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General circulation of the atmosphere and climatic zones Radiation Conduction Shallow layer heated by contact Convection Clear day Overcast day Clear night Terrestrial Long wave Clear night Terrestrial Long wave Absorption by water vapour and CO2 Large net loss Overcast night Solar heating 2% absorbed 20% absorbed CLOUDS AND DUST 35% reflected CLOUDS, ICE SNOW, WATER 43% reaches earth Why is it coldest at the ‘poles’..... ... and warmest in the tropics ? Area heated Depth of atmosphere N Pole Equator ‘Still’ earth ‘Still’ earth 40N 20N 0 20S 40S Rotating earth Polar Disturbed Temperate Warm Temperate Arid Sub-Tropical 40N 20N Tropical 0 Equatorial Tropical 20S Arid Sub-Tropical Warm Temperate 40S Disturbed Temperate Polar Rotating earth Any questions? Air Masses Air Masses Definition: A large body of air (covering thousands of square miles) in which, at a given level, temperature, lapse rates and humidity characteristics are almost uniform Air Mass - Origins Polar Tropical Maritime Track Evaporation moistens air raises dew point Maritime Track Heating (from below) raises temperature makes air unstable Maritime Track Cooling (from below) lowers temperature makes air stable Continental Track Evaporation has little effect Continental Track Heating (from below) increases temperature makes air unstable Continental Track Cooling (from below) lowers temperature makes air stable traps pollutants Tracks into Britain Polar Maritime (Arctic) Tropical Maritime Polar Continental Tropical Continental Returning Polar Maritime Air Mass - Tracks Polar Maritime Polar (or Arctic) Maritime Polar Continental Returning Polar Maritime Polar Continental Tropical Continental Tropical Maritime Tropical Continental Polar Maritime Cold air over warm sea Very Long Sea Track Polar Maritime Iceland Scotland Polar Maritime Cold and dry Warming from below -05 /-10 Increasingly unstable Moistening -02/-07 00/-03 Iceland Scotland 06 07 08 09 08 Polar Maritime Iceland Scotland Polar Maritime • • • • Unstable air mass Sct Cu/Cb cloud Showers of rain or snow Visibility very good, except in showers Iceland Scotland Tropical Maritime Warm air over cold seaVery Long Sea Track Tropical Maritime Azores SW England Tropical Maritime Warm Rather moist Cooling from below 23/15 Increasingly stable Moistening 19/16 16/16 Azores SW England 22 20 18 17 16 Tropical Maritime •Stable •Ovc StSc •Drizzle, hill/coast fog Azores •Poor Polar Continental Summer Warm air over warm Short Sea Track land Polar Continental Summer 24/06 25/05 16 Kent English Channel 25/05 26 North France 27 Germany Poland Polar Continental Warm air over cold seaLong Sea Track Summer Polar Continental Summer 16/14 14 Lincolnshire 21/09 North Sea 17 25/05 Baltic North Germany 16 Latvia Polar Continental Stable Ovc StSc Drizzle, hill/coast fog Moderate/poor visibility North Sea Lincolnshire Summer Baltic North Germany Latvia Polar Continental Winter Cold air over cold Short Sea Track land Polar Continental Winter -09/-17 -12/-20 07 Kent English Channel -12/-20 -15 North France -15 Germany Poland Polar Continental Stable Nil cloud Haze Moderate visibility Kent English Channel North France Winter Germany Poland Any questions? Wind Terminology • Veering – Direction changing in a clockwise sense • Backing – Direction changing in an anti-clockwise sense 260 deg 180 deg VEERING 110 deg 200 deg BACKING Pressure gradient low pressure high pressure By sloshing water in a bath a horizontal pressure gradient is created on the bottom of bath, and as a result a flow is established from high to low pressure (this is analogous to pressure gradients in the atmosphere) ‘Still’ earth 100 W 100 W 60N 500 mph 60N 30N 870 mph 30N Imagine a southerly airflow blowing over the coordinate system As a result of the rotating coordinate system the southerly airflow veers PGF = pressure gradient force GF = geostrophic force (or coriolis force) PGF PGF 986mb PGF V 988mb 990mb B PGF GF 992mb 994mb 996mb GF GF PGF GF 998mb 1000mb A At B, PGF and GF are equal but opposite in direction, The resultant wind (V) is the geostrophic wind Buys Ballot’s Law In the northern