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Aviation Meteorology Part 2

Aviation Meteorology Part 2. AVN-1020 Fall 2010 Session 12. Overview. Winds Stability Clouds Weather Hazards. Review: Combined Wind Effects. Pressure gradients – perpendicular to isobars Coriolis force – shifts toward parallel Surface friction – further shifts

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Aviation Meteorology Part 2

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  1. Aviation Meteorology Part 2 AVN-1020 Fall 2010 Session 12

  2. Overview • Winds • Stability • Clouds • Weather Hazards

  3. Review: Combined Wind Effects • Pressure gradients – perpendicular to isobars • Coriolis force – shifts toward parallel • Surface friction – further shifts • In Summary (in Northern Hemisphere): • High Pressure System/Ridge Wind Flow: • Downward, outward and clockwise around • Low Pressure System/Trough Wind Flow: • Upward, inward and counterclockwise around • Friction causes winds to shift even more the closer we get to the surface

  4. Review: RH / Dewpoint RH = Actual amount of moisture in air Total capacity • Amount of moisture air holds depends on temperature • Dew Point: when RH = 100% • Condensation or sublimation • Dew, frost, cloud formation 4

  5. Review: Stability / Lapse Rates • Stability: resistance to vertical motion • Greatest stability: Dry air at lower temperatures • Greatest instability: Moist air at higher temperatures • Standard ICAO lapse rates: • Based on 58°F, 29.92” Hg at sea level (15°C) • 3.5°F and 1” Hg per 1,000 ft (2°C) for “standard” air • 4°F per 1,000 ft (2.5°C) for moist air • “K Index”: Comparative measure of stability • Higher the number, the more unstable the air • Inversion: when lapse rate reverses

  6. Lapse Rate Applications • Finding the Freezing Level • At 1,500 MSL, temp is 48° F; where is freezing level? 48° - 32 ° = 4,000 ft 4,000 + 1,500 = 5,500’ MSL 3.5 ° per 1000” • Finding the Cloud Level If surface is 82° F and dew point is 62° F; where do clouds form? • Assume 4° F / 1000’ lapse rate for moist air 82° - 62 ° = 5,000 ft 4° per 1000”

  7. Cloud Types • Dependent on stability of airmass and RH • Classified by: • Base altitude: • Low: surface to 6,500’ AGL • Middle: 6,500’ to 20,000’ AGL • High: > 16,000’ AGL to tropopause • Characteristics • Stratus: “layered” – steady precipitation • Cumulus: “accumulation” – showery precipitation • Nimbus: “raincloud” - downpours • Vertical Development

  8. Stability vs. Clouds • Stability of air mass directly influences weather:

  9. Fog • Basically a Stratus with base w/in 50ft of ground • How to make fog: • Cool the air to the dew point • Add moisture to cool air • More prevalent where condensation nuclei abundant: • Industrial areas (smog) – refineries, cities • Salty / dusty areas (haze) – desert coastlines

  10. Fog Types • Radiation Fog • On clear, calm, humid nights on low, flat surfaces • Ground cools adjacent air by radiation; reaches dew-point • Advection Fog (aka, “sea fog”) • Warm moist air moves over cooler surface • Prevalent in coastal areas or by rivers and lakes in winter • Upslope Fog • Moist stable air, forced up sloping land mass by wind • Precipitation-Induced fog • Air is saturated after rain • Ice Fog • Temperatures below -20o F

  11. Middle Clouds

  12. High Clouds

  13. Clouds with Vertical Development

  14. Thunderstorms • Three requirements for formation: • Lifting action • Uneven surface heating • Fast moving fronts • Upsloping terrain • Unstable air • Non-standard lapse rate • High moisture content • Close temp/dew point spread

  15. Thunderstorm Stages Updrafts Heavy Rain Lightning Light Rain Downdrafts 15

  16. Thunderstorm Hazards • Gusty winds & turbulence • Tornadoes • Lightning • In-cloud, cloud-to-cloud, cloud-to-ground & clear air • Hail • 20 nm downwind from storm under anvil • Wind shear zone / gust fronts • At surface from downdrafts spreading out 16

  17. Icing • Induction Icing • Chokes off air supply to engine • Structural Icing: • Loss of lift • Increase in weight • Control problems • Conditions: • Visible moisture / humidity and viable temperature range • Temp of the aircraft skin is below freezing (structural) • Mitigation: • Climb / descend or run away!

  18. Types of Icing • Rime Ice • Opaque, milky – caused by quick-freezing of water • Primarily in stratus clouds • Roughens surfaces; increases drag • Clear Ice • Transparent – caused by slow-freezing of water • Primarily in cumulus clouds • Adds weight • Mixed Ice • Combination of both! • Frost • Same effects as Rime/Clear ice

  19. Turbulence “Duration – Intensity – Frequency” DURATION: “Occasional” - less than 1/3 of the time “Intermittent” - 1/3 to 2/3 of the time “Continuous”- more than 2/3’s INTENSITY: “Light” - slight changes in altitude/attitude/heading “Moderate” - greater changes in altitude, attitude, heading “Severe”- large changes in altitude, attitude, heading “Extreme”- violent, uncontrollable 19

  20. Wind Shear • Drastic shift in wind speed and/or direction • Any altitude, vertical or horizontal plane • Sudden updrafts & downdrafts • Sudden gain or loss of altitude or airspeed • Places: • Jet stream boundaries • Frontal zones • Low-level temperature inversions • Convective precipitation - MICROBURSTS 20

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