Aviation Weather Meteorology for Pilots PDF

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aviation weather meteorology weather patterns atmospheric science

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This document contains notes on aviation weather meteorology, including topics such as the atmosphere, circulation, pressure systems, and various weather phenomena. The notes cover basic weather theory and provide an overview of key concepts and elements.

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Aviation Weather Meteorology for Pilots Chapter 6 Section A Basic Weather Theory The Atmosphere  Mixture of gases surrounding the earth  Fairly uniform in proportions up to approx. 260,000 feet  Divided into layers that are defined by other criteria...

Aviation Weather Meteorology for Pilots Chapter 6 Section A Basic Weather Theory The Atmosphere  Mixture of gases surrounding the earth  Fairly uniform in proportions up to approx. 260,000 feet  Divided into layers that are defined by other criteria The Atmosphere  Troposphere  Tropopause  Stratosphere  Mesosphere  Thermosphere The Atmosphere  Troposphere – Surface to approx. 36,000 feet  Higher in summer than winter  Higher at equator than poles – Where most of the weather is  Tropopause – top of troposphere, jet stream, turbulence, top of thunderstorms  Stratosphere – to approx. 160,000 feet  Mesosphere and Thermosphere Composition of the Atmosphere  Gases – Nitrogen 78% – Oxygen 21% – Other 1% – Water vapor 0% to 4%  Pollutants Atmospheric Circulation  Why is there movement of the air? – Atmosphere fixed to earth by gravity – Rotates with earth  What upsets the equilibrium? – Unequal temperatures at the earth’s surface Circulation - theory  Temperature is affected by exposure to sun – Length of time – summer versus winter – Angle at which sun strikes the surfaces – equator versus poles  Air compensates for unequal heating by convection – Warmer air is less dense, rises - equator – Cooler air is more dense, sinks - poles – and replaces warmer air by flowing to equator Circulation - reality  Three-cell pattern – Hadley Cell – Ferrel Cell – Polar Cell Atmospheric Pressure  Unequal heating causes – Changes in air density – Circulation – Results in pressure changes  Altimeter settings are different in different locations On the Weather Maps  Isobars – Lines connecting points of equal pressure  Pressure gradient – change in pressure over distance – Close together or widely spaced isobars indicate strong or weak gradient Isobars Identify Pressure Systems  High pressure system  Low pressure system  Ridge  Trough  Col Air Flow  From cool, dense air of high pressure  To warm, less dense air of low pressure  Pressure gradient force – Strong pressure gradient (isobars close together) = strong wind – Weak pressure gradient (isobars far apart) = light wind Coriolis Force  Air does not go in a straight line directly from high pressure to low pressure  Rotation of the earth causes path to deflect – To right in northern hemisphere – To left in southern hemisphere – No deflection at equator, most deflection at poles – The greater the speed the greater the deflection Coriolis Force  Deflection continues until Coriolis Force and Pressure Gradient Force are equal  Air flows parallel to isobars  Clockwise flow around a high pressure area  Counterclockwise around a low pressure area Frictional Force  Friction slows air near surface of earth  Less Coriolis force because of slower speed of air  Pressure gradient force is greater and air flows toward low pressure Global Wind Patterns Local Wind Patterns  Wind patterns are affected by: – Terrain variations – Water  Warmer air rises - cool air replaces warm air – Same as global patterns – smaller scale Sea Breeze  Day time heating of land  Causes air to rise  Cooler air from over water flows in to replace warmer air  Return flow above sea breeze  10 to 20 knots  1,500 to 3,000 feet AGL Land Breeze  Land cools faster than water at night  Reverse of daytime sea breeze  Temperature contrasts less at night than during day so land breeze not as strong  1,000 to 2,000 feet AGL Valley Breeze  Mountain slopes heated by sun which heats adjacent air  Warmed air flows up the valley  5 to 20 knots  Maximum winds several hundred feet above surface Mountain Breeze  At night, terrain cools  Becomes cooler than the air  Pressure gradient reverses  Air flows down the slopes and valley  5 to 15 knots, max 25 knots Katabatic Wind  Downslope wind  Stronger than mountain breeze  Either warm or cold Cold Downslope Winds  Over areas of ice or snow air becomes extremely cold  Shallow dome of high pressure forms  Pressure gradient force pushes cold air through gaps in mountains  If through a narrow canyon, speeds can exceed 100 knots  Named in some locations – bora (Croatia), mistral (France), Columbia Gorge wind (US) Warm Downslope Wind  Warm airmass moving over mountains can form trough of low pressure on lee side  Causes downslope wind to develop  As descends, compresses and warms  Can increase over 20º in an hour  20 to 50 knots, as much as 100 knots  Named – Chinook (eastern slopes of Rockies), foehn (Alps), Santa Ana (So. Calif) Meteorology for Pilots Chapter 6 Section B Weather Patterns Atmospheric Stability  Stability – resistance to vertical motion  Stable atmosphere makes vertical motion more difficult  Generally smooth air  Unstable air – turbulent, rising air, large vertical movement  