70
Turbulence
  • Mechanical Turbulence
turbulence71
Turbulence
  • Mechanical Turbulence
turbulence72
Turbulence
  • Convective Turbulence
turbulence73
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.
turbulence74
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.
the end
The End
  • Any questions??
  • Next week we will be covering Weather Products!
    • Make sure you review this material before Monday…