Anticyclone

An anticyclone (or high-pressure system) is a region where the atmospheric pressure is higher than in its surroundings. It is characterized by isobars that are more widely spaced than in depressions, indicating weak pressure gradients and generally light winds.

Visual representation of circulation in a high pressure area.

Air Dynamics

The movement of air in an anticyclone is characterized by:

1. Aloft (Convergence): Air converges at the upper levels of the troposphere.
2. Center (Subsidence): Air descends slowly. During this descent, the air is compressed and warmed adiabatically. This warming evaporates clouds and increases atmospheric stability, often creating subsidence inversions.
3. Surface (Divergence): Air flows outwards from the center of high pressure. Due to the Coriolis force, this flow is clockwise in the Northern Hemisphere.

Types of Anticyclones

There are mainly two thermal categories (cold and warm) and specific configurations such as blocking highs or ridges.

1. Cold Anticyclones

Typically form over continents in winter (e.g., Siberian High) or over polar regions.

• Cause: Radiative cooling of the surface which cools the lower layers of air, making it denser.
• Vertical Structure: They are shallow systems.
  • They weaken with height.
  • Isobaric surfaces curve upwards at low levels (high pressure), but because cold air is compressed vertically, pressure falls rapidly with height. At high levels, they may become low-pressure areas (isobars curve downwards).
• Weather: Strong inversions, severe frosts, clear and very cold air at the surface.

2. Warm Anticyclones

These are deep, dynamic systems, often semi-permanent (e.g., Azores High).

• Cause: Dynamic subsidence of warm air from the upper troposphere.
• Vertical Structure: They are deep systems.
  • They intensify with height.
  • Isobaric surfaces curve upwards at all levels. The warm air expands the atmospheric column, maintaining high pressure even at high altitude.

3. Blocking Highs

These are anticyclones (usually warm) that remain quasi-stationary or move very slowly, "blocking" the usual westerly flow.

• They divert frontal depressions towards higher or lower latitudes.
• They can persist for weeks, causing prolonged periods of dry weather in summer or cold/fog in winter.

4. Ridge

An elongated extension of high pressure projecting from a main anticyclone.

• Usually located between two depressions.
• Isobars are always rounded ("U" shape), never "V" shaped.
• Provides a brief and transient interval of fair weather.

Associated Weather

Weather in an anticyclone is generally stable due to subsidence, but varies by season and surface type.

Feature	Summer	Winter
Clouds	Very few or clear skies. Occasionally small cumulus or clouds at the edges.	Often extensive stratus or stratocumulus trapped under the subsidence inversion (Anticyclonic Gloom).
Precipitation	None.	Generally none, possible light drizzle from thick stratiform clouds.
Visibility	Moderate with haze due to pollutant accumulation under the inversion.	Moderate to poor. Conducive to radiation fog (over land) and mist.
Temperature	Warm (in warm anticyclones). Large daily temperature range.	Relatively mild in warm anticyclones; very cold in continental cold anticyclones.
Wind	Light and variable.	Light, calm in the center.

Note on Fog: In winter, anticyclones favor radiation fog over land at night due to radiative cooling under clear skies and calm winds. Near the coast, if the wind blows from the warmer sea towards the cold land, advection fog may form.