L9 High Pressure Systems

Flashcards about High Pressure Systems, Anticyclones, and Ridges. These flashcards cover key concepts, characteristics, and weather phenomena associated with these meteorological systems, focusing on their impact on flying conditions and atmospheric phenomena.

What is an anticyclone?

An area of high pressure.

Where do warm and cold anticyclones typically form?

Warm anticyclones form over tropical and sub-tropical climates, and cold anticyclones form over polar regions.

At what rate does air subside in an anticyclone?

Approximately 2000 ft per day.

What happens to air as it subsides in an anticyclone?

The air is compressed and warms.

At what rate does dry air warm as it sinks in an anticyclone, according to the Dry Adiabatic Lapse Rate (DALR)?

3°C per 1000 ft.

What is the general movement of air within an anticyclone?

Air sinks near the surface and then diverges, with the wind flowing clockwise in the northern hemisphere.

What type of winds are typically associated with anticyclones due to the slack pressure gradient?

Light winds.

What type of cloud formations are associated with high-pressure systems or anticyclones?

Layered or small cumulus clouds with no great vertical extent.

What type of air movement is associated with high-pressure systems?

Descending air.

What causes the formation of cumulus clouds in an anticyclone?

Local warming of the surface causing rising air to cool and form clouds.

What are the small cumulus clouds that form in an anticyclone often called?

Fair-weather cumulus (cu).

What type of air does a summer anticyclone in Europe often bring?

Tropical continental air from the south.

What flying conditions are typically associated with the arrival of anticyclonic conditions?

Light winds, clear skies, and generally calm conditions.

At what latitudes can extensive areas of high pressure (subtropical Highs) be found over oceanic areas?

Around 30°N/S.

What conditions are typically found in high-pressure areas during summer in Central Europe?

Wide isobar spacing, calm winds, and the formation of local wind systems.

What conditions can be expected in high-pressure areas during winter?

Calm winds and widespread areas of fog.

When does a flat pressure pattern typically occur?

When an anticyclone begins to weaken.

During what season is a flat pressure pattern most frequent?

During the summer.

What type of clouds can warm air parcels turn into when they reach their dew point during hot days?

Cumulus clouds with significant vertical extent.

What type of turbulence may be experienced beneath cumulus clouds with significant vertical extent?

Moderate to severe turbulence with strong updrafts.

When are thermal thunderstorms most likely to occur?

Between mid-afternoon and late evening.

When can thunderstorms triggered by frontal activity develop?

At any time of the day.

What is the best course of action regarding thermal and frontal thunderstorms?

Give them a very wide berth.

What is a temperature inversion?

A layer of the atmosphere through which the temperature rises with increasing altitude.

What acts as a 'lid' on the atmosphere in a temperature inversion?

The top of the inversion.

What happens to rising thermals from the ground when they reach the top of a temperature inversion?

They stop on reaching the top of the inversion.

How does an inversion affect wind flow above and below it?

Wind flow above and below it can be markedly different.

What happens to pollutants as an inversion layer becomes established?

Smoke, dust, and haze are concentrated in it.

What happens to visibility and the top of the inversion layer over a period of days?

The visibility within the inversion layer will worsen, and the top of the inversion layer will gradually rise.

What events can cause the persistent situation of worsened visibility in an inversion layer to change?

The anticyclone breaks down or a depression moves in.

What conditions can a winter anticyclone initially bring for flying?

Clear skies and calm conditions.

What potential hazards can occur if temperatures fall below freezing in a winter anticyclone?

Frosty or icy runways and aircraft.

What happens after a few days of a winter anticyclone regarding visibility?

The inversion layer appears and the resulting haze reduces visibility.

What will probably cause a winter anticyclone to persist until it changes?

A strong and fast-moving depression moves in.

What conditions within an anticyclone can encourage the formation of radiation fog in spring and autumn?

Slack winds and clear skies.

What causes the formation of a low-level inversion in spring and autumn anticyclones?

Cooling of the ground by radiation.

What does the cloud tops' height (generally 2500 ft) indicate in the context of the METFORM forecast example?

The level of the inevitable inversion layer.

What is the visibility typically like below the inversion in anticyclonic conditions?

Less than 1000 m.

What type of air movement is associated with a high-pressure anticyclone system?

Descending air.

What type of cloud formations are associated with a high-pressure anticyclone system?

Layered or small cumulus clouds with no great vertical extent.

What is a ridge?

An elongated area of high pressure.

What type of weather does a ridge usually bring?

Very similar weather to that of an established anticyclone.

What is a particular feature of 'weak' ridges during winter weather?

They often appear between successive depressions and bring a short period of fine weather.

What is a col?

An area of stagnant air, sandwiched between two highs and two lows.

What can be said with certainty about the winds within a col?

Light winds.

What often influences the weather situation within a particular col?

The strongest pressure system nearby.

What weather phenomena can occur in cols during spring/autumn and summer, respectively?

Fog may form, and thunderstorms may break out.

What is the key consideration when forecasting weather in and around cols?

