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Met Revision Fact sheet
The Atmosphere
Oxygen 21%, Nitrogen 78% and all the rest 1% incl water vapour
Troposphere/Stratosphere/Mesosphere/Thermosphere (Ionosphere)
Tropopause Average 36,090 feet/11km -56.5C. At Poles = 26000feet/8km -50C. At Equator = 60000 feet/18km -75C.
Stratosphere 50km, temp 0C. Isothermal layer (-56.5C) up to 20km then increases.
Mesosphere 80km/-90C.
Thermosphere 2000C. Ionoshpere layer will affect radio waves so nav eqpt
ISA: Temperature: +15 C; Pressure: 1013.25; Density: 1.225kg/m
ISA Temperature lapse rate (uniform)
1.98 C/1000’ or 0.65 C/100m up to a height of 11km/36,090’.
ISA Pressure lapse rate (non-uniform)
1013.25 hPa/500 hPa (about half) 18,000/300 hPa 30,000’
Lapse rate greater closer to surface. Pressure is assumed to decrease at 1hPa for every 27 feet up to 10,000ft.
ISA Density (non-uniform) lapse rate
1.225kg/m3 Surface/0.609kg/m3 (about half) 22,000/0.302(quarter) 40,000’
Heating the Atmosphere
Scales: Celsius 100C, Kelvin 373 and Fahrenheit 212F.
Stevensons screen, placed about 4’ above the surface
Earth heated by Insolation.
Rates of heating vary by surface, latitude, time of year.
Energy transferred from surface in to atmosphere by:
Long wave Radiation, Conduction, Convection, Turbulence/Advection, Latent Heat
Diurnal variation of temperature
Hottest/coldest time of day
Inversions
Ground, Valley, Subsidence, Frontal, Tropopause Inversion
Atmospheric Pressure
ISA surface pressure: 1013.25hPa/29.92insHg/760mmHG.
Features on barometric chart : Isobars, H, L, COL, TROUGH, and RIDGE
QFF: MSL pressure in actual conditions (varies)
QNH: MSL pressure calculated using ISA lapse rate (27’/hPa)
QFE: Pressure at field elevation (Relationship between QFE & QNH is airfield elevation)
SPS: 1013.25hPa – Pressure altitude
Comparing QFF to QNH only possible if temperature given: amsl , warmer than ISA QFF will be lower value than QNH.
To find true altitude : use 27’/hPa for pressure correction. High to Low - Look Out Below.
Temperature Error Correction (TEC): 4% (of altitude) per 10°C of ISA deviation.
Warmer than ISA – true altitude will be more than indicated (under-read/safe)
Colder than ISA – true altitude will be less than indicated (over-read/danger)
Cloud Formation
Triggers Convection, Orographic, Turbulence, Convergence, Frontal
Rising air …….. Adiabatic cooling ……. Condensation …….. Cloud
Hygroscopic nuclei required for vapour to change to liquid or ice.
0C to -10C = mostly large SuperCooledWaterDroplets
-10C to -40C = smaller SCWD and ice particles
Below -40 = almost all ice particles.
Stability and instability:
DALR – 3 deg C/1000’ or 1 deg C/100m ;
SALR – 1.8 deg C/1000’ or 0.6 deg C/100m
ELR – actual lapse rate measured using radiosonde data so varies.
Why DALR greater than SALR – release of latent heat with condensation reduces rate of cooling. SALR increases at higher altitudes due less vapour content so less condensation.
Weather characterisitics due stability of atmosphere
Unstable: Cumuliform, showers, good vis, mod – severe turb. Cu unstable
Stable air: none or stratus, continuous rain, poor vis, light/mod turb. St stable
Cloud Classification
Clouds form when triggers (Terrain, frontal, convection, convergence) cause air to rise, air cools adiabatically.
If saturation (dew point) reached then condensation occurs, clouds form.
Type of cloud formation depends on stability of atmosphere
Cloud base : lowest level of cloud measured using ceilometer.
Cloud ceiling: lowest level of cloud obscuring +50% of sky (BKN/OVC)
Cloud Tops measured by radar, satellite, visual obs or triangulation.
