L9 High Pressure Systems

Anticyclones

• An anticyclone is an area of high pressure.

• Two types:

  • Warm: Typically form over tropical and subtropical climates.

  • Cold: Form over polar regions.

• Air subsides slowly at around $2000$ ft per day.

• As air subsides, it is compressed and warms, similar to how a bicycle pump heats up when used.

• Dry air warms at about $3°C$ per $1000$ ft (Dry Adiabatic Lapse Rate - DALR) as it sinks.

• Air movement: Sinks near the surface, then diverges (spreads out) from the center.

• Wind flows clockwise in the northern hemisphere.

• Pressure gradient is generally slack with widely spaced isobars, leading to light winds.

• Associated with descending air, so cloud formations are layered or small cumulus with limited vertical extent.

• Subsidence discourages cloud formation, though local heating can cause small cumulus clouds (fair-weather cumulus, or "cu").

• Stable atmosphere usually prevents significant growth of cumulus clouds.

• Summer anticyclones in Europe often bring heat waves due to clockwise flow bringing tropical continental air from the south.

• Strong anticyclones are slow-moving or stationary, causing prolonged conditions.

• Pilots generally experience light winds, clear skies, and calm conditions.

• Extensive subtropical highs exist over oceanic areas around $30°N/S$ throughout the year.

• Central Europe in summer high-pressure areas: wide isobar spacing, calm winds, and local wind systems.

• Winter high-pressure areas: calm winds and widespread fog.

Flat Pressure Pattern

• Occurs when an anticyclone weakens.

• Most frequent in summer.

• Can persist for several days.

• Warm air parcels rise until they reach their dew point, forming cumulus clouds with vertical extent.

• Moderate to severe turbulence with strong updrafts may occur beneath these clouds.

• Thermal thunderstorms are most likely in mid-afternoon to late evening when uplift is strongest.

• Frontal thunderstorms can develop at any time.

• Both thermal and frontal thunderstorms can be heavy, occasionally with hail, and should be avoided.

Anticyclones and Temperature Inversion

• Subsidence within an anticyclone may cause a temperature inversion.

• Temperature inversion: Temperature rises with increasing altitude (reverse of normal).

• Example:

  • Surface: $+20°C$

  • $1000$ ft: $+21°C$

  • $2000$ ft: $+22°C$

  • $3000$ ft: $+23°C$

  • $4000$ ft (Top of inversion): $+23°C$

  • $5000$ ft: $+20°C$

  • $6000$ ft: $+17°C$

• The top of the inversion acts as a 'lid' on the atmosphere.

• Tropopause acts as a large-scale inversion, keeping water content and weather in the troposphere.

• Inversion prevents rising thermals from the ground from rising any further. It separates the air above it from the friction layer.

• Wind flow above and below the inversion can be markedly different, with possibility of stronger winds above the inversion than below.

• Inversions can affect VHF radio waves.

• Smoke, dust, and haze are concentrated in the inversion layer.

• Lack of strong winds and precipitation means pollutants aren't dispersed or washed out.

• Visibility within the inversion layer worsens over days.

• The top of the inversion layer will gradually rise.

• This persists until the anticyclone breaks down (often with thunderstorms and heavy showers) or a depression moves in, leaving clearer air.

• In winter, clear skies and calm conditions can bring very good (if cold) flying conditions initially.

• Temperatures may fall below freezing, causing frosty or icy runways and aircraft.

• After a few days, an inversion layer appears and haze reduces visibility.

• Persistent layer of turbulence cloud may make VFR flight below the inversion difficult.

• A winter anticyclone will likely persist until a strong and fast-moving depression moves in.

Anticyclones and Radiation Fog

• In spring and autumn, slack winds and clear skies can encourage radiation fog formation, which may persist for several days.

• Cooling of the ground by radiation makes the air in contact with it colder than the air above, encouraging a low-level inversion.

• The separation of wind flow above the inversion from the friction layer below leads to noticeable windshear at the top of the inversion.

• Example: Early December anticyclone over the UK for about $10$ days.

• METFORM forecast: Zone A covers most of England and Wales.

• VFR flight is not possible.

• Cloud tops (generally $2500$ ft) indicate the inversion layer level.

Typical Anticyclonic Conditions

• Southern England view: Air above stratocumulus turbulence cloud is clear and smooth.

• Cloud top is at the inversion level at $2500$ ft.

• Below the inversion: Visibility is less than $1000$ m.

• Example case: Top of control tower at Ronaldsway Airport ($115$ ft AGL) obscured by cloud.

Clouds

• High pressure anticyclone systems are associated with descending air.

• Cloud formations: layered or small cumulus clouds with limited vertical extent, caused by local warming of the surface.

• Rising air cools, forming clouds as moisture condenses.

Ridges

• A ridge is an elongated area of high pressure, bringing similar weather to an established anticyclone.

• Weak ridges are common in winter, appearing between successive depressions.

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

Cols

• A col is an area of stagnant air, sandwiched between two highs and two lows.

• Winds are light due to the lack of isobars.

• Weather is influenced by the strongest nearby pressure system.

• Requires a meteorologist with knowledge of the upper atmosphere to make a sensible forecast.

• In spring and autumn, light winds can encourage fog.

• Thunderstorms may break out in summer.

• Difficult to generalize about weather in and around cols, as each one is an individual case.