synoptic charts weather discussion 2009 sarah kew
TRANSCRIPT
Synoptic ChartsWeather 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
1362
18321
495
Pressure (1018.3 hPa)
Pressure tendency (rising 2.1 hPa in 3 hours)
Weather(Fog)
Wind (30 kt, WNW)
Cloud cover (8 oktas)
The Station Circle
1362
18321
495
Pressure (1018.3 hPa)
Pressure tendency (rising 2.1 hPa in 3 hours)
Weather(Fog)
Wind (30 kt, WNW)
Cloud cover (8 oktas)
The Station Circle
1362
18321
495
Pressure (1018.3 hPa)
Pressure tendency (rising 2.1 hPa in 3 hours)
Weather(Fog)
Wind (30 kt, WNW)
Cloud cover (8 oktas)
998.4 hPa
The Station Circle
1362
18321
495
Pressure (1018.3 hPa)
Pressure tendency(rising 2.1 hPa in 3 hours)
Weather(Fog)
Visibility(12 km)
Medium-levelcloud (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
http://atschool.eduweb.co.uk/kingworc/departments/geography/nottingham/atmosphere/pages/gfx/
Weather/Precipitation
• Mark rain (light/strong)• Mark fog• Mark Thunderstorms
fog light rain strong rain thunderstorm
snow cb
shower
Pressure tendency
• Mark pressure fall and rise
28
17
38
8
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
http:
//ap
ollo
.lsc.
vsc.
edu/
clas
ses/
met
130/
note
s/ch
apte
r11/
grap
hics
/of_
co_s
chem
.jpg
http://www.qc.ec.gc.ca/meteo/images/Fig_8-9_a.jpg
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
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 10-15 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 23-03-09 18 UTC
Get practising!