Classification of cloud coverage
Appearance (3 main shapes) :
cirriform Stratiform Cumuliform
10 Basic Types (Genera)
Ci, Cc, Cs, Ac, As, Ns, Sc, St, Cu, Cb
High Cloud (ice clouds) – 16500 to 45000 (20000 to 60000 tropics). No precipitation
(Ci) Cirrus – wispy, Cs Cirrostratus – Halo. Cc Cirrocumulus – more bumpy.
Med Cloud (mixed clouds) – 6500 to 23000
As Altostratus - Layered cloud. Light precipitation – virga.
Ac Altocumulus – Lumpy blanket. Light precipitation, rain or virga.
Low Cloud (pure water clouds) – surface to 6500 (all regions)
Ns Nimbostratus – grey layered cloud, rain (continuous).
St Stratus – greyish white layered cloud
Sc Stratocumulus
- layered with lumps, patchy coverage.
Cu Cumulus – Cauliflower, build vertically, flat base, showers.
Ice pellets (PL); Like snow pellets but larger and made of clear ice.
Sleet: Mixture of rain and snow. Serious threat as can easily freeze onto airframe/runways.
Hail (GR) Small Hail (GS): Formed in CBs from frozen droplet caught in up and down draughts. Diameter Hail 5 – 50mm. Largest reported weighed 1kg. Small hail less than 5mm.
Freezing Rain (FZRA): Supercooled rainfall. Forms when cloud in warmer air (Ns) produces rain which falls into colder air (below freezing). Typical example is a warm front.
Precipitation and cloud type
Precipitation Clouds
Rain Sc, Ns, As, Cu, CB
Freezing rain Sc, Ns, As
Drizzle St
Freezing drizzle St
Snow Sc, Ns, As, Cu, CB
Snow grains St
Snow pellets Sc, CB
Hail & Small hail CB
Ice crystals St
Ice pellets Ns, As
Reporting Precipitation Continuous = 60 minutes or more usually stratus (As or Ns) Intermittent = continuous with breaks usually As or Sc. Snowfall likely Sc Showers = Short outbreaks associated with cumulus clouds
Thunderstorms
NEED: Deeply unstable air A lot of moisture A trigger to cause the air to rise.
Types – Heat (thermal)/Air mass/Frontal/Squall
Building Phase – updraughts, 20 minutes
Mature phase - Updraughts and downdraughts, minutes
Dissipating phase – Downdraughts, 1 – 2.5 hours.
Self-propagating storm - super-cell/Multi-cell (squall, cluster)
THUNDERSTORM HAZARDS
Microbursts Gust Front Turbulence & Windshear Icing Hail Tornadoes Water ingestion Virga Lightning Static Instrument error
Tornadoes: Most common mid-west USA in spring/summer;
- windspeeds 100-200kts in funnel;
- groundspeed 20 – 40kts;
- Average diameter 100 – 150m;
- Lasts up to 30mins.
Airborne Weather Radar (AWR) - sensitive to wet precipitation. Can mask multiple cells of precipitation.
TS avoidance: 20nm over 20,000’ 10nm below.
Icing
Effect on aircraft: centre of gravity , stall speed , lift, drag, thrust, control surfaces, instrument sensors, engine intake , carburettor , visibility
Factors: Speed , Wing shape, Temp, SCWD (0°C to -40°C), highest risk = 0 to -10°C.
Cloud types High risk of severe icing in cumuliform clouds in the range 0°C to – 20°C.
High (Ci, Cc, Cs) – IC, low icing risk
Med (As, Ac) – IC & small SCWD low to mod icing risk
Low (St, Sc) – large & small SCWD, poss some IC, low to mod icing risk
Ns, Cu, Cb – large SCWD at colder temps than normal, mod to sev icing risk
Orographic effect - Greatly increases icing risk
FOG = water droplets and solids, vis less than 1000m (999m).
MIST = water droplets and solids, vis 1000 and 5000m.
HAZE = solid particles, vis less than 5000m.