Hemisphere standing with your back to the wind, the area of low pressure is on your left In which direction does the wind blow ? L H 984 988 992 1032 1028 1024 Surface Wind Speed To determine the surface wind speed over the land: Multiply the geostrophic wind speed by 0.5 (by day) Multiply the geostrophic wind speed by 0.3 (by night) To determine the surface wind speed over the sea: Multiply the geostrophic wind speed by 0.75 Surface Wind Direction To determine the surface wind direction over the land or sea: Subtract 20o from the geostrophic wind direction (over land) Subtract 10o from the geostrophic wind direction (over sea) Gusts To calculate gusts, multiply the surface wind by one of the following factors: Open Sea Isolated hill tops Flat open country Rolling country (few wind breaks) Rolling country (numerous wind breaks, 1.3 forests, towns, outskirts of large cities) 1.9 Centres of large cities 2.1 1.4 1.6 1.7 Beaufort Wind Scale Force Speed (knots) Description 0 <1 Calm 1 1-3 Light air 2 4-6 Light breeze 3 7-10 Gentle breeze 4 11-16 Moderate breeze 5 17-21 Fresh breeze 6 22-27 Strong breeze 7 28-33 Near gale 8 34-40 Gale 9 41-47 Severe gale 10 48-55 Storm 11 56-63 Violent storm 12 >64 Hurricane Practical • Calculate surface winds at different locations – Measure isobaric spacing – Use geostrophic wind scale to estimate geostrophic wind – Correct for friction Geostrophic wind scale Diurnal variation of wind Day Night H (ft) 2000 Surf H geostrophic= 30 kt 10 20 30 (ft) 2000 Surf V (kt) geostrophic= 30 kt 10 20 30 V (kt) Summary • Friction:– leads to the surface wind blowing across the isobars towards low pressure – is negligible above 2000 ft (i.e. above the friction layer) – varies daily, as a result of thermal, turbulent mixing Local winds • The cause of a local irregularity in the pressure pattern might be ? – Topography (shape of the land) – Heating/cooling – Combination of the above LAND AND SEA (LAKE) BREEZE CIRCULATIONS ACROSS A SHORELINE Day DURING ANTICYCLONIC WEATHER Night Clear night and a light wind (i.e. slack pressure gradient) Air in contact with ground cools by conduction and its density increases, causing it to flow down the slopes of the valley Katabatic wind Met Office Anabatic wind The reverse argument for katabatic wind Pressure pattern Air funnels through valley at greater speed Summary • The causes of local winds are:– Topography – Heating or cooling – A combination of the above • Examples of local winds are:– – – – Katabatic Anabatic Valley Sea breeze Any questions? Fronts and Frontal Depressions Low 988 Front and frontal surface Frontal Surface Tropical Air Mass Polar Air Mass North Front Development of a Frontal Depression Cold Air Low Cold Air Warm Sector Frontal depression, with warm/cold fronts Development of a Frontal Depression Polar Air Falling Pressure Polar Front Tropical Air New Low Low Day 4 Day 3 Day 2 Low 988 995 Low 979 Low 984 Low Day 5 Occlusion disappearing Well occluded (slow moving low) Low 1011 Day 1 Frontal wave (developing) Warm Sector Depression (deepening) Partly occluded (low starting to fill) Life cycle of warm sector depression. Frontal slopes and surfaces Frontal slopes and surfaces Direction of Movement 30,000 FT Average Slope 1:150 20,000 FT Warm Air 10,000 FT Cold Air 0 NM Warm Front 200NM 400NM 600NM Earth’s Surface Frontal slopes and surfaces Direction of Movement 30,000 FT Average Slope 1:50 20,000 FT Warm Air 10,000 FT Cold Air 150NM 0 NM Cold Front Earth’s Surface Frontal slopes and surfaces Direction of Movement 30,000 FT Average Slope 1:50 Average Slope 1:150 20,000 FT Warm Air 10,000 FT Cold Air Cold Air 150NM 0 NM Cold Front 0 NM Warm Front 200NM 400NM 600NM Earth’s Surface Pressure changes Pressure changes 1000 1000 1002 1004 Movement 1002 1004 Pressure changes Plan View 1000 1000 1002 1002 A 1004 B 1004 Cross Section B A 1004 1002 1001 1001 1002 1004 Wind changes at a front Wind changes at a front Front moves Warm front Warm front 40,000 ft Tropopause 30,000 ft Tropical Air 20,000 ft Slope: 1:150 0 nm 200nm Polar Air 400nm 10,000 ft 600nm Warm front 40,000 ft Ci 30,000 ft Cs As 20,000 ft Ns 10,000 ft Cu Sc St 0 nm 200nm 400nm 600nm Warm