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Weather theory

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

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  1. Weather theory Kyle Black Carol Cushman

  2. 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?

  3. The Atmosphere • 99% located within 100,000 feet of the earth’s surface

  4. 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

  5. Atmosphere Composition • How are the gases of the atmosphere proportioned? What percentage? • Nitrogen? • 78% • Oxygen? • 21% • Other Gases • 1%

  6. 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.

  7. 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

  8. Atmospheric Circulation • Global Wind Patterns • Local Wind Patterns • Terrain Variations • Mountains • Valleys • Water

  9. 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)

  10. 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

  11. Atmospheric Circulation – Katabatic Winds • Generally , any downslope wind • Usually refers to downwind flows which are stronger than mountain breezes • Usually given special names

  12. 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.

  13. 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

  14. Weather patterns

  15. 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)

  16. 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’

  17. 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’

  18. Stability • Temperature Inversions • Temperature increase with altitude • Below the inversion visibility is restricted by pollutants and weather – fog, haze, smoke, low clouds

  19. 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

  20. 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

  21. Stability - Clouds • Types of Clouds • Low Clouds (Surface to 6500’ AGL) • Middle Clouds (6500’ to 20,000’ AGL) • High Clouds (Above 20,000’ AGL)

  22. Stability - Clouds • Low Clouds • Stratus • Nimbostratus • Stratocumulus

  23. Low Clouds

  24. 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

  25. Stability - Clouds • Middle Clouds (6500’ to 20,000’ AGL) • Altostratus • Altocumulus

  26. Stability - Clouds • High Clouds (Above 20,000’ AGL) • Cirrus • Cirrostratus • Cirrocumulus

  27. Stability - Clouds • Vertical Development • Cumulus • Towering Cumulus • Cumulonimbus

  28. 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

  29. 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.

  30. 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.

  31. AIRMASSES • Classifications • Airmasses are classified according the the regions where they originate.

  32. 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

  33. 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.

  34. 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.

  35. Modification • Cooling From Below

  36. 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

  37. 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.

  38. 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

  39. Cold Front

  40. 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.

  41. 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.

  42. Warm Front

  43. 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.

  44. 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.

  45. 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.

  46. Occlusions

  47. 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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