Synoptic Charts Weather Discussion 2009 Sarah Kew

Synoptic charts Synoptic charts are drawn every 3 hours (00, 03, 06 UTC etc) Measurements from about 400 land/sea stations plotted on Europe chart Drawn onto chart: – Isobars (every 5 hPa) – fronts (cold, warm, occluded) – Highs (H) and Lows (T) Cold front Warm front Occluded front

Overview over current weather situation What’s going on Good starting point for making forecast Why? Old fashioned?!

Outline Decoding the measurements Drawing isobars and fronts Synoptic charts for typical Alpine weather situations

What is measured? Pressure Pressure tendencies Air and dewpoint Temperature Wind speed and direction Current and past weather Cloud cover, type, height Visibility

The Station Circle Pressure ( hPa) Pressure tendency (rising 2.1 hPa in 3 hours) Weather (Fog) Wind (30 kt, WNW) Cloud cover (8 oktas)

The Station Circle Pressure ( hPa) Pressure tendency (rising 2.1 hPa in 3 hours) Weather (Fog) Wind (30 kt, WNW) Cloud cover (8 oktas)

The Station Circle Pressure ( hPa) Pressure tendency (rising 2.1 hPa in 3 hours) Weather (Fog) Wind (30 kt, WNW) Cloud cover (8 oktas) hPa

The Station Circle Pressure ( hPa) Pressure tendency (rising 2.1 hPa in 3 hours) Weather (Fog) Visibility (12 km) Medium-level cloud (Ac with Ns) Dew point Temperature (9 C) Past Weather (rain) Low cloud base height (500 m) Low cloud Type (Sc) Low cloud Cover (4/8) Air temperature (13 C) Wind (30 kt, WNW) Cloud cover (8 oktas)

How? Remember drawing by numbers?

Advanced drawing by numbers 1005

Identifying highs and lows Pressure minimum Anticlockwise winds Fronts Pressure maximum Clockwise winds

How to find the front? Clouds Precipitation Wind change Isobars Temperature Dew Point Visibility Satellites

Weather/Precipitation Mark rain (light/strong) Mark fog Mark Thunderstorms fog light rainstrong rainthunderstorm snow cb shower

Pressure tendency Mark pressure fall and rise

How to find the front? Warm front: Pressure decrease ahead of front, then constant Increase in temperature Increase in dew point Wind jump at front Cirrus ahead of front Light rain and then increasing but usually not convective

How to find the front? Cold front: Pressure decrease ahead of front Strong pressure increase behind front Decrease in temperature Wind jump at front Convective clouds Strong rain, showers, thunderstorms

How to find the front? Occlusion: Joining of cold and warm front Heaviest rain usually at occlusion point

20/03/02 00 UTC T T H Finished Chart

Typical Alpine Weather Situations Recognise patterns in synoptic charts  predict local effects missed by models Recognise patterns in synoptic charts  predict local effects missed by models

West-wind Straight westerly upper flow Succession of waves on polar front, 1-2 day separation Few days - 1 week Autumn to Spring Variable weather, particularly Alpine North side

West-wind Extensive cloud band on the warm front, thicker as front approaches Isolated clouds in warm sector Narrow cloud band on cold front – Thunderstorms in summer Convective clouds behind cold front (daytime)

East – Northeast flow called the “Bise” Low level: air channelled between Alps and Jura – Wind speed increases as separation narrows Above: large scale subsidence Summer: dry air, fine weather, pleasant temperatures Winter: fog, if relative humidity high enough Bise

Warm dry subsiding air above, separated from moist air below by inversion Turbulence due to high wind speed, but rising air trapped by inversion Height of fog base depends on humidity Solar heating insufficient to evaporate fog (winter)

South Foehn South – southwest flow over Alps Cold front over east France “S”-form of isobars is typical Pressure in Zurich hPa lower than in Locarno Hours - days

South Foehn South side: Ascending air cools, clouds form accumulate and thicken, and it starts to rain North side: Drier descending air warms, 10C warmer than south side, clouds evaporate, “Foehn-hole” Lee side: Buoyancy waves - Lenticular clouds, rotors

North Foehn Moist air from North-Sea approaches Alps Pressure in Zurich 15 hPa higher than in Locarno Clouds pile up on north-side Precipitation mainly in central and east parts of Alpine North side West still influenced by stable high Clear skies in Ticino and Bergell Lee-waves and rotors to South

High Pressure Very light winds Sinking air warms, lowers relative humidity, evaporates clouds Days – several weeks Summer: fine weather Winter: radiation fog – 200 m thick – Nov-Jan not always evaporated

Flat Pressure Distribution Only weak extrema large spacing between isobars, light winds Typical in summer Absence of subsidence allows convective clouds to develop Thunderstorms over Jura and Alpine foothills

What do we have now? Mo UTC

Get practising!