Pressure systems

Anticyclones & Cyclones

Global circulation:

Part 1

Part 2

Part 3

Global circulation

Seasonal pressure movement

Seasonal pressure movement

Anticyclone

• High pressure area or ”high”

• Subsidence dissolving clouds.

Types of highs

Ridge

Blocking anticyclone

Cyclone

Low pressureareas or ”low”.

Convectiveweather, CU, CB

Types of lows

Trough

Non-Frontal Depressions

• Thermal

• Orographic

• Polar Air Depression

• Secondary Depression

Thermal Depression

• Over land in the

summer

• Balochistan low

Orographic Depression

Wind flows around the

edge rather than the top.

Polar Air Depressions

• Extremely cold Arctic

air moves south,

leading to convection.

• Only during winter.

• Different from Polar

Front Depressions!

Secondary depressions

1) On a trailing front from

an occluded primary.

2) On a trailing cold front

well within the primary

circulation.

Secondary depressions

3) At the tip of the

warm sector of a partly

occluded depressions.

Cols

Cols in frontal systems

Air masses

Air masses

• An airmass is a large body of air with the same properties.

• Develop in source regions.

Source regions

Moving air masses

Moves over warmersurface =

Defined as cold air mass!

• Becomes warmer in lowerlayers

• Becomes more unstable

• RH decreases

Moves over colder

Surface =

Defined as warm air mass!

• Becomes cooler in lower

layers

• Becomes more stable

• RH increases

Typical weather - Warm Air mass

Inversion

Typical weather - Cold Air mass

Air mass classification

C = Cold

W = Warm

Examples:

cTw

mAc

UK

Fronts and Occlusions

Fronts• Border between 2 air masses

Front Definitions

• Warm front: Warm air displacing cold air.

• Cold front: Cold air displacing warm air.

• Occluded front: Forms when the cold front overtakes the warm front.

• Warm front occlusion: The air ahead of the occluded front is the coldest.

• Cold front occlusion: The air behind the occluded front is the coldest.

• Stationary front: Essentially no movement acrossthe frontal zone.

Fronts Definitions

The warm front is red

The cold front is blue

The occluded front is violet

The Quasi stationary front is

grey. One cold and warm front

opposing each other

Warm Front

Warm front movement

• 2/3 of geostrophic

wind

• At right angles to itself

Ahead of the warm front

At the warm front

Cold front

Cold front movement

• Geostrophic wind

• At right angles to itself

At the cold front

Behind the cold front

Squall Lines

10 - 50 ( 150 ) NM

CB

CB

Gust front Warm humid

air

Ahead of a cold front

or a trough line the

outflow of cold air can

be fairly uniform and

at about the same

distance from the

mother clouds a line of

new CB clouds forms –

a squall line (SQL).

Warm sector

The area between the fronts

Stationary front

• No or little movement

• Calm but long lasting weather

Occluded fronts

Occlusions – General

• The cold front

catches up to the

warm front

• Warm sector raised

off the ground

• Hard to predict

movement and

weather

Warm occlusion

Cold occlusion

Polar front depressionsA hemispheric sea level

pressure chart reveals the

presence of several

extratropical cyclones around

the middle latitudes.

Extratropical

cyclones develop

near the polar front. They have a

significantly smaller scale than

features of the global circulation.

Frontal Development

A disturbance causes a bulge of mTw air in the mPc

air, cerating an area of relatively lower pressure.

Air starts to blow towards the low pressure centre,

but is reflected to the right due to the Coriolis force.

The low pressure centre moves east due to the

general circulation. The fronts follow accordingly.

The cold front will eventually catch up to the warm

front, creating an occluded front.

The two cold airmasses start to mix, effectively

removing the frontal zone.