General Met viz (MOR – Met Optical Range) expressed in M to 5000 then KM
Types of Fog
Radiation: most common type inland fog. Forms over night.Ground cooling. Need: Clear skies (great diurnal temp range); High RH; Light wind 2 – 8 kt. Average depth 500’agl, can extend to 1500’. Often isothermal fog layer, inversion above. Often in valleys ( Valley fog ), due cold air descends into moist valley air. Dispersed by Sun/Wind - form St/Sc.
Advection Fog: Warm, moist, stable air over colder surface. Affects coasts (sea/land temps). Need: Wind up to 15 kt to move mass along; High RH; Cooler surface. Dispered by change of air mass, wind increase/direction change. Can lift to form st/Sc
Evaporation/Steam Fog/Arctic sea smoke: Opposite to advection fog. High latitudes in winter.
Frontal Fog: Approaching warm front. 200 miles wide and hundreds of miles long. Dispersed with passage of front.
Orographic/Hill fog/Cap cloud: Stable, humid air. Condenses into cloud/fog. Dispersed by increase in temp/wind.
Haze: Reduction in visibility due to solid particles (5000m). Dispersed by winds to lift smog or daytime heating to break inversion
Volcanic Ash: Damage to airframe and engines is the more significant threat.
Dust and Sandstorm : Wind exceeds 15 to 20 knots – 3000ft. Harmattan & Haboob = dry dusty wind, N Africa – can affect up to 15,000’.
Windshield Icing: Reduces visibility. Need to apply anti-icing.
Rising and setting sun: Glare and sun can reflect off top of clouds, haze or fog.
Slant or Air-to-ground visibility Oblique vis range in fog. Shallow angle, reduced vis range.
Reporting visibility: Metres. 2nd^ vis given if area less than 50% prevailing or below 1500m.
RVR: Surface vis in direction of landing. Transmissometer (IRVR). 7.5’ up (cockpit height).
IRVR 3 readings : threshold (300m), mid (1000-1500m), stop (300m), updates 60 seconds.
Averaged over 1 min ATC, 10 mins METAR and SPECI.
‘U’ upward tendency
‘D’ downward tendency.
Vertical Visibility. Fog/Haze. Measured in feet using e.g. VV002 = 200ft.
Wind
Surface wind measured at 10m by anemometer. Upper winds measured by tracked radiosonde/pilot reports
Units : knots, m/s, k/h. 1 kt = 0.52m/s = 1.85k/h.1 MPS = 1.94kts = 3.6k/h
Calm: less than 1kt;
Lull: sudden drop in wind;
Gust: sudden increase of +10kts;
Gale: wind speed 34 – 47 kts;
Storm: 48 – 63 kts;
Hurricane: +64 kts
Air moves high to low pressure = Pressure Gradient Force ( PGF ).
Coriolis force (CF) deflects fluid in motion to right in N Hemi and to left in S Hemi.
Increase in latitude (towards poles) causes increase in Coriolis effect.
Coriolis proportional to speed of object/wind.
CF = 2ΩρV sin Φ
CF = 2x angular rotation of earth x density x wind speed x sin of latitude.
Geostrophic wind = PGF and CF equal and opposite. Straight isobars.
Gradient wind = PGF, CF and centrifugal force. Curved isobars.
Centrifugal force acts with PGF around high but against PGF around low therefore wind faster around high for equal isobar spacing.
Surface wind & friction layer: N Hemi – backs and reduces at surface /veers & increases with altitude. Opposite in S Hemi.
Over land: direction of surface wind changes by 30 deg and speed reduces by 50% of 2000FT wind.
Over sea: 10 degree change at surface and speed reduces by 25%.
Over night friction layer shallows, windspeed reduces more.
Sub-Tropical jet - (FL400, 30 – 40N/S), west to east, all year round.
Polar Front jet - (FL300, 50 – 60N/S) - west to east all year round.
Seasonal jets – Arctic (west to East), winter, 70N (50N USA), FL200;
Equatorial/easterly jet – east to west, summer, 10-20N, FL500.
Jet Stream forms in warmer air mass below tropopause.
- About 200 miles wide, 2 miles deep, 2000 miles long
- Fastest at core
- Travels at speeds above 60kt, only reported on charts if over 80kt.
- Clear Air Turbulence (CAT) - most severe on cold air side.