Significant cloud development, hazardous weather Adiabatic Heating/Cooling  Air moving up – expands due to lower pressure  Air moving down – compressed, high pressure  As pressure changes so does temperature  Process is adiabatic heating (compression) or cooling (expansion Lapse Rate  Lapse rate – rate of temperature decrease with increase in altitude  Average is 2ºC (3.5ºF) per 1,000 feet Water Vapor and Lapse Rate  Water vapor is lighter than air – Moisture decreases air density – causes air to rise – Less moisture – air is more dense – air descends  Moist air cools at a slower rate than dry air  Dry adiabatic lapse rate is 3ºC (5.4ºF) per 1,000’  Moist adiabatic lapse rate is 1.1ºC to 2.8ºC (2ºF to 5ºF) per 1,000’ Temperature and Moisture  Combined, determine the stability of air  Warm, moist air = greatest instability  Cold, dry air = greatest stability  Lapse rate can be used to determine the stability of the atmosphere Temperature Inversions  Temperature usually decreases with altitude  Inversion is when temperature increases with altitude  Usually in shallow layers  Near surface or at higher altitudes  Lid for weather and pollutants  In stable air with little or no wind and turbulence  Visibility usually poor Temperature Inversion  Clear, cool night, calm wind  Terrestrial radiation – Ground cools, lowers the temperature of air near ground  Cooler layer of air next to ground Frontal Inversions  Cold front – Cool air forced under warm air  Warm front – Warm air rides up over cold air Moisture  In terms of flight hazards – Very moist air – poor or severe weather can occur – Dry air – weather will usually be good State of moisture  Solid, Liquid, Gas  Evaporation  Condensation  Sublimation  Deposition  Melting  Freezing Latent Heat  Extra heat in changing state – either absorbed or released – 32º water to 32º ice  Every physical process of weather is accompanied by a heat exchange  Page 6-19, Latent heat diagram Humidity  Moisture in the air  Relative humidity – Actual amount of moisture in air compared to total amount that could be at that temperature  Amount of moisture in the air depends on air temperature Dewpoint  Temperature to which air must be cooled to become saturated – can hold no more water  Calculate cloud bases Temp ºF – Dewpoint ºF x 1,000 4.4 ºF Dew and Frost  Surface cools to temp below the dewpoint of surrounding air – Dew if dewpoint is above freezing – water vapor condenses – Frost if dewpoint is below freezing – water vapor changes directly to ice Frost and Airplanes  Frost – Spoils smooth surface of airfoil – Spoils the smooth airflow over wings – Decreases lift – Increases drag  Thou shall not fly an airplane with frost on it. Clouds  Air cools to saturation point  Condensation and sublimation changes vapor into visible moisture  Clouds, fog (clouds near surface)  Very small droplets or ice crystals  Condense or sublimate onto small particles of solid matter in the air – condensation nuclei Cooling of Air Clouds and Fog  Anticipate by noting temperature/dewpoint spread  Less than 4ºF (2ºC) of spread and decreasing – favorable for fog, clouds Types of Clouds  Grouped by families according to altitude  Low, fog  Middle  High  Clouds with vertical development Low Clouds  Surface to about 6,500 feet  Stratus – Layered, stable, uniform appearance, cover wide area  Nimbostratus – Nimbus means rain producing – Widespread areas of rain, thick layer, heavy icing if below freezing  Stratocumulus – White, puffy clouds Fog  Low cloud  Base within 50 feet of the ground  Ground fog if less than 20 feet deep  Classified by way forms – Radiation fog – clear, calm, humid nights – Advection fog – warm, moist air moves over cooler surface – Upslope fog – moist, stable air forced up sloping land – Steam fog – cold, dry air moves over warmer water, turbulence and icing hazard Middle Clouds  6,500 to 20,000 feet AGL  Altostratus – Flat, dense, uniform color, min. turbulence, mod. ice  Altocumulus – Patchy, uniform appearance, over wide area, often from altostratus clouds breaking up, light turbulence High Clouds  Above 20,000 feet AGL  Cirrus – Wispy, indicate stable air, white, patches or bands  Cirrostratus – Thin, white, long bands or sheets, low moisture content  Cirrocumulus – White, patchy, look like cotton, light turbulence Clouds with Vertical Development  Cumulus – In convective currents from heating of earth’s surface, flat bottoms, dome-shaped tops, fair weather cu’s, turbulence, little icing or precip  Towering cumulus – Large mounds of cotton, deep area of unstable air, heavy turbulence, icing, pre-thundestorm  Cumulonimbus – Thunderstorms, large, vertically developed, very unstable air, large amounts of moisture, heavy turbulence, icing, hail – many flight hazards Precipitation  Water, liquid or solid, that falls from the atmosphere and reaches the ground  Aviation problems – Visibility – Engine performance – Increased braking distance – Wind – shift direction, velocity – Icing Precipitation Causes  Need – Saturation of atmosphere – Growth of water or ice particles to point where atmosphere can not support them Precipitation Causes  Condensation/deposition  Coalescence  Slow and inefficient Precipitation Causes  Super-cooled water droplets  H2O in liquid form to temperatures as low as -40ºC  Water vapor from these droplets cause ice crystals to grow more quickly Types of Precipitation  Drizzle

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