The Climate of the British Isles
• Learning Objectives:
1. Know the characteristics of the climate and
British air masses
2. Understand depressions
3. Understand anticyclones
4. Know a case-study of a storm event
1.
Know the characteristics of the climate and British air masses
Precipitation - Clouds
Clouds are formed by the condensation
of invisible water vapour in the
atmosphere. In being forced to rise, the
air temperature falls below dew point
temperature and water droplets result.
Humidity and Dew point
Humidity - a measure of the amount of water vapour in the atmosphere. Warm air can
hold more water vapour than cold air. In humid weather the amount of water vapour
being held is between 80-99% of the maximum it can hold i.e. it is saturated at 100%.
Absolute humidity – The mass of water vapour in a cubic metre of air (g/m3)
Relative humidity – The amount of water in the air at a given temperature (% of
maximum e.g. 50% or below is dry air.
Dew point – The temperature at which unsaturated air becomes saturated and water
droplets begin to form (condensation).
Precipitation – Frontal rainfall
Tropical air
warm air
LCL
cold air
Polar air
What are the stages in the formation of frontal rainfall?
1. Warm, moist air meets cooler, denser air.
2. Warm air is lighter, less dense and lower pressure so rises over the colder air,
3. The warm air cools as it rises, becomes saturated and rain begins to fall.
Precipitation - Relief rainfall
LCL
LCL
Rain shadow
What are the stages in the formation of relief rainfall?
1. Air is forced to rise over a relief barrier.
2. Air cools as it rises and becomes saturated.
3. Water vapour condenses, forms droplets and falls as rain.
4. On the leeward side air warms as it sinks and precipitation ceases.
What is the drier area on the leeward side called?
the rainshadow
Precipitation - Convectional rainfall
LCL
What are the stages in the formation of
convectional rainfall?
1. Ground is heated and conducts heat to adjacent
air.
2. Warm air rises and latent heat is released.
3. Air cools as it rises and becomes saturated.
4. Water vapour condenses, forms droplets and falls
as rain.
The beginning of unstable
cumulo-nimbus cloud
developing in summer
Quick information:
The gulf stream influences the British climate.
Water takes longer than land to heat but is slower to
cool.
Mountains and hills can cause relief rainfall
Heat can cause convectional rainfall
Know the characteristics of the climate and British air masses
1. Describe and explain the pattern of rainfall
in the British Isles.
8 marks
What will the 8 marks be awarded for?
•Description –
•Explanation -
Average yearly rainfall in Britain
Know the characteristics of the climate and British air masses
Average summertime surface temperature in Britain.
Average wintertime surface temperature in Britain.
1. Describe the differences in temperature between summer and winter in Britain.
2. Explain the differences in temperature in the British summertime
Can you explain why in the winter the isotherms run roughly east-west and in the summer they
run roughly north-south?
AQA A2 page 48 fig 2.7
On a map of the British Isles use figure 2.7 and the text
about wind to annotate your map with information
about wind in Britain.
Influencing air masses in the British Isles
The UK weather continually changes because of the different
weather systems brought by different air masses.
Am
What are the main air masses
affecting the British Isles?
• Polar maritime (Pm)
Pm
Pc
• Tropical maritime (Tm)
• Tropical continental (Tc)
• Polar continental (Pc)
• Arctic maritime (Am)
Tm
Tc
Prevailing wind direction
AQA A2 page 50.
Know the characteristics of the climate
and British air masses
Discuss the relative importance of the
physical factors that influence the
climate of Britain.
40 marks
Use the essay generic mark scheme on the P:drive
Essay style question – make notes, plan and write.
Understand depressions
What do you know?
What are depressions?
Isobars
Large, LP surface weather systems. They
are linked to upper air movements (Rossby
waves) and are part of the global
atmospheric transfer of energy.
Which air masses are most likely to bring
depressions to the UK?
Tropical maritime/Polar maritime
Cold front
Warm front
The synoptic chart for a midlatitude depression
In a northern hemisphere depression, in
which direction do the winds blow?
Anticlockwise. They usually follow the
isobars (lines of equal pressure).
Depression from above
Depressions
Satellite image of a mid-latitude
depression over the Atlantic.
Synoptic chart of a mid-latitude
depression over the Atlantic.
Warm air
pushed up as
its less dense.
Travelling faster than
other side.
Tom Abbott, Biddulph High School and
made available through
www.sln.org.uk/geography and only for
Tom Abbott, Biddulph High School and
made available through
www.sln.org.uk/geography and only for
Tom Abbott, Biddulph High School and
made available through
www.sln.org.uk/geography and only for
Tom Abbott, Biddulph High School and
made available through
www.sln.org.uk/geography and only for
Depressions
How and why do mid-latitude depressions form?
Depressions originate at the POLAR FRONT where the Polar Easterlies (cold air mass from the
Poles) blow parallel to the Westerlies (warm air
mass from the tropics). The depression begins
when the cold air begins to undercut the warm
air.
cold
air
warm
air
A wave develops on the Polar Front. The warmer, less dense air is forced to rise in a
spiral movement (due to coriolis effect). This means there is less air at the surface
creating lower pressure. Winds begin to blow around it.
