Fronts and Mid-latitude Cyclones

FrontsThe boundary between two different air masses is called a front.It is a region of significant horizontal gradients in temperature or humidity.Typically 100 to 200 km wide – very sharp transitions are uncommon.

Fronts are a dominant feature of mid-latitudes. In particular fronts associated with low pressure systems (mid-latitude cyclones, extra-tropical cyclones, depressions).The movement of fronts is responsible for much of the day-to-day variability in weather conditions. Northwest Europe receives many different air mass types, with frequent frontal passages – results in very variable weather.

Warm Frontwarm air

cool air

movementof front

cool air

warm airnimbo-stratus

alto-stratuscirro-stratus

cirrus

~300 km ~500 km

• Warm air flows up over denser cold air

• Inclination of frontal surface is very shallow: 0.5 to 1

• Approach of front signalled by high cirrus or cirrostratus, cloud base lowering as surface front approaches.

• Rain starts ahead of surface front, is widespread and persistent

• Skies clear quickly after passage of surface front

~10

km

Cumulo-nimbus

Cold Frontcold air

warm air

movementof front

cold airwarm air

~70 km ~200 km

• Dense cold air pushes forward into warmer air, which is forced upward

• Steeper than warm front: ~2• Deep convective clouds form above

surface front, heavy rain in narrow band along surface front

• Behind front cloud base lifts, eventually clearing

• Near the surface the cold air may surge forward, producing

a very steep frontal zone

~10

km

Stationary Fronts• There is no fundamental difference

between the air masses either side of warm and cold fronts – the front is defined by the direction of motion

• When the boundary between air masses does not move it is called a stationary front

• Note that the wind speed is not zero – the air individual masses still move, but the boundary between them does not

cold air

warm air

Occluded Fronts

movementof front

• In general cold fronts move faster than warm fronts, and may thus catch up with a warm front ahead – the result is an occluded front

• There are two types of occluded fronts: warm and cold, depending on whether the air behind the cold front is warmer or cooler than the air ahead of the warm front

• Cold occlusions are the more common type in the UK

• Occlusion is part of the cycle of frontal development and decay within mid-latitude low pressure systems

cold air

warm air

cool air

Warm Occlusion• In both warm and cold occlusions,

the wedge of warm air is associated with layered clouds, and frequently with precipitation

• Precipitation can be heavy if warm moist air is forced up rapidly by the occlusion

cold air

warm air

cool air

Cold Occlusion

Mid-latitude Cyclones• Low pressure systems are a

characteristic feature of mid-latitude temperate zones

• They form in well defined zones associated with the polar front – which provides a strong temperature gradient – and convergent flow resulting from the global circulation

31-08-2000

• Low pressure forms at surface over polar front due to divergence aloft

• As rotation around initial low starts, a ‘wave’ develops on the polar front

• Mass balance: inward flow compensated by large-scale lifting cooling cloud formation

cloud

• Surface low is maintained (or deepens) due to divergence aloft exceeding convergence at surface

• Flow is super-geostrophic: cold sector air pushes cold front forward; warm sector air flows up warm front – warm front moves slower than cold

• Cold front overtakes warm front to form an occlusion, which works out from centre

• Depression usually achieves maximum intensity 12-24 hours after the start of occlusion

• Low starts to weaken as inflowing air ‘fills up’ the low pressure

• Low continues to weaken, clouds break up

A

BA

B

A

BA

B

Ana-Fronts• Air is rising with respect to both

frontal surfaces• Clouds are multi-layered and

deep, extending throughout the troposphere

tropopause

cold

warm

cold

Kata-Fronts

tropopause

Sc Sc

subsidence inversion

• Air aloft in the warm sector is sinking relative to the fronts

• Restricts formation of medium & high-level clouds. Frontal cloud is mainly thick stratocumulus, it’s depth limited by the subsidence inversion

• Precipitation is mostly light rain or drizzle.

cold

warm

cold

Ana-cold fronts may occur with kata-warm fronts, and vice-versa.Forecasting the extent of rain associated with fronts is complicated

– Most fronts are not ana- or kata- along whole length, or at all levels within the troposphere

upper wind

A

B

C

D

0°

30°

60°

Polar Front

Mid-latitudeJet Stream

Tropicaljet

30

60

80

Major Frontal Zones Northern Hemisphere Winter

Atlantic Polar Front

Pacific Polar Front

CanadianArctic Front

Atlantic/AsiaticArctic Front

MediterraneanFront