Turbulence
Reporting Criteria (Above FL150)
Light turbulence/chop : slight strain against seatbelts, Food service continues.
Moderate turbulence/chop : definite strain against seatbelts, Food service difficult.
Severe turbulence : severe strain against seatbelts, Food service/walking impossible.
Mechanical : Mountains, strength affected by wind speed, height of obstacle.
Thermal: day time (most severe at 15:00), over land, unstable air.
Vertical Windshear - change in wind speed/direction with altitude, kts/100ft
Horizontal Windshear – wind change in horizontal plane, kts/1000ft
Worst windshear associated with TS; also Frontal passage, inversions.
Mountain Waves/Standing Waves/Lee waves:
- turbulence 50 - 100nm downwind.
- Stable air at top of mountain,
- wind speed min 15/20kts
- wind direction 90 degrees to mountain range.
- Cap, lenticular, rotor or roll cloud can develop,
- vertical windspeed 1000’/min, waves 1500’ deep.
Pressure Systems
(L) Low Pressure/Depression : Convergence, rising air, cloud, precipitation. Air flow anti- clockwise N Hemi/clockwise S Hemi.
(H) High Pressure/Anti-cyclone : Divergence, sinking air, calm, haze, poss fog (winter), TS (Summer). Air flow clockwise N Hemi/anticlockwise S Hemi.
Trough of Low Pressure: increased convergence, cloud, precipitation.
Ridge or Wedge of High Pressure: calm, same as (H).
Col: between 2xH, 2xL. Calm, haze, fog, poss TS over land in summer.
Warm anti-cyclone/High Pressure: warmer air at core so pressure levels bulge upwards away from surface e.g. Azores High, Pacific High.
Blocking anti-cyclone/High Pressure: (often warm H i.e. Azores) blocks path of Polar Front Depressions forcing them to deviate from usual track.
Cold anti-cyclone: colder air at core so pressure levels bulge upwards close to surface, downwards at altitude e.g. Siberian High/Canadian High.
Temporary Cold Anti-cyclone: forms between two travelling polar front depressions (mid- latitudes), relative High in cold polar air mass.
Tropical Continental ( summer) N Africa/S Europe. WX: Clear skies, hot and hazy. Poss fog in English channel. Poss Cbs/Cu on S Coast UK, TS late afternoon.
Polar Continental (winter) - Russia, very cold, low humidity. WX: Clear skies, Low temps, frost poss snow showers.
Returning Polar Maritime – PM tracks S (unstable layer) then N (lower stable layer). WX: low level St, poor vis. Winter drizzle, summer coastal mist, poss Cb.
Fronts
Front is where two different air masses meet e.g. the Polar Front between polar and tropical air masses. Located approx. 50 - 60N & S, July Canada to Norway/January Florida to SW UK.
Mediterranean Front only exists in winter between Pc over Europe and Tc over N Africa.
Arctic and Antarctic Fronts sit between arctic and polar continental air masses in winter.
ITCZ less defined (25 – 300nm wide), moves north in N Hemi summer months and south in N Hemi winter.
Stationary front: nil/stable weather, poss stratiform cloud and precipitation if humid enough. Precipitation will be long lasting/continuous and can be moderate or heavy.
Polar Front Depressions (Westerly Waves)
Cold polar air forms a cold front (slope 1:50). Warm tropical air forms warm front (slope 1:150). Wedge of warm tropical air in between is the warm sector.
Warm Front Cloud Formation – stratiform; Ci, Cs, As, Ns, St. WX: Rain/snow (Ns), poss fog ahead of the front. Wind southerly. WX extends 500-700nm belt ahead of surface position. Warm sector weather – TM air; low St/Sc. WX: fog or drizzle, poor vis. Wind westerly.
Cold Front Cloud Formation – cumuliform; Cu, Cb. WX: Rain showers, poss TS & squalls. Wind northerly. WX extends 50- 100nm belt behind surface position.
Cold Sector Weather - PM or PC. Cumuliform cloud. WX: showers, good vis (except in SH). Wind north westerly.
Pressure decreasing with approach of warm front, steady in warm sector, increasing behind cold front; Wind southerly ahead of warm front, veers at warm front, veers at cold front.
