CLEAR SKIES CAUSE FROST

HIGH PRESSURE High Pressure means that air tends to sink. Sinking air is compressed, warms up as a result and its relative humidity falls below saturation. Any clouds evaporate. Rainfall is unlikely, apart from occasional short, intense convectional storms due to insolation with lack of clouds in daytime.

� In Britain, high pressure systems have clear skies, little or no wind, little rainfall and tend to be stable and slow moving.� Visibility is intially good, but rapidly deteriorates as dust is trapped by sinking air and is not washed out by rainfall.� Cloud cover is slight, resulting in a high diurnal ranges of temperature (hot days, cold nights). Due to the trapped dust particles and cold nights, dew, frost, fog or smog are common.� Air quality is low as all forms of pollution are retained in the lower atmosphere.

FEW CLOUDSVISIBILITY IS

POOR

FOG & SMOG IS COMMON

VISIBLITY REMAINS BETTER IN MOUNTAINS - LESS POLLUTION

CLEAR SKIES

LITTLE WIND

•Credit 03Credit 03

How to answerHow to answer …… ……

A Weather QuestionA Weather Question

How to answerHow to answer …… ……

A Weather QuestionA Weather Question

Synoptic Chart for the British Isles at

0700h on 31 August

A yacht race from Wick to Stornoway was due to start from Wick harbour at 8.00am on 31 August

At 7.00am the Met Office advised the race organisers to cancel the race.With reference to the synoptic chart, explain why this advice was given.

5 marks

A yacht race from Wick to Stornoway was due to start from Wick harbour at 8.00am on 31 August

At 7.00am the Met Office advised the race organisers to cancel the race.With reference to the synoptic chart, explain why this advice was given.

5 marks

What are the key words?What are the key words?place : see map

direction : see maptime : compare

with map

type of boat : think!

change of plan!use the

information on the map

number of points to make

Link question to the mapLink question to the map

Where?

0700 h

31 August

When?

Wick and Stornoway

From Wick to Stornoway

Direction?

Race due to start at 0800h

Map is for 0700h

Study the map for Study the map for informationinformation

Centre of low pressure : likely to bring stormy weather

Occluded front:

broad belt of very heavy rain, low cloud and poor visibility Tightly

packed isobars : strong winds

South westerly winds

Associated weather is moving west to east

Warm mT air

Cold mP air

Back to the question ….Back to the question ….Why was the race cancelled?Why was the race cancelled?

• Competitors would risk very strong winds Competitors would risk very strong winds and therefore high seasand therefore high seas

• They would be sailing straight into the wind They would be sailing straight into the wind so it would be difficult to make progressso it would be difficult to make progress

• Temperatures would be lowTemperatures would be low

• There would be heavy rainThere would be heavy rain

• There would be thick cloud and probably There would be thick cloud and probably poor visibilitypoor visibility

But this is not enough. Why not?

““Explain.. With Explain.. With reference to the reference to the mapmap””

• Strong winds – Strong winds – close isobarsclose isobars

• Head winds – Head winds – wind direction SWwind direction SW

• Low temperatures – Low temperatures – mP air massmP air mass

• Heavy rain – Heavy rain – occluded frontoccluded front

• Low cloud and poor visibility – Low cloud and poor visibility – occluded frontoccluded front

5 marks

Study the synoptic chart below and describe the likely weather conditions in Glasgow (G) and Lyon (L)

Miss Anything?Miss Anything? remember to include remember to include wind, rain and temperature. What type wind, rain and temperature. What type of air pressure are they experiencing? of air pressure are they experiencing? Look at the spacing of the isobars.Look at the spacing of the isobars.

• GlasgowGlasgow will be experiencing will be experiencing periods of heavy rain with a short dry periods of heavy rain with a short dry period in between. The winds will be period in between. The winds will be high.high.

• LyonLyon will be experiencing dry, sunny will be experiencing dry, sunny weather with clear calm skies. There weather with clear calm skies. There will be low temperatures and will be low temperatures and possibly frost.possibly frost.

Other topics to reviseOther topics to revise

• Air massesAir masses

• Weather instrumentsWeather instruments

• Station circlesStation circles

• HP and LPHP and LP

Each of 4 zones has issues:Each of 4 zones has issues:

• Deforestation in the RAINFORESTDeforestation in the RAINFOREST

• Desertification in the DESERTDesertification in the DESERT

• Oil development in the TUNDRAOil development in the TUNDRA

• Tourism in the Mediterranean and Tourism in the Mediterranean and ocean pollution.ocean pollution.

• LOOK over notes and check LOOK over notes and check

• http://www.scalloway.org.uk/phye1.hhttp://www.scalloway.org.uk/phye1.htmtm