L8 Low Pressure Systems
Low Pressure Systems - Depressions
Definition of a Front
Fronts are typically associated with depressions, which are areas of low pressure.
A front is the boundary between two different air masses.
The concept was introduced by a Norwegian meteorologist during World War I, drawing an analogy to a battlefront.
In the North Atlantic, the polar front marks the boundary between cold polar maritime air from the north and warm tropical maritime air from the south.
Warm and Cold Fronts
A front is classified based on whether it signifies a transition from warm to cool air or vice versa.
A cold front brings cold air, replacing warmer air. The 'spikes' are on the leading edge of the front indicate the direction of movement.
A warm front brings warm air, replacing cold air. The 'bobbles' are on the leading edge of the front indicate the direction of movement.
Direction of a Front
A warm front occurs when warm air replaces cold air.
A cold front occurs when cold air replaces warm air.
A front represents the boundary between two different air masses.
Areas of high terrain can impede frontal progression.
If a front reverses direction, its type changes, even if other conditions remain constant.
Fronts are often associated with cloud and precipitation, especially those linked to depressions.
Understanding the development and life cycle of a depression is crucial to understanding frontal systems.
Development and Life-Cycle of a Classic Warm-Sector Depression
Meteorology is an inexact science due to the chaotic nature of weather systems.
No two depressions or fronts are identical.
Describing a 'standard' depression provides a basic understanding for assessing real-life situations.
Depressions affecting western Europe originate over the North Atlantic along the polar front.
The polar front's position varies daily, generally around 60°N in summer and 50°N in winter.
Formation of a Depression
A depression starts as a semi-stationary front with cold air to the north and warm air to the south.
A 'bulge' of warm air pushes into the cold air, creating a 'kink' in the front.
Mature Depression
A depression begins as an unstable wave along the polar front.
Fine-scale mechanisms are complex and related to jet streams above the polar front.
Jet streams are fast-moving air streams at high altitude over transition zones like the polar front.
The formation and movement of the depression are closely linked to the jet stream.
Pressure at the bulge's center decreases as air rises, initiating a circular motion.
As pressure continues to deepen, the depression rotates with defined boundaries (fronts) between warm and cold air.
Cross Section of a Warm Sector Depression
The initial sign of a depression is a warm air 'bulge' into colder air.
Warm air rises, causing surface pressure to fall.
Inflowing air curves, initiating the 'winding up' of the low.
Ascending air cools and condenses, forming cloud, and releasing energy that strengthens the depression.
The center of the low deepens, and airflow becomes circular.
Well-defined warm and cold fronts mark the boundaries between air masses.
The warmer air between fronts is called the "warm sector."
The depression moves eastwards, steered by the jet stream, and continues to deepen.
The Warm Front
In a classic warm-sector depression, the warm front signals the initial approach.
The warm front has a gentle slope, typically around 1:150.
Cirrus clouds appear at the top of the frontal slope, near the tropopause, up to 500 nm (925 km) ahead of the surface front.
As the front nears, cloud thickens and lowers.
Rain or snow falls from altostratus clouds around 15,000 ft but often evaporates before reaching the ground.
Slight precipitation reaches the surface 100-200 nm (185-370 km) ahead of the front, increasing to moderate and continuous.
Pressure steadily falls as the front approaches.
The surface front has the lowest cloud (nimbostratus), possibly reaching the surface.
The transition is gradual, even if the frontal zone is narrow (25 nm/40 km), and can be up to 90 nm/150 km.
Early Signs of a Warm Front
Thickening high-level cloud and persistent aircraft contrails indicate an approaching depression.
Warm Front Approaching
Visibility decreases and light rain starts.
Cloud continues to thicken and lower.
The sun disappears completely.
Warm Front Arrives
Rain becomes moderate, pressure continues to fall, and cloud thickens and lowers.
Visibility decreases significantly.
After the warm front passes, the weather begins to clear, visibility and cloud base improve, and temperature rises.
The wind veers by 90° and becomes stronger and gustier.
The first sign of the passage of a warm front may be a veering of the surface wind, followed by a steadying of the pressure and an increase in both temperature and dew point.
Precipitation should diminish, but low cloud, drizzle, and poor visibility may persist behind the front.
Characterisitcs of the Warm Front - Summary
Slope: Average 1:150, but may be steeper for more active fronts
Pressure: Falls increasingly as the front approaches. Generally becomes steady in the warm sector but continues to fall if depression is deepening.
Surface Wind: Tends to back (anticlockwise) and increase ahead of the front. Veers (clockwise) on frontal passage
Temperature: May rise on passage of the front, but not necessarily as rain depresses temperature
Cloud: Increasing amounts of high cloud (starting 400 - 450 NM ahead of the frontal surface), thickening and lowering with the approach of the front
Weather: Slight rain approximately 100 - 200 NM ahead of the surface front becoming moderate close to the surface front, ceasing after frontal passage. Scattered outbreaks of slight rain or drizzle may occur in the warm sector
Dewpoint: Starts to rise ahead of the front, leveling off on passage of the front
Visibility: Good ahead of the front, becoming moderate in precipitation, occasionally poor in the warm sector
The Cold Front
The cold front's slope is steeper than the warm front, averaging 1:50.