Warm front travels at 2/3 speed of cold front so cold catches up with warm front = occluded front. Mix of cold and warm front cloud formations, poss embd CBs.
Warm occlusion: (why warm = looks like letter ‘y’) cold air ahead of warm front is colder than air behind cold front so less cold air (PM?) forced to rise over coldest air (PC?). More often in winter. Precipitation falls ahead of occluded front (warm front characteristics).
Cold occlusion: cold air behind the cold front is colder (more dense) than the air in front of the warm front it will undercut the less cold air. More often in summer. Narrow rain belt both sides but mostly ahead or surface occlusion. Narrow belt of precipitation both sides but mostly ahead of surface occlusion.
Back-bent occlusions: Extend from centre of depression where occlusion starts. Belt of rain in cold air behind cold front, sometimes TS.
Non-frontal Depressions
Secondary Depressions: Form within circulation of primary low; can be thermal or orographic
Orographic Depressions: strong wind, mountains, low on leeward side. If unstable air can expect Cu or Cb. Showers, TS, Hail. Worse with cold front passing over e.g. Alps in N Italy in winter – squalls, heavy showers.
Thermal Depressions: due thermal lifting causing low pressure at surface. If unstable air, can be active up to the tropopause, Cu, Cb, TS, hail.
- Over water lows (Winter): PC air over relatively warmer seas/lakes – Med, Black Sea. - Polar Air Depression/Polar Lows: Arctic Maritime air - Norwegian, Barents, Sea of Japan, Gulf of Alaska.
Ch 21 Remote Sensing/Met Observation
Weather Radar: Reflects wet precipitation. Ground based weather surveillance radar (WSR). Airborne weather radar (AWR): mounted in aircraft. Tilt Control to assess returns depth.
Limitations: - Blind Alley effect: threats between weather cells;
- Masking: heavy precipitation may cloak weather cells behind. - TS avoidance: Above 20,000’ – 20nm. Below avoid by 10nm. Lightning Detectors: Stormscope/Strikefinder if not integral to AWR.
Geostationary Satellites: 35,786km above equator, e.g. GOES, Meteosat, and EUMETSAT. View whole planet but images relatively poor resolution. Identify TRS activity.
Polar Orbiting Satellites: Orbit twice daily 850 km. e.g. NOAA, MetopA, Meteor and RESURS. Better resolution and detection of weather fronts.
Visual Imagery: reflects sunlight; shows cloud patterns/storm systems/snow cover/weather fronts. Disadvantages: daylight only; Difficult to distinguish low from high clouds and snow from clouds in winter.
Infra-red Imagery: senses heat (day and night). High altitude cloud = cold, low-level cloud = warm. Helps identify location of clouds, fronts, jet streams.
Most effective use of imaging is to compare visual with infra-red.
Ch 22 Reporting and Forecasting
Met Services: ICAO set up World Meteorological Organisation (WMO) to standardise weather reporting and data dissemination then WAFS in 1982 for digitalised reports and forecasts.
Information Source Organisations within WAFS: World Area Forecast Centres (upper air SIGWX). London & Washington Meteorological offices (TAF) All over world Meteorological Watch Offices (SIGMET) Meteorological Stations (METAR, SPECI, TREND) Aerodrome based Volcanic Ash Advisory Centre (VAAC) 9 across the world Tropical Cyclone Advisory Centre (TCAC)
Data collected from aircraft in flight (AMDAR)
Pre-flight planning/briefing sources: Routine Reports: METAR (issued every 30 or 60 mins); Landing forecast/Trend (as required); SPECI (change of operational significance, as required.)
Routine Forecasts: TAF (every 3 or 6 hours. Valid 9-36hrs); SIGWX charts (every 6 hrs, upper levels); GAMET (Forecast low-level, text format); GAFOR (Low level vis/cloud (VFR))
En route reports: VOLMET (METAR for up to 10 aerodromes. Wind True ) ; ATIS (wind Magnetic ) ; Weather Warnings (SIGMET/AIRMET)
N.B. Surface wind speed and direction averaged over 10 minutes before METAR update.
Given in °True in METAR and TAF but in degrees °Magnetic by ATC & ATIS.
