L8 Low Pressure Systems

Flashcards on Low Pressure Systems - Depressions, covering fronts, warm/cold fronts, development, life-cycle, and associated weather phenomena, based on lecture notes.

What is a front?

The boundary between two different types of air mass.

What air masses meet at the polar front in the North Atlantic?

Cold polar maritime air from the north and warm tropical maritime air from the south.

Along what front are depressions born that affect Western Europe?

The polar front.

In the summer, where does the polar front generally lie that affects Western Europe?

Approximately the 60°N parallel

In the winter, where does the polar front generally lie that affects Western Europe?

Approximately the 50°N parallel

What does a depression begin as along the polar front?

An unstable wave

What are jetstreams?

Fast-moving streams of air at high altitude that tend to form over transition zones such as the polar front.

Jetstream

Warm sector depressions are steered by what?

What is the 'warm sector'?

The area of warmer air between the warm and cold fronts in a warm sector depression.

What type of clouds are found at the top of the frontal slope of a warm front?

Cirrus-type clouds

What is the commonly-quoted figure for the slope gradient of a warm front?

1:150

Rain or snow first falls from what type of cloud at around 15,000ft ahead of a warm front?

Altostratus

What is the lowest cloud type associated with a warm front, often reaching the surface?

Nimbostratus

What are the first signs of the passage of a warm front?

A veering wind, followed by a steadying of pressure, and an increase in temperature and dew point.

How does the Surface wind trend ahead of a warm front

Backs (anticlockwise)

How does the pressure trend as a warm front approaches

Falls increasingly

What is the average slope of a cold front?

Average 1:50

What type of cloud is likely to be included in a cold front zone?

Cumulus-type cloud

What marks the actual passage of a surface cold front?

Heavy precipitation and 'squally' conditions.

What type of turbulence is associated with the passage of a cold front?

Frontal turbulence.

In a classic warm sector depression, how does the speed of the cold front compare to the warm front?

It moves faster.

What are the two types of occluded fronts?

Cold occlusion or warm occlusion.

What does the occluding of a depression mark?

The start of the end of its life-cycle.

What is the typical speed of fronts within a warm sector depression?

Traveling at 20-30 knots.

What symbols are included on weather charts to indicate a quasi-stationary front?

Alternating cold and warm front symbols

What can form within the 'old' primary depression along a trailing cold front?

A secondary depression.

From surface observations, what indicates the direction and speed of a depression's movement?

Isobars in the warm sector.

Traveling depressions tend to steer around what established weather systems?

Anticyclones.

Besides the veering of wind, what is another reliable indicator of the passage of a front?

Dew Point

What atypical cloud formations can warm fronts sometimes have?

It can form embedded cumulonimbus clouds associated with it, giving thunderstorms.

What sort of non-frontal lows sometimes form over Continental Europe in summer?

Heat Lows

A cold front brings what type of air to replace warmer air?

Cold

A warm front brings what type of air to replace colder air?

Warm

What geographical feature can hold up frontal progression/movement?

Areas of significantly high terrain

If a front reverses direction, the type of front , even though all other conditions remain the same.

Changes

The depressions affecting what geographical area are born over the North Atlantic?

Western Europe

Fine-scale mechanisms involved in the formation of depressions are complex and are associated with what?

The jetstreams flowing above the polar front

A cross section through a ‘classic’ warm-sector depression, the process is cumulative, and the consequence is an increasing mass of air.

Ascending

By now the depression is moving _, steered by the jetstream and gathering energy as it continues to deepen.

Eastwards

The slope of a is generally quite gentle, the commonly-quoted figure is a slope gradient of about 1:150.

Warm Front

At the top of the frontal slope of a warm front, close to the tropopause, are cirrus-type clouds which can be up to _ ahead of the surface front.

925 km

Mid-morning on the 19th February near __, UK showed high cirrus cloud increasing from the west.

Newcastle

The slope of the cold front tends to be steeper than that of the warm front, the average cold front having a slope of _.

1:50

The passage of a cold front is often marked by a _ change in weather.

Dramatically

The passage of a cold front is often marked by a dramatic change in weather when what is introduced?

A polar maritime air mass

Where the cold front has _ the slower moving warm front, the air behind the surface front is colder than that ahead, this is known to meteorologists as a 'cold' occlusion.

Undercuts

The low gradually _ and it becomes slower moving as the occluding of a depression marks the beginning of the end of its life-cycle.

Fills

Because a stationary front is going nowhere, the conditions it may be bringing (e.g. low cloud, poor visibility and precipitation) are obviously going to _ for some time.

Persist

The greater the temperature difference and the _ the temperature gradient across a front, the more active and sharply defined it will be.

Steeper

What is another name for a 'heat low'?

Thermal Low

The concept of a front was coined during WW1 using what analogy?

Battle Front

Cold fronts have 'spikes' on its leading edge pointing towards what?

