Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Weather Theory 6 April 2005 Heating of the Earth 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. 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 highpressure areas when flying west.) 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. 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 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. Fronts Frontal passage is denoted by: Change in temperature Decrease in pressure followed by an increase A shift in wind direction, speed, or both 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 Stable Air Unstable Air Stratiform clouds Cumuliform clouds/fog Smooth air Rough air Continuous Showery precipitation precipitation Poor visibility Good visibility Clouds Two general types: Cumuliform stratiform Clouds 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 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 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 Thunderstorms Three prerequisites: Water vapor Unstable lapse rate (unstable air) Lifting action Three stages: Cumulus Mature Dissipating Thunderstorms 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 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 Wind shear 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 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) Icing Dangerous! Increases weight Increases drag Decreases lift Decreases thrust Increases stalling speed 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 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! Next Week… - Weather services