Weather Theory – 1 Quick note: water vapor is lighter than air, moisture decreases air density causing it to rise - Convection – vertical transport of atmospheric properties Advection – horizontal transport of atmospheric properties 78% N2 & 21% O2 Water Vapor – air contains some water vapor, varying from 0 – 5% by vol (small amount is responsible for major changes in weather) Troposphere – 20,000 ft – 4 – 12 miles over northern/southern poles (48,000 over equatorial regions) avg temp decreases at rate 2 Celsius every 1,000 ft of altitude gain Tropopause – boundary located at top of troposphere that traps moisture/associated weather in troposphere (alt of tropopause varies with latitude/seasons (takes on elliptical shape) - Commonly associated with location of the jet stream & possible clear air turbulence Stratosphere – 160,000 ft – temp increases (ozone absorption, heat source is sun) Mesosphere – 280,000 ft – temp decreases (relatively no ozone, fewer molecules to absorb heat) Thermosphere – until space Temp increases (O2 absorbs short wavelength solar energy, but feels cold – fewer molecules in air) Atmospheric circulation & Weather – atmosphere is in constant motion due to uneven heating of Earth’s surface, creates changes in air movement and atmospheric pressure (in short: movement of air around surface, results when warm air rises and is replaced by cooler air) Air movement – warm air rises because heat causes molecules to spread apart. As air expands, it becomes less dense and lighter than the surrounding air. As air cools, molecules are more packed together, being denser and heavier than warm air - Cool, heavy air sinks and replaces warm, rising air Atmospheric pressure – Measured in inches of mercury (Hg) by aneroid barometer, is unequal heating of earth’s surface is cause of weather: modifies , creates circulation patterns, causes changes in air pressure or force exerted by weight of air molecules, result patterns reveal pressure gradient - Lines of equal pressure are isobars, when spread apart considered weak, close = strong Pressure differs with altitude, temperature, and density of air Standard Sea Level Pressure – 29.92 Hg, standard temp of 59 F (15 C) also 1,013 mb - ISA – International standard temperature Standard lapse rate - 1 Hg / 1000 altitude gain/loss Temp: 2 deg C / 1000 altitude gain/loss and 3.5 deg. F / 1000 altitude gain/loss Coriolis Force – force created by rotation of earth. Deflects air to the right in the Northern hemisphere, causes it to follow a curved path instead of straight line - Amount of deflection differs depending on latitude (0 at equator) also differs with speed of moving body – greater speed = greater deviation General flow to break up into three distinct cells in each hemisphere - 30 to equator, 30-60 latitude, between 60 and the poles Hadley cell – air still rises at equator and flows toward poles, rotation only reaches to 30 laitude where air sinks again Ferrel cell – rotates between 30 – 60 latitude Polar cell – rotates between 60 latitude and the poles Frictional Forces – factors that affect circulation patterns are seasonal changes, difference between the surfaces of continents/oceans/frictional forces - Are caused by topography of earths surface that modify the movement of air in atmosphere (within 2,000 feet of the ground, friction between surface and atmosphere slows moving air) Altitude and Flight Higher altitudes = decreased atmospheric pressure = T/O & landing distances are increased, while climb decreases Winds and Currents – in the Northern Hemisphere, the flow of air from areas of high to low pressure is deflected to the right and round - Highs – clockwise flow, anti-cyclonic circulation, dry descending air associated with good weather Lows – air flows toward a low and is deflected to create counterclockwise, cyclonic circulation, bad weather due to rising air that creates clouds/precipitation Weather Theory – 25 Atmospheric stability – stability (determined by Ambient lapse rate) of the atmosphere depends on its ability to resist vertical motion, temperature and moisture determining the stability of air - Stable atmosphere – makes vertical movement difficult Unstable Atmosphere – small vertical air movements become larger, resulting in turbulent airflow and convective activity Instability – leads to turbulence, extensive vertical clouds, and severe weather Temperature Inversion – an increase of temperature with altitude, usually shallow layer, commonly smooth air close to the ground, occur in stable air with little or no wind or turbulence (acts as lid for pollutants and weather) Moisture: - Evaporation – water to vapor – liquid to gas – absorbs latent heat Condensation – vapor to water – heat