UK Weather (SQA National 5 Geography): Revision Note

Specification checklist

This page covers what you need to know from the SQA National 5 geography specification to answer questions on:

• Within the context of the United Kingdom:

  • The characteristics of the five main air masses affecting the UK

  • The characteristics of weather associated with depressions and anticyclones

Five air masses affecting UK weather

What is an air mass?

• An air mass is a large body of air that has similar moisture, density and temperature characteristics

• A front is the boundary separating two air masses

Which air masses affect the UK?

• Air masses are classified by the region where they form:

  • Arctic or Polar cold air

  • Tropical warm air

  • Maritime wet air

  • Continental dry air

• Britain’s weather is dominated by Atlantic low-pressure systems (depressions or cyclones) that move across the British Isles from west to east

• These bring a mix of warm tropical maritime air and cold polar maritime air

• Cold air is denser, so it moves more quickly and undercuts warmer air at the rear of the depression or blocks its movement in at the leading edge of this feature

IMAGE needed
Air masses affecting the UK

Weather associated with different air masses

Arctic Maritime (mA)

• The source of the air mass is the Arctic Ocean

• The weather characteristics are:

  • Very cold temperatures

  • Not always wet due to low humidity

Polar Continental (cP)

• The source of the air mass is Northern Europe and Russia

• Weather associated with this air mass is more common in winter than summer

• The air mass starts over North Europe and brings cold conditions in winter and milder conditions in summer

• In winter it can bring unstable air (which leads snow) but usually brings cool, dry conditions

Tropical Continental (cT)

• The source of the air mass is Southern Europe and North Africa

• Weather associated with this air mass usually occurs in summer

• Warm, dry air from North Africa and the Mediterranean brings warm, dry weather

• Dust from the Sahara can be carried on the light winds and deposited in the UK

Tropical Maritime (mT)

• The source of the air mass is the Atlantic Ocean

• The weather associated with the air mass is warm and wet

• Mild conditions in winter and wet conditions in summer

• It brings cloud, drizzle and fog

Polar Maritime (mP)

• The source of the air mass is the North Atlantic

• The weather associated with the air mass is cold and wet

• It is the most common air mass in the UK

• It brings unstable air which creates clouds

• Can cause rainfall in the summer

Weather associated with anticyclones

• An anticyclone is an area of high pressure, which brings long periods of settled weather

• The air in a high-pressure area is sinking, which means clouds do not form, so conditions are dry

• Characteristics of high-pressure systems on a synoptic chart include:

  • Widely spaced isobars (bringing gentle winds)

  • No fronts (so no clouds)

  • Isobars showing pressure increasing towards the centre and above 1008mb

  • Winds circulate clockwise, following the isobars

Weather associated with high pressure systems (anticyclones) in summer

• Anticyclones during the summer lead to high temperatures

• Clear skies meansthere are no clouds to shade the surface of the Earth

  • It is exposed to the sun’s energy throughout the day 

  • This increases the temperature

• Evaporation occurs at the surface, but due to the high pressure and sinking air, it cannot move up and spread out in the atmosphere 

• This can cause intense thunderstorms to occur in the afternoon or early evening

• Coastal areas on the east coast of Britain can be affected by sea fog (or haar) as warm, moist air travels over the relatively cool North Sea

  • The sinking cold air of the high-pressure system traps the warm, moist air below it, leading to fog formation

The impact of high-pressure systems in summer

• Drought conditions reduce the yield of farmers’ crops

  • This can reduce the food supply and affect farmers’ profits

• Hosepipe bans can make it difficult for gardeners and people with allotments, whose produce suffers in dry weather

• People can suffer from sunburn and dehydration, especially infants and the elderly, who are most vulnerable

• Wildfires can spread rapidly due to dry conditions

  • This endangers wildlife and buildings and increases the workload for the fire service

• Severe thunderstorms from convectional rainfall can lead to localised flooding and travel disruption

• Travel disruption can be caused by railway lines buckling in the intense heat

  • This can lead to trains being  derailed

Weather associated with high-pressure systems (anticyclones) in winter

• Anticyclones that occur during winter lead to:

  • Very low temperatures

  • Frost 

  • Fog

• The clear skies allow heat radiating from the earth to escape into the upper atmosphere

  • This lowers the temperature at the surface

• Frost can form on the ground due to the low temperatures

• In the early morning, there may be fog 

  • This is because moisture in the air is trapped near the surface by the sinking air

  • It is unableto move up and spread out in the atmosphere or be distributed by winds.

