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!