Warm air

Warm fronts have 'bobbles' on its leading edge pointing towards what?

Cold air

In general terms, the summer polar front is along what parallel of latitude?

60°N

In general terms, the winter polar front is along what parallel of latitude?

50°N

At the _ the first sign of a depression forming is a bulge of warm air into the colder air.

Surface

The combination of curvature and ascent causes the low to begin to 'wind up'. The process is cumulative, and the consequence is an increasing mass of ascending air. As might be expected, this cools and condenses to form _.

Cloud

In a classic warm sector depression, a warm front will bring the first signs of its approach. The warm front, the commonly-quoted figure is a slope gradient of about 1:150 what front will have the first signs of its approach?

Moving warm front

A warm front as depicted on a METFORM chart. According to the forecast it will bring poor visibility, low stratus cloud down to _ above sea level in places and rain.

200 feet

The surface wind often veers sharply and becomes _ with the passage of a cold front as a polar maritime air mass is introduced.

Turbulent

What backs slightly (anticlockwise) ahead of a cold front?

Surface Wind

A narrow band of rain around the surface frontal position, some heavy, especially at the front, is linked to what front?

Cold

What happens with speed, relative to warm and cold fronts?

Speed

Structure of occluded front tends to be _ and difficult to generalize.

Complex

In the cases of Manchester airport, there seems to be what change, if any across the occulsion?

Little

The fronts that exist within a ‘typical’ warm sector depression, what is mentioned about travel?

Speed it tends to travel

A depression that is still _ (meaning that the pressure at the centre is falling) brings more active fronts and generally worse weather.

Deepening

In meteorology it is __ to try to lay down hard and fast rules about the type of conditions that certain atmospheric conditions will cause.

Difficult and dangerous

A cold front can have extensive layered stratus cloud, what atypical cloud formations will cold fronts have?

Cumulus-type clouds

A trough is formed on the of a mountain and is more specifically an orographic or low formation?

Lee side

Most depressions bringing fronts to western Europe form over the North Atlantic in low pressures, what type of depressions/low occur, albeit less frequently?

Non-Frontal

The boundary between two different air masses is called a _.

Boundary

Areas of significantly high terrain can hold up the progression.

Front

At the _ a bulge of warm air pushes into the cold air, causing a ‘kink' in the front.

Surface

In a classic warm-sector depression, the warm front will bring the first signs of its approach, close to the _, are cirrus-type clouds.

Tropopause

Thickening of high level cloud, and persistent aircraft contrails, are often the first visual signs of an approaching depression, as seen in _airport.

Newcastle airport

Less commonly the cold front _ over the slope of the warm front, leaving less cool air behind the surface front than ahead, in which case the result is a 'warm' occlusion.

Rides up

In either case, a slow-moving occluded front can bring ___ low cloud and heavy (and prolonged) precipitation.

Persistent

On a weather chart a 'quasi-stationary' front will be shown with cold and warm front symbols on either side of the front.

Alternating

The sharper the wind veer associated with a front (as seen by a sharp 'V' in the isobars at the front), the more active the front will be. An acute 'V' in the isobars implies a strong wind.

Shear

Intense surface heating leads to a widespread ascent of air, creating a small 'closed' low, what type of low is this?

Heat Low

The concept of 'front' came from who during the First World War?

The Norwegian meteorologist

The 'bobbles' on a front schematic indicate the front is a _ front.

Warm

Jet streams are fast moving streams of what?

High Altitude Air

As more air flows into the surface, at the first sign of a depression forming, it acquires what characteristic?

A curve

The frontal zone as the front passes is rarely sharp, even if the frontal zone is narrow, what area is mentioned?

40 km

Which front tends to be steeper?

Cold Front

The __ of a depression marks the beginning of the end of its life cycle.

Occluded

The low gradually fills, marking the end of its life. The fronts become less active and what happens to the winds?

Slower Moving

What designation is used for slow-moving fronts?

Quasi-Stationary

The track of the depression is largely guided by what air formation?

Jetstream

Active cold fronts are particularly good at producing what type of weather effect?

Windshear

There is often an area of very cold, clear air just behind a cold front, bringing _ for a few hours until cumulus clouds and showers move in.

Clear skies

A trough formed on the lee side of a mountain occurs because the _.

Air column is 'stretched'

During what war was the concept of a front first coined?

First World War

What is the ratio of surface to height in a warm front?

1:150

The initial formation and the subsequent movement of the depression are closely linked with the existence of what?

Jetstream

As the centre of the low deepens (the pressure reduces) the air flow becomes circular. Soon there are well-defined warm and cold fronts marking the boundary between the differing air masses. What is happening to air masses as the press reduces?

Ascending

By now the depression is moving eastwards, by the Jetstream?

Steered

Visibility 8000 m, light rain, how many oktas at 1300 ft?

1-2 oktas