Impact of high-pressure systems in winter

• Frost/ice leads to more people falling over 

  • This increases the workload for emergency departments

• Fog can create travel disruption due to poor visibility

• People’s moods improve as a period of bright, settled weather gives people the opportunity to see direct sunlight during the short winter days

• People have to get up earlier to defrost vehicles before travelling, or may be late for work/school

• Low temperatures can cause frost and lead to burst pipes, causing damage to property

Weather associated with the passage of a depression

• A depression is an area of low pressure which moves from west to east in the northern hemisphere

  • In a depression the air is rising 

• Low-pressure systems can be identified from a synoptic chart due to:  

  • Cold fronts 

  •  Warm fronts 

  • Possible occluded fronts 

  • Tightly packed isobars, which means that winds are strong 

  • Isobars showing pressure decreasing towards the centre from about 1004mb 

• As a depression passes over the weather follows a particular sequence

Stage 1

• The depression approaches from the west

Weather changes

• In the area ahead of the warm front is the cold sector

• The weather is bright and sunny

• High level cloud (cirrus and cirro-stratus) begins to develop

• Winds are light and south-westerly, they are beginning to increase in strength

Reasons for the changes

• There are no fronts so the weather is bright and sunny

• As the warm front approaches clouds start to appear due to warm moist air rising above the cold air

• As the isobars get closer together the winds start to increase in strength

Stage 2

• The warm front arrives

Weather changes

• At the warm front, long periods of rain are experienced

• There is dense cloud cover (stratus or strato-cumulus)

• Wind speeds increase and may reach storm/gale force

• The wind direction changes at the front switching to westerly

Reasons for weather changes

• A warm air mass e.g. tropical maritime is pushing in and moist air is forced to rise above the cool air ahead of it

• The air cools, condenses and creates rainfall (frontal rainfall)

• Wind speeds increase because isobars are close together

• Air circulates anti-clockwise, so winds are now westerly

Stage 3

• In the warm sector

Weather changes

• The rain eases

• The temperature rises

• The warm air passes over

• It may be dry or there may be drizzle

• Winds remain strong and westerly

Reasons for weather changes

• This is a warm air mass e.g. tropical maritime which leads to higher temperatures

• With distance from the warm front, cloud cover and rain decrease

• Winds are still strong because the isobars are close together

Stage 4

• The cold front arrives

Weather changes

• There is heavy rain for a short time as the cold front passes

• Temperatures drop as the cold air follows behind the cold front

• Winds are now from the north-west as the front passes bringing in colder weather

Reasons for the weather changes

• The cold front is a steeper gradient than the warm front so warm. moist air is forced to rise more rapidly

• The air cools and condenses quickly, leading to heavy rainfall

• The temperature drops because the cold front is introducing a cold air mass e.g. polar maritime (winds from the North Atlantic)

Stage 5

• Behind the cold front

Weather changes

• Winds are still from the north-west as the front passes, bringing in colder air

• Winds from the north-west are blustery at first, but wind speed decreases as the front moves away

• Calmer conditions follow

Reasons for the weather changes

• 
  Behind the cold front is a cold air mass

• Wind speed, cloud cover and precipitation decrease as the depression, with its associated fronts, move away

Synoptic charts

• A synoptic chart is a detailed weather map which shows what type of weather system is moving over an area

  • The term synoptic means summary

IMAGE - Synoptic Chart

• The main weather element shown on a synoptic chart is air pressure

• Isobar lines, which join up areas of equal air pressure, are drawn on these maps

  • The pattern these lines make shows areas of low or high pressure

  • The isobars are also indicate wind speed

  • Isobars which are closer together indicate higher wind speeds 

• There are several other features on these maps including isobars, low pressure systems (and the associated fronts) and high pressure systems

Station circles

• Synoptic charts may also include station circles

• These are used to show the temperature, cloud cover, wind direction and speed at specific weather stations