Depressions – characteristics
•
•
•
•
Low pressure <1000mb
Steep pressure gradient from the outside to the centre
Usually move rapidly W to E across UK
Strong winds blow inwards in anticlockwise manner
Depressions – formation
• Dense, moist, cold pm air
meets light, moist, warm tm
air
• Warm air is forced to rise
over the cold air
• Rotational forces of the
Earth cause the rising air to
twist
• Twisting vortex creates a
wave at ground level in the
polar front
•
•
•
•
cold front
warm sector
warm front
Warm front is at the leading edge of the depression
Behind this is the warm sector
Following on is the cold front
As the depression moves east (due to Rossby waves), the cold front catches up with
and overtakes the warm front, creating an occluded front
Depressions – weather
Rainfall
• Warm front: As the warm Tm air rises over the cold Pm air, it cools via uplift and
contact to dew point, there is condensation and rainfall. The rainfall intensifies as the
WF approaches
• Warm sector: rainfall will stop
• Cold front: The cold air undercuts the warm air and causes more rapid uplift, therefore
higher intensity and shorter duration rainfall
Temperature
As the warm air passes there will be a sudden increase in temperature. Once the CF has
passed it become much colder
Winds
Veer from S/SE to SW to NW, becoming very strong as CF passes. Anticlockwise
circulation into the centre
Clouds
Thicken and lower towards the WF, become scattered in the warm sector and become
towering at the CF
Pressure
Decreases steadily to the center of the depression
An Integrated Approach page 230-233
Read and take notes from the book about Depressions over the UK.
The passage of a depression over
the UK leads to predictable weather
change over a period of time.
Outline the changes that would
occur over Liverpool in a 12 hour
period if a mature depression
passed from west to east.
Refer to:
Changes in wind speed and
direction, cloud cover, temperature
and precipitation.
Why these changes occur (the
processes responsible).
Depressions – Carlisle floods 2005
Case-study: storm events
The UK, due to it’s mid-latitude position, is
regularly hit by depressions that form out in the
Atlantic and are driven westwards by Rossby
waves (Polar front jet stream).
On January 6Th 2005 an unusually low pressure
weather system (depression) hit the UK. Gales
and torrential rainfall lashed the NW of England
causing extensive flooding, the loss of life and
millions of £’s of damage. Carlisle, a town close
to the Scottish border, and the surrounding area
was particularly badly affected.
Continuous rainfall for two weeks in late
December was followed by the extreme
weather from 6th to 8th January. 15% of the
average rainfall fell in 36 hours (about
220mm). Winds of 90mph felled trees and
by 2am on the 8th the rivers could no longer
hold the vast amounts of water and Carlisle
began to flood.
Carlisle floods - Effects
• 3 people died,1,600 homes flooded, estimated cost
£100 mill
• 6,000 people had to be re-housed because of
displacement
• 1,150 trees felled in the gales
• 70,000 people without power for 3 days
• Rail services cut for 2 days, roads and bridges closed
for 2 days
• 70 buses lost worth £3 million
• Ind. Est.flooded damaging 325 business propertiescost £100 million
• Fire station flooded, relocated to Carlisle Castle
• Emergency centres set up in a school and church hall to receive rescued residents, ran
for 5 days
• Phone exchange flooded, mobile phone mast down, Radio Cumbria becomes main
source of information and broadcast throughout the crisis
• RAF, Coastguard and Mountain Rescue teams evacuate people by boat and helicopter
Carlisle floods – The future
• Reappraisal of flood defence needs
for the city, works expected to last 3
years costing £20 million
• Plans to rebuild Police station on a
new site
• Plans to significantly alter city centre
• Many small family firms closed down
• A year on some flood victims still in temporary housing
• Increases in household and car insurance premiums throughout the Carlisle
postcode area
Home learning: Complete some additional research about the Carlisle floods in 2005, get
some photos and annotate them.
What other storm events have occurred? How were they dealt with?
Group exam answer
Create notes from your work and home learning.
Plan out the essay
write the essay.
• Discuss the impact of storm events in the
British Isles and evaluate the responses to
them.
40 marks (A2 exam 2010)
Understand anticyclones
Anticyclones
An area of high pressure. Bring stable conditions with clear skies. In the UK they are
associated with extended warm, dry conditions in the summer and cold, frosty and
sometimes foggy conditions in the winter. An anticyclone may be 2500 km in diameter,
although many are smaller.
clockwise
1032
1028
The synoptic chart for a mid-latitude
anticyclone
The satellite image shows the cloudless skies
associated with high pressure systems. They may
remain for a number of weeks causing heat waves
across many parts of Europe during the summer.
Anticyclones
Anticyclonic Weather In The UK
Winter Conditions (Polar Source)
Impacts
Conditions
Summer Conditions (Tropical
Conditions
Impacts
Ice. Increase in
accidents particularly
elderly increased power
use.
Cold day time
conditions. Usually
below freezing.
Hot daytime conditions
over 23º.
Increase in heart
attacks/heat stroke.
Sales of lettuce/ice
cream/ soft drinks
increase. Hose pipe
bans/drought.
Roads need gritting.
Very cold at night with
frosts.
Warm night
temperatures.
Sleeping difficulties.
Temperatures remain
low.
Clear skies.
Generally clear skies.
Sunburn/ sunstroke.
Car accidents.
Disruption to transport
(aircraft, ferries).
Stable conditions may
produce fog.
Some early morning
mists especially at
coast.
May take a while to
clear impacted tourists.
Breathing difficulties.
High levels of pollution
due to lack of wind.
Thunderstorms may
form in evenings due to
convection.
Flash flooding effects
homes and transport.
Hospital admissions
increase.
Subsiding air traps
pollutants.
Photo-chemical smog
and low ozone level
trap heat.
Increase in deaths from
asthmas/breathing
conditions.
European heat wave of 2003
Use the website to create
a study sheet about
anticyclone weather and
the heat wave of 2003.
Winter anticyclones
• Read page 54 AQA A2 text book.
• State the characteristics of a winter
anticyclone and give reasons for these.
– You can draw labelled diagrams to help you
answer.