front 40,000 ft Tropopause Tropical Air Ci Cs 30,000 ft Jet Stream As 20,000 ft Polar Air Ns 10,000 ft Cu Sc St 0 nm O deg C isotherm 200nm 400nm 600nm Cold front Cold front 40,000 ft Tropopause 40,000 ft 30,000 ft Tropical Air 20,000 ft Slope: 1:50 10,000 ft Polar Air 400nm 200nm 0 nm Cold front 40,000 ft 40,000 ft Ci Cs 30,000 ft As 20,000 ft Cb Ns Cb 10,000 ft Cu Sc St 400nm 200nm 0 nm Cold front 40,000 ft 40,000 ft Ci Tropical Air Jet Stream Cs Tropopause 30,000 ft As 20,000 ft Polar Air Cb Ns Cb 400nm Cu O deg C isotherm 200nm 10,000 ft Sc St 0 nm Describe the weather likely to be produced by: Warm front Cold front Low 988 When a warm front passes? Low Wind: Backs ahead Veers astern Pressure: Falls ahead, Steadies astern Temperature: Rises Dew Point: Rises Cloud Base: Falls Weather: Rain to drizzle Visibility: Deteriorates When a cold front passes? Wind: Backs ahead veers astern Pressure: Falls ahead rises astern Temperature: Falls Dew Point: Falls more Cloud Base: Rises Weather: Rain dies out Visibility: Improves Low Mobile anticyclone and frontal depression Any questions? Visibility and Fog Upslope (Hill) Radiation Fog Formation •Clear skies •Light winds •Moist air •Long cooling period •Low-lying ground Met Office Advection Fog Formation Requires: Warm, moist air Cold surface Some wind Occurs: Over sea and windward coasts Late spring and early summer Advection Fog Formation Occurs: Overland In winter After severe frost and/or snow Any Questions? SOURCES OF WEATHER INFORMATION Inshore Waters Forecast Inshore Waters Forecast to 12 miles offshore from 0500 UTC to 1700 UTC. From Cape Wrath to Rattray Head including Orkney. 12 hour forecast: Wind: variable 2 to 4 becoming southwest 5 or 6, possibly 7 around Cape Wrath. Weather: fair then patchy rain and drizzle. Visibility: moderate or good. Sea State: moderate or rough, but slight Inner Moray Firth. Outlook for the following 24 hours: Wind: west or southwest 5 or 6, occasionally 4 Inner Moray Firth. Weather: patchy light rain and drizzle, mainly north. Visibility: moderate or good. Weather on the Web The UK Met Office http://www.meto.govt.uk Has satellite pictures from around the globe, pressure charts (00, 24,48hrs) but without a wind scale. The aviation chart F215 has good weather, cloud and visibility forecasts (need to register (free)) Bracknell Fax Charts http://weather.noaa.gov/fax/otherfax.shtml http://www.wetterzentrale.de/topkarten/tknfax.html Both sites have Met Office charts with wind scale for 00, 24, 36,48,60,72,96,120 hours ECMWF Charts http://www.ecmwf.int/services/forecast/index.html Surface pressure charts to 6 days but without a wind scale, covers N. Atlantic, northern and southern hemispheres Satellite Picture http://www.met.fu-berlin.de/wetter/meteosat/D2.jpg The latest infra-red picture, covers Europe and south to Dakar Any Questions? Chart interpretation Interpret surface meteorological charts Movement of lows • Developing depressions move at near 100% and move parallel to warm sector isobars • Developed depressions move at 80% • Occluded depression will slow down and curve polewards Movement of Anticyclones • Anticyclonic movement is difficult to calculate past history is a useful guide. • Mobile ridges (those between fronts) tend to move at right angles to their axis Movement of fronts • Warm fronts – 65% over sea – 50% over land • Cold fronts and troughs – near 100% • Occlusions become slow moving • Fronts move 90º to their orientation • Measure along axis of front apply to geostrophic scale at the correct latitude then apply correction Measurement of winds • Take a mean between isobars apply to geostrophic scale then apply correction: – Over the sea • Direction back by 10º, speed 75% of geostrophic – Over the land • Direction back by 20º, speed 50% , 30% at night Visibility terms • Good – 10 - 20km • Moderate – 4 - 10km • Poor – 1 - 4km • Fog – < 1km Any Questions? Summary Temperature=8.6C Intermittent moderate rain Visibility=3000M 086 Pressure=1012.2 mb 122 30 28 Past weather = rain and drizzle 082 Dew point=8.2C Pressure tendency = rising strongly then falling, at 2.8mb in3 hr 8/04 Wind=140 10 KT Low cloud=Stratus 8/8 at 400 FT