1 / 21

Weather Theory

Weather Theory. 6 April 2005. All weather on earth is caused by differential (unequal) heating of the earth’s surface! Large-scale atmospheric circulation (warm air rises; cooler air settles)

umeko
Télécharger la présentation

Weather Theory

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Weather Theory 6 April 2005

  2. All weather on earth is caused by differential (unequal) heating of the earth’s surface! Large-scale atmospheric circulation (warm air rises; cooler air settles) Add rotational effects of the Coriolis Force and winds are deflected to the right (in the Northern Hemisphere) Coriolis force causes wind to flow parallel to isobars. Heating of the Earth

  3. Circulation and Wind • Air circulates clockwise around a high and counterclockwise around a low. • When isobars are closer together, wind speed increases. • Surface friction causes winds to not exactly parallel the isobars but rather move outward from high pressure areas. • (e.g., more favorable winds will be found south of high-pressure areas when flying west.)

  4. Temperature • Standard Temperature is 15 Celsius • Temperature decreases an average of 2 degrees per 1000 feet. (Standard lapse rate) • When temperature increases with altitude, there is a temperature inversion. • The most frequent ground-based TI is produced by radiation on a clear, still night.

  5. Moisture • Key Terms • Relative humidity: percentage of air’s maximum saturation • Dewpoint: temperature to which air must be cooled to become saturated (100% RH) • Adding and removing moisture: • Evaporation • Sublimation

  6. Airmasses and Fronts • Air masses are formed when a large body of air sits on top of a region for a long time, taking on its moisture and temperature contents. • Two airmasses are separated by a front. • Ridges are lines of high pressure • Troughs are lines of low pressure.

  7. Fronts • Frontal passage is denoted by: • Change in temperature • Decrease in pressure followed by an increase • A shift in wind direction, speed, or both

  8. Atmospheric Stability • Stability is the resistance of the atmosphere to vertical motion • A stable atmosphere resists vertical motion • An unstable atmosphere allows convective currents to develop. • Stable air is often associated with a temperature inversion

  9. Clouds • Two general types: • Cumuliform • stratiform

  10. Cumuliform Form in unstable air Are vertically developed (billowy) Showery precipitation Stratiform Form in unstable air Flat/even Continuous light rain Cirrus High clouds composed of ice crystals Clouds

  11. Bases of cumuliform clouds can be determined by: Subtracting temperature (Celsius) from dewpoint and dividing by 2.5 (temperature per 1000 feet that temp. and dewpoint converge) This is the height (in thousands of feet) of the base of the clouds Clouds

  12. Turbulence • Found in convective currents (e.g. when cumulus clouds are present) • Strongest in areas with large updrafts (towering cumulus/cumulonimbus) • When severe turbulence is encountered, reduce airplane’s airspeed to maneuvering speed and attempt to maintain a level flight attitude

  13. Three prerequisites: Water vapor Unstable lapse rate (unstable air) Lifting action Three stages: Cumulus Mature Dissipating Thunderstorms

  14. Lightning always present Hail frequently created in up/down drafts Significant windshear turbulence Embedded thunderstorms are those that are within large cloud areas and cannot be seen Squall-line thunderstorms create the most hazardous conditions and develop ahead of cold fronts Thunderstorms

  15. Wind Shear • Any change in direction/speed in a short distance • Low-level wind shear most hazardous • Temperature inversions • Around thunderstorms • Found when wind speed between 2000’ and 4000’ is at least 25 kts

  16. Two hazardous conditions: Loss of tailwind Airspeed initially increases Aircraft pitches up Altitude increases, causing a requirement for less power Loss of headwind Airspeed decreases (dangerous if flying slowly) Aircraft pitches down and begins to sink Wind shear

  17. Icing • Two types: • Clear ice – hard /glossy, cumuliform clouds • Rime ice – stratiform clouds/small drops • Occurs when water contacts airplane parts that are below freezing • Requires visible moisture • Ice pellets indicate freezing rain above (most dangerous)

  18. Icing • Dangerous! • Increases weight • Increases drag • Decreases lift • Decreases thrust • Increases stalling speed

  19. Fog • Radiation fog • Terrestrial radiation cools the ground, cooling air touching ground • Advection fog • Warm, moist air moves over cold ground • Upslope fog • Moist, stable air is cooled to dewpoint • Preciptation fog • Caused by evaporating precipitation • Steam fog • Cold, dry air moves over relatively warm water

  20. Frost • Ice deposits formed by sublimation on a surface • Formed when collecting surface is at or below the freezing point • Frost causes loss of aerodynamic efficiency due to early flow separation • Do not take off with frost on your airplane!

  21. Next Week… • Weather services

More Related