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Meteorology Tim Freegarde www.uskgc.co.uk Meteorology • what you need for the Bronze exam • useful knowledge for flying • terminology • principles • how it all works Q. Questions from Bronze & Beyond 2 Terminology • • • • • • • • • • • • • • • • • • air mass anabatic anticyclone cloud (common types) convection convergence Coriolis force cyclone, high, low depression dew point diurnal drizzle fog Föhn effect front (warm, cold, occluded) geostrophic hail humidity • • • • • • • • • • • • • • • • • • inversion katabatic lapse rate lee waves mist orographic pressure rain ridge saturation sea-breeze/-front snow synoptic chart temperature thermal trough visibility wind/-gradient/-shear 3 Principles of meteorology 1. 2. 3. 4. 5. hot air rises the Earth rotates solar power drives everything warm air can hold more moisture Newton was right 4 The Earth’s atmosphere • layer of gas surrounding earth • weather occurs in troposphere • at sea-level pressure, ~8.5 km thick PRESSURE /hPa 0.1 1 10 250 1013 Randy Russell, UCAR PRESSURE • 1 mbar = 1 hPa • 1 mbar per 27 ft 5 Convection & thermals • • • • sunlight heats the ground ground heats the air hot (less dense) air rises atmosphere is transparent THERMAL 250 WIND 1013 Randy Russell, UCAR 6 Global circulation • • • • • • • more dense sunlight heats the ground ground heats the air hot (less dense) air rises atmosphere is transparent warm air rises at the equator cool air descends at poles but… less dense Remote Sensing Tutorial, NASA THERMAL 250 WIND 1013 Randy Russell, UCAR 7 Coriolis effect H Coriolis 300 m s-1 L • • • • pressure gradient L 460 m s-1 tendency to turn right in N hemisphere to observer moving with Earth, appears as virtual force pressure gradient balances Coriolis & centrifugal forces at altitude, flow follows isobars around high/low pressure 8 Buys-Ballot’s law H Coriolis 300 m s-1 L pressure gradient L • with the wind on your back, in the N hemisphere, the low pressure is on the left 460 m s-1 Low High 1. What does Buys-Ballots law state for the northern hemisphere? 9 Global circulation H L Coriolis pressure gradient L • Coriolis effect breaks up the circulation into cells and cyclones • prevailing/trade winds • weather & climate • real situation complex & dynamic HIGH LOW COLD HIGH NOAA WARM 10 Global circulation HIGH LOW LOW • sea, land, ice affect heating • UK rarely far from low pressures HIGH LOW COLD HIGH NOAA WARM 11 Global circulation OCCLUDED FRONT HIGH PRESSURE WARM FRONT LOW PRESSURE COLD FRONT • sea, land, ice affect heating • UK rarely far from low pressures HIGH LOW COLD HIGH NOAA WARM 12 Global circulation OCCLUDED FRONT HIGH PRESSURE WARM FRONT LOW PRESSURE COLD FRONT • sea, land, ice affect heating • UK rarely far from low pressures HIGH LOW COLD HIGH NOAA WARM 13 Global circulation SUMMARY • convection • sunlight warms earth • hot air rises at tropics (LOW) • cool air descends at poles (HIGH) • spinning Earth • Coriolis effect breaks up circulation into cells and cyclones • weather where air masses converge • fronts tend to distort and twist Low High HIGH LOW COLD HIGH NOAA WARM 14 Wind 360 090 270 AT ALTITUDE • geostrophic wind • follows isobars • Buys-Ballot’s law • speed from isobar spacing NEAR GROUND • friction slows air movement • reduced centrifugal/Coriolis force • flow from High to Low pressure • wind veers with height STRENGTH & DIRECTION • bearing from which wind comes • speed in knots (1 kt = 1.15 mph) 180 24012KT 12 knots from SW 04515G25KT 25 knot gusts 3. does the wind direction vary youbydescend to which grounddirection level from 2. How If a due westerly wind is forecast toas veer 10°, from will2000'? it then come? 15 Synoptic charts OCCLUDED FRONT HIGH PRESSURE WARM FRONT GEOSTROPHIC WIND • closer isobars → higher Coriolis force → stronger wind LOW PRESSURE COLD FRONT • isobars: pressure contours (4 hPa) • cold/warm fronts divide air masses AIR MASSES • cold/warm; dry/humid 16 Synoptic charts 36 00 THU 19 MAR 2015 H 1028 L 990 • note times of forecast & validity • …but why the weather? SUMMARY • geostrophic wind • follows isobars • Buys-Ballot’s law • strength from isobar spacing • low pressure • cyclone • wind, rain, cloud • high pressure • anticyclone • calm, clear/fog • fronts & air masses • cold/warm • dry/humid 17 Weather fronts 36 00 THU 19 MAR 2015 H 1028 L 990 WARM COLD convection stops at warm air WIND COLD FRONT WARM FRONT WARM FRONT • shallow slope • slower than wind • cloud layers • descending cloudbase • steady rain/drizzle • poor visibility COLD FRONT • steep slope • moves with wind • towering cloud • rising cloudbase • squalls & showers WIND • strengthens & veers as cold front passes 18 Warm front CIRRUS CIRROSTRATUS ALTOSTRATUS WARM FRONT • shallow slope • slower than wind • cloud layers • descending cloudbase • steady rain/drizzle • poor visibility NIMBOSTRATUS 12. In the summer, what is often the first sign of an approaching warm front? Cloud Appreciation Society 19 Cold front CUMULONIMBUS CUMULONIMBUS COLD FRONT • steep slope • moves with wind • towering cloud • rising cloudbase • squalls & showers • wind strengthens & veers as cold front passes CUMULUS 17. It is July and the weather feels fresh. A ridge of high pressure is forecast. What weather would you expect? Cloud Appreciation Society 20 THERMAL WIND 1000’ height Convection TOP OF THERMAL RISING AIR SURROUNDING AIR 3°C temperature LAPSE RATES /1000’ • fall in temperature with increasing altitude • clear, rising air dry adiabatic lapse rate 3°C • actual temperature environmental lapse rate • condensing air saturated adiabatic lapse rate 1.5°C AIR RISES • if it is warmer than the surrounding air RISING AIR • expands as the air pressure falls EXPANDING AIR • cools as it pushes the air aside COOLING AIR • continues to rise if it is warmer than the air around 9. What are the environmental lapse rate, the dry adiabatic lapse rate and the saturated lapse rate? 21 height Convection THERMAL WIND INVERSION TOP OF THERMAL RISING AIR UNSTABLE SURROUNDING INVERSION AIR temperature LAPSE RATES /1,000’ • fall in temperature with increasing altitude • clear, rising air dry adiabatic lapse rate 3°C • actual temperature environmental lapse rate • condensing air saturated adiabatic lapse rate 1.5°C STABILITY • inversion if environmental lapse rate < dry adiabatic i.e. rising air cools more quickly than air around • unstable air mass if environmental LR > adiabatic LR THE AIR AROUND • is heated by mixing with the warm air IF CONDITIONS STEADY • convection continues until environmental = dry adiabatic lapse rate HEATING CAN CHANGE • cloud cover • time of day • geography • radiative cooling AIR MASS CAN CHANGE • fronts • sea breezes • downdrafts 22 height Water TOP OF THERMAL RISING AIR THERMAL WIND SURROUNDING AIR temperature CONDENSATION NUCLEI • condensation generally requires a surface or seed particle (salt crystal, pollution) • in clear air, supersaturation AIR CONTAINS WATER • above a certain amount, water vapour will condense • saturated air contains maximum water • warm air can hold more water than cold RISING AIR • cools until saturation • vapour condenses • dew point 8. What is the dew point? 14. What is the cause of radiation fog? 23 height Water TOP OF THERMAL RISING AIR THERMAL WIND SURROUNDING AIR dew point temperature CUMULUS CLOUD • flat base, where rising air reaches dew point • latent heat gives extra buoyancy CONDENSING RISING AIR • releases latent heat • cools less quickly • saturated lapse rate • condensing air will rise higher than dry air BRADBURY RULE • height of cloudbase = (max-min temp) x 400’ • if inversion lower, thermals will be blue 10. The maximum temperature is forecast to be 22 °C, and the dew point is 7 °C. Roughly how high will the cumulus cloudbase be? 15. What would be the effect on thermals of an inversion layer in an anticyclone in the summer? 24 Convective cloud THERMAL WIND GROUND LEVEL WINDS • flow along ground towards thermal → local changes in wind strength and direction • convection mixes lower and upper air → wind picks up by day, slackens over night 5. What causes the windspeed to increase in the morning and decrease in the evening? WHAT COULD POSSIBLY GO WRONG? • powerful convection • strong downdrafts • wind shifts, turbulence • lightning • heavy rain, hail • visibility 25 Convective cloud THERMAL WIND 13. You are landing your glider near a thunderstorm. What is your greatest risk? 16. You notice that the cumulus clouds are rising to great heights above their bases and flattening at their tops. What could happen next? 19. You are on a gliding holiday in the Pyrenees and because thunderstorms are forecast, you decide not to fly that day. Should you de-rig the glider? WHAT COULD POSSIBLY GO WRONG? • powerful convection • strong downdrafts • wind shifts, turbulence • lightning • heavy rain, hail • visibility 26 Topography AIR CANNOT PASS THROUGH ROCK • wind blows along valleys • wind rises over hills (lift), falls into valleys (sink) • air rising over hills can form orographic cloud SUNSHINE HEATS ROCK • air flows up sun-facing slopes (anabatic wind) • air flows down unlit slopes (katabatic wind) • warm terrain creates thermals • mountain ranges draw in air WEATHER can be very local 11. You are ridge soaring and find that cloud is forming over the hill. What is the type of cloud called? 21. In which direction would the wind tend to blow in the mountains in high summer during the afternoon? 27 Topography AIR CANNOT PASS THROUGH ROCK • wind blows along valleys • wind rises over hills (lift), falls into valleys (sink) • air rising over hills can form orographic cloud THE WHOLE ATMOSPHERE CAN RIPPLE • strong wind over mountains can cause wave • lenticular cloud can mark wave crests • strong, smooth lift and sink aloft • violent rotor below Gordon Boettker 4. There is a strong cross-wind. You want to land alongside and downwind of a row of trees and large hangar. What might happen? 20. What might you experience near a rotor cloud? 28 Sea breezes SEA HEATS & COOLS VERY SLOWLY • warmer land by day forms sea breezes • cool air from sea suppresses convection • front forms inland where cool and warm air meet • night air leaves cooler land as land breeze 6. What effect will a sea-breeze front have on your plans if you have to fly across one? 7. What effect will the passage of a sea-breeze front have had if you make a field landing? 29 Satellite photographs Analysis, midday 24 hour visible satellite view 18. There has been a shower while your glider has been at the front of the winchlaunch queue but flying is about to restart. What should you do? 30 Books and websites • • • • • www.metoffice.gov.uk/learning/learn-about-the-weatherhow it all works www.metoffice.gov.uk/learning/library/publications/factsheets www.weatherjack.co.uk www.meteoblue.com www.xcweather.co.uk [email protected] 31