Each one is an individual case.

What are the two main types of anticyclones, based on their formation location?

Subtropical and Polar.

In which direction does air flow in an anticyclone in the Northern Hemisphere?

Clockwise in the Northern Hemisphere.

What spacing of isobars is characteristic of an anticyclone?

Isobars are widely spaced.

What is a common name for the small cumulus clouds found in anticyclones?

They are often called 'fair-weather cumulus'.

How does a summer anticyclone in Europe influence air mass movement?

It tends to bring tropical continental air from the south.

How long may a flat pressure pattern last?

It may persist for a prolonged period, e.g., several days.

Beneath the cumulus clouds that may occur during flat pressure patterns, what kind of air currents are likely?

Strong updrafts are likely.

What action is advisable concerning thunderstorms generated by either thermal or frontal activity?

They are best given a very wide berth.

What effect does the top of a temperature inversion have on atmospheric conditions?

It acts as a 'lid' on the atmosphere.

Regarding wind flow, what effect does a temperature inversion have?

Wind flow above and below it can be markedly different.

Over a period of days under a temperature inversion, how will the visibility change?

The visibility within the inversion layer will worsen.

What type of fog is encouraged by slack winds and clear skies in an anticyclone during spring and autumn?

Radiation fog.

What wind-related phenomenon is particularly noticeable in spring and autumn anticyclones due to the low-level inversion?

The separation of the wind flow above the inversion from the friction layer below, leading to marked windshear.

Based on the METFORM forecast example given, what is the possibility of VFR flight?

VFR flight is, to all intents and purposes, not possible.

Where is the top of the cloud in typical anticyclonic conditions?

At the top of the inversion level at 2500 ft.

Define the term: Ridge (in meteorology).

An elongated area of high pressure.

What is a Col in relation to highs and lows?

It is an area of stagnant air sandwiched between two highs and two lows.

What role does an anticyclone play in the formation of a temperature inversion?

The eventual subsidence within an anticyclone may also cause it to form.

In winter weather, when do ‘weak’ ridges often appear?

They often appear between successive depressions.

From where was the weak ridge of high pressure building-up over France and the UK?

An anticyclone centered over Spain.

In typical anticyclonic conditions, what is the visibility below the inversion?

Below 1000 meters.

In typical anticyclonic conditions, what is the state of the air above the unbroken layer of stratocumulus turbulence cloud?

Clear and smooth.

What are high pressure systems also known as?

Anticyclones.

Besides anticyclones, what other high pressure system exists, which is an elongated area of high pressure?

Ridges.

Why does a summer anticyclone often bring 'heatwave' weather in Europe?

The clockwise flow tends to bring tropical continental air from the south.

Why do cumulus clouds formed in an anticyclone not normally grow to any great extent?

Because the atmosphere in an anticyclone is usually stable.

As an inversion layer becomes established, how will the visibility change, and what will happen to the top of the inversion layer?

Worsen, and the top of the inversion layer will gradually rise.

How does the cooling of the ground by radiation affect the air in contact with it, and what does it encourage?

The air in contact with it is much colder than the air above, which encourages the formation of a low-level inversion.

What kind of weather do weak ridges typically bring?

They will bring a short period of fine weather between depressions.

In the view over southern England on the day of the Metform forecast, what is a notable detail about the control tower at Ronaldsway Airport?

The top of the control tower at Ronaldsway Airport is obscured by cloud.

What is the primary characteristic of air movement within an anticyclone that leads to its associated weather conditions?

Subsidence.

What type of depression is likely to end a winter anticyclone?

Strong and fast-moving.

The atmosphere in an anticyclone is usually what?

Stable.

A flat pressure pattern can occur when an anticyclone begins what?

Weakening.

Subsidence within an anticyclone may cause these to form.

Inversions.

Temperature inversions can have profound effects on what type of radio waves?

VHF radio waves.

The air in an anticyclone is doing what?

Subsiding slowly.

Air warms as it subsides because it is what?

Compressed.

When air sinks to the surface in an anticyclone, it diverges or what?

Spreads out.

Because it, light winds are a feature of areas of high pressure.

The pressure gradient within an anticyclone.

What may break out in summer within a Col?

Thunderstorms.

A weak ridge brought a brief spell of clear weather before what?

The next depression moved in.

If temperatures fall below freezing, what may runways and aircraft be?

Frosty.

The winds in Cols are light, as witnessed by the lack of what?

Isobars.

Why will cloud formation be limited in areas of high pressure?

Subsidence discourages the formation of cloud.

Cloud formations will be either layered or small cumulus clouds with no great what?

Vertical extent.

The general discourages the formation of cloud.

Subsidence.

A summer anticyclone in Europe often brings weather.

Heatwave.

What do anticyclones initially bring that's great for flying?

Clear skies.

If air is _, it will be warming at about 3°C per 1000 ft as it sinks.

Dry.

The eventual subsidence within an anticyclone may also cause an inversion to form close to the _.

Surface.

Isobars show lines of equal _ pressure.

Horizontal.