The zone of cloud and rain is narrower and more likely to include convective cumulus-type cloud.
Passage of the surface front may involve heavy precipitation and squally conditions.
Active cold fronts can exhibit a line of cumulus or cumulonimbus clouds, potentially hidden by lower clouds.
The passage of a cold front often brings a dramatic weather change.
Cloud and rain often clear quickly, leading to drier and clearer conditions.
Temperature and dew point decrease, while pressure increases.
Surface wind often veers sharply and becomes turbulent, known as 'frontal turbulence'.
Characteristics of the Cold Front - Summary
Slope: Average 1:50, but may be steeper for more active fronts
Pressure: Starts to fall as front approaches, rising quickly on frontal passage and behind the front
Surface Wind: Backs slightly (anticlockwise) ahead of the front. Veers sharply (clockwise) on frontal passage
Temperature: Usually falls but may rise due to insolation in clearer air behind the front
Cloud: Thick layers of stratiform cloud. Some active cold fronts have occasional embedded cumulonimbus. Cloud becoming convective and well-broken behind the front
Weather: A fairly narrow band of rain around the surface frontal position, some heavy, especially at the front. Risk of hail and thunder if cumulonimbus are present
Dewpoint: Fall on passage of front
Visibility: Moderate in precipitation, improves rapidly to good or excellent behind the front
Occluded Fronts
In a 'classic' warm-sector depression, the cold front moves faster than the warm front.
Eventually, the cold front overtakes the warm front, lifting the 'warm sector' air off the surface.
The faster-moving cold front 'undercuts' the warm front.
A temperature difference exists across the occluded front, though less distinct than in warm or cold fronts.
A 'cold' occlusion: air behind the surface front is colder than that ahead.
A 'warm' occlusion: the cold front rides up over the warm front, leaving less cool air behind the surface front than ahead.
The structure of occluded fronts tends to be complex and hard to generalize.
Passage of an Occluded Front
An occlusion combines the cloud and precipitation characteristics of both warm and cold fronts.
Slow-moving occluded fronts can cause persistent low cloud and heavy precipitation.
Often, there is little change in wind velocity across the occlusion.
The occluding of a depression marks the end of its life cycle.
The low gradually fills, slows down, and the fronts become less active.
Stationary Fronts
Fronts in a 'typical' warm sector depression usually travel at 20-30 knots.
A front may slow down or stop, becoming a 'quasi-stationary' front.
Quasi-stationary fronts bring persistent conditions like low cloud, poor visibility, and precipitation.
These fronts can have 'waves' moving along them, similar to a fluttering flag.
Weather charts depict quasi-stationary fronts with alternating cold and warm front symbols.
Secondary Depressions
A slow-moving depression may leave a long cold front.
A 'secondary' depression can form within the 'old' primary depression along this front.
Secondary depressions can develop rapidly, causing bad weather.
General Properties of Warm-Sector Depressions
It is difficult to establish strict rules about atmospheric conditions due to complex interactions.
Fronts can vary in intensity, from causing extensive cloud and rain to passing almost unnoticed.
Warm fronts don't always bring stratus-type cloud, and cold fronts don't always bring cumulus build-ups.
Warm fronts can (rarely) have thunderstorms, and cold fronts can have stratus cloud.
The jet stream largely guides the track of depressions.
Surface observations indicate the direction and speed of a depression.
Depressions tend to steer around anticyclones.
Occluded depressions become less active as they fill up.
Deepening depressions bring more active fronts and worse weather.
Further Properties of Warm-Sector Depressions continued
Ridges of high pressure often occur between successive depressions, bringing brief periods of fine weather (often overnight).
Frontal weather is worse near the center of the depression and improves away from the low or where fronts cross high-pressure areas.
Greater temperature differences across a front result in more active fronts.
Temperature changes at the surface during frontal passage can be influenced by cloud cover or sunshine.
Veering wind and changes in dew point are more reliable indicators of frontal passage.
Sharper wind veers, indicated by a sharp 'V' in the isobars, suggest more active fronts and potential windshear.
Warm Sector Depressions Synoptic Chart
Fronts may be weak (inactive) or strong (active). A strong warm front may be followed by a weak cold front, and vice versa.
A 'dry slot' of very cold, clear air often follows a cold front, bringing clear skies for a few hours.
Troughs (lines of low pressure) on weather charts can cause front-like weather without air mass changes.
Orographic troughs, found on the lee side of mountains, result from the stretching of the air column as it descends.
Non-Frontal Lows
Most depressions with fronts affecting western Europe originate over the North Atlantic.
Other types of lows, without fronts, can also occur, such as heat lows over continental Europe in summer.
Intense surface heating leads to widespread air ascent.
Inflow of air at low levels is curved, potentially forming a small 'closed' low.
Heat lows are often shallow and have little weather impact.
Unstable air within heat lows can lead to cumulonimbus clouds and thunderstorms.
A 'heat low' is also known as a 'thermal low'.