released – releases latent heat Melting – solid to liquid – absorbs latent heat Sublimination – solid to gas – absorbs latent heat Humidity – amount of water vapor present (if does exist, wise to add 10% to takeoff distance and anticipate a reduced climb rate Relative Humidity – actual amount of moisture in air Dew Point – temperature at which air can hold no more moisture Atmospheric Stability – (Clouds) warm, moist air rising in a convective current – clouds often form at the altitude where temperature/ dewpoint reach same Fog (FG) - is a cloud that is on the surface (visibility less than 5/8 mile – AIM) occurs when temperature of air near ground is cooled to the air’s dew point (caused by Water vapor in the air condenses and becomes visible) - Radiation – when ground cools rapidly due to terrestrial radiation, and surrounding air temp. Reaches its dew point Advection – occurs when a layer of warm, moist air moves over a cold surface Upslope – when moist, stable air is forced up sloping land features like a mountain range Steam – cold, dry air moves over warm water Ice – occurs in cold weather when the temp. Is below freezing and water vapor forms direct into ice crystals Ground Fog – less than 20 feet thick (up to 6 feet thick) Clouds – small droplets of water or ice crystals, droplets condensed/sublimated on condensation nuclei (usually form when air becomes saturated) - Low – SFC – 6500 Middle – 6500 – 20,000 High – 20,000 – Higher clouds with vertical development Precipitation – (contributes to weather aviation problems) any form of water particle, liquid/solid, that falls from atmosphere and reaches ground - Can reduce visibility, create icing situations, affect landing/takeoff performance of aircraft Causes: condensation/deposition, coalescence, ice crystal process – causes most precip Virga – mass of streaks of rain appearing to hang under a cloud and evaporating before reaching ground Airmasses – large body of air with uniform temperature/moisture content Fronts – boundary between airmasses (cold - blue, warm - red, stationary – red/blue, occluded -purple) Frontal discontinuities – can be abrupt/occur over more distance, Temperature – one of most easily recognized, wind – most reliable indicator – shifts to right in Northern, pressure – as front approaches pressure decreases – most important to set altimeter Thunderstorms – three conditions needed: Unstable lapse rate (unstable atmosphere), Lifting action – rising terrain (orographic) fronts and heating of earth’s surface(convection), high moisture content of atmosphere (turbulence and hail are greatest thunderstorm hazard to aircraft) - Three stages: cumulus – lifting action of air begins Mature – more updrafts than downdrafts (most violent part of cycle) begins when rain reaches earth’s surface Dissipating – the downdrafts spread out/replace updrafts needed to sustain the storm Squall lines – most intense weather hazard o an aircraft, narrow band of active thunderstorms, develops ahead of cold front and contains steady state thunderstorms Tornadoes – draw air that has a rotating motion into their cloud bases, can occur with isolated and squall line thunderstorms, tornado vortex – LOC and a/c structural damage - Funnel cloud – if cloud doesn’t reach surface, tornado – touches land, waterspout – cloud touches water Turbulence – present in all thunderstorms, strongest turbulence within the cloud occurs with shear between updrafts/downdrafts. - Outside loud, shear turbulence has been encountered several thousand feet above & 20 miles laterally from a severe storm “Roll cloud” - located on leading edge of storm, signifies an extremely turbulent zone Low level turbulence – (Below 15,000 MSL) Mechanical – obstructions, Convective – thermals caused by heating of air at surface, Frontal – fronts Wake Turbulence – caused by wingtip vortices (greatest danger: large, heavy, low speed, high AOA, clean config. Clear Air Turbulence (CAT) - above 15,000 MSL normally but can be at any altitude – no visual warning. Caused by interaction of layers of air with differing wind speeds, convective currents/obstructions, jet stream Mountain Wave Turbulence – possible as stable air moves across a ridge at 40 Knots or greater, may extend 100 miles downwind, cruise at least 3-5,000 above peaks, approach ridge at 45 degree angle - If winds at planned altitude exceed 30 knots, FAA recommends o flight for small a/c Icing – 2 conditions needed to form, visible moisture and temperature of aircraft surface at or below 0 degrees C - - Effect on aircraft – thrust reduced, stall speed/drag/weight incr., and lift/performance decreased Rime – stratus clouds with instantaneous freezing of tiny supercooled water droplets Clear ice – cumulus/freezing rain below warm front inversion – large supercooled drops slowly freeze and glaze the aircraft surfaces (most serious form – adheres/hard to remove) most likely 0 – 10 C Mixed Ice - -10 to -15 C Hail – supercooled drops above freezing level begin to freeze. Once a drop has frozen, other drops latch on and freeze to it. Large hail occurs – with severe thunderstorms with strong updrafts that have built to great heights. Lightning – always associated with thunderstorms Restrictions to Visibility – HAZE, smoke, smog, dust, volcano, fog (AIM – chap. 8) Windshear – any rapid change in wind direction/velocity. Can occur horizontally/vertically. At low or high altitudes. Lasts 5 – 15 minutes - Usually occurs in topographical conditions, temperature inversions, sea breezes, frontal systems, strong surface winds, thunderstorms, and wind showers Microburst - Has a diameter of 1-2 miles at 30-90 knots, almost any convective activity can be experienced and longer than 15 minutes Concentrate/powerful downdrafts that can be as strong as 6,000 fpm. Extreme danger near ground Avoid: look for clues in LLWAS alerts, ATC, weather reports Recognizing: abrupt change in airspeed/altitude, AC 00-54, pilot Windshear Guide Aviation Weather Services 1-800-WX-Brief METAR – Routine aviation Weather Report, an observation of surface weather that’s reported in a standard format AWOS – Automated weather observing system - AWOS-A altimeter setting only AWOS-AV altimeter and visibility AWOS-1 altimeter setting, wind data, temp, dew point, density altitude AWOS-2 ETC. AIM 7-1-12 ASOS – Automated service observing system – will provide minute by minute observations and perform the basic observing functions necessary to generate an aviation routine weather report (METAR) ATIS – automated Terminal Information Service – the continuous broadcast or recorded non-control information in selected high activity terminal areas - Includes airport, phonetic letter, time of last weather sequence, weather consisting of: wind direction, visibility, obstructions to vision, present weather, instrument approach, runway in use Upper Air Observation – Radiosonde observations, PIREPs: pilot weather reports, Aircraft Meteorological Data Relay (AMDAR) , meteorological Data Collection and Reporting system (MDCRS) Radiosonde = Balloon Observation – a small cubic instrumentation package that is suspended below a six-foot hydrogen/helium filled balloon, rises 1,000 fpm, gathers air temp, moisture, pressure, wind speed/direction, then relayed to ground stations Pilot report – PIREPS – only real time source of information about turbulence, icing, cloud heights - UA: routine - UUA – Urgent (includes hazardous weather such as icing and severe turbulence Radar Observations – provide precipitation and wind, doppler radar, FAA terminal doppler weather radar (TDWR), FAA airport surveillance radar, airborne radar Flight Service Station (FSS) - primary source for preflight weather information, 1-800-WX-BRIEF anywhere in US and PR, provides weather briefing services/advisories at 122.2 Telephone information Briefing service (TIBS) - provided by FSS, automated recordings of meteorological/aeronautical info, provides area and route briefings, airspace procedures, and special announcements Transcribed Weather Broadcast (TWEB) - Alaska only Primary Weather report – aviation weather product that meets the regulatory requirements/safety needs for use in making flight related, aviation weather decisions (FSS) Supplementary Weather Product – aviation weather products that may be used for enhanced situational awareness (aviation weather center) Preflight briefings – available 24/7 Preflight weather briefing – Always ID yourself as a pilot and provide: type of flight, A/C tail number and type, dep, route, destination, altitude, ETD and ETA. (will need a rough plan of desired flight so agent will know area and time required for briefing. Standard Weather Briefings (SWB) - shoulf be requested when planning the trip and has not obtained a preliminary weather, is the most complete WX briefing since it assumes that your not familiar with weather, briefer automatically provides (adverse conditions, VFR flight not recommended) - Adverse conditions, VFR not recommended, synopsis, current conditions, en route forecast, destination forecast, NOTAMS, ATC delays, on request information Abbreviated Briefing – when information to supplement mass disseminated data is needed, update a previous briefing, when you need on or two specific items Outlook Briefing – an Outlook briefing whenever your proposed time of departure is 6 or more hours from the time of briefing, planning purposes only, obtain a standard briefing prior to departure
