Lecture 11 Midlatitude Weather and Climate

Isobars

every 4 millibars on surface pressure charts

Airmass Characteristics (6)

• a large body of air that has uniform horizontal properties of 

  • temperature (either warm or cold)

  • moisture (either dry or moist)

• horizontal extent = thousands of kilometres

• extend vertically throughout the troposphere

• have the properties of their source regions

• move slowly from source region but are modified en-route

• boundary between two airmasses is called a front

Airmass Source Regions

• c -> continental, dry

• m -> maritime, wet

• P -> polar, cold

• T -> tropical, warm

• A -> arctic, very cold

• e.g. cT, mP, A

British Airmasses and Fronts (6)

• affected by a series of fronts, usually separating polar maritime and tropical maritime air

  • Arctic Maritime (mA), brings snow in winter

  • Polar Continental (cP), hot air brings dry summers, cold air brings snow in winter

  • Polar Maritime (mP), brings cold showery weather

  • Returning Polar Maritime (mP), brings clouds and rain showers

  • Tropical Maritime (mT), brings clouds, rain, and mild weather

  • Tropical Continental (cT), bring hot weather in summer

Fronts (7)

• transition zone between two airmasses

• extends vertically through troposphere

• temperature contrast on either side of front

• air converges on either side of the front to produce strong winds

• warm air rises over cold air at a front

• as the air rises it cools and condenses to form clouds and produce rainfall

• surface pressure lowers

Warm Fronts (5)

• occurs when a warmer airmass catches up to a colder airmass

• warm moist air rises, condenses producing clouds and precipitation ahead of and at the surface front boundary

• clouds are thicker and lower nearer to the front boundary and extensive

• gentle slope -> ~1:150 - 1:200

• speed -> 25 km/hour

Cold Fronts (6)

• occurs when a colder airmass moves into a warmer airmass

• warm moist air condenses into high clouds ahead of the surface front boundary

• uplift more rapid than for warm fronts, cumulonimbus form

• thick clouds, narrow, short-lived band of precipitation (can be thunderstorms) close behind front boundary

• steep slope -> 1:50 - 1:70

• speed typically twice as fast as warm fronts -> 50 km/hour

Occluded Fronts (6)

• low pressure front, storms

• occur where warm and cold fronts meet

• cold front moves about twice as fast as warm front and catches up to warm front

• rising air along occluded front with cold air either side

• part of life cycle of mid-latitude cyclone

• low pressure centre of mid-latitude cyclone at tip of occluded front as rising air lowers mass of air column

Mid-Latitude Cyclones (5)

• area of low pressure in the mid-latitudes associated with warm, cold, and occluded fronts

• brings stormy weather

• air converges at surface

  • cyclonic motion

  • anticlockwise in NH, clockwise in SH

• typically form between 30-60° along polar front

  • boundary between cold polar air and warm tropical air

• above the polar front is the polar front jet stream

  • strong winds

Mid-Latitude Cyclone Characteristics (5)

• motion: ~1000 km/day

• size: ~2000 km/day

• lifetime: ~5 days

• pressure gradient: ~35hPa/1000km

• central pressure: ~959hPa

Cyclone Formation

• situation of converging surface air and divergence aloft

• if upper-level divergence is stronger than surface convergence -> surface pressure drops -> surface low deepens

UK Storm Centre

• Met Office website set up for winter 2015/2016

• to raise awareness and aid communication of approaching severe weather through media and government agencies

• storms named when they had the potential to cause an amber ‘be prepared’ or red ‘take action’ wind/rain warning

Cyclone Case Study: Storm Eowyn (8)

• January 2025, 22nd/23rd/24th

• severe winds over Scotland and Northern Ireland

• Red warnings issued for wind in these regions with amber warning covering the northern UK

• wind gust speeds of over 80mph

• ‘the UK’s most powerful storm in a decade’

• Jan 24th

  • 6am, Eowyn off the coast of Ireland

  • very tight isobars = high winds

  • ‘explosive cyclogenesis’ 

  • 50hPa drop in 6 hours

  • due to interaction with the jet stream

  • intensified by cold temperatures in North America

  • deepened to 927hPa by 12pm

  • low pressure centre over the Central Belt with extensive clouds associated with fronts

• lots of rain in the early morning, some continued in North

• last storm with such gusts in 2014/2015

Storm Eowyn Impacts (4)

• schools closed, transport disruption

• fallen trees and debris

• over 1 million homes in Scotland and Northern Ireland without power

• 2 fatalities

Storm Eowyn Wind Speeds (3)

• 58mph (50 knots) mean wind speeds across Northern Ireland and Scotland

• 81mph (70 knots) peak gust speeds at ~20 stations

• 92mph (80 knots) at ~5 station