Anticyclones

When we talk about cyclones, most people immediately think of storms, winds, and destruction. But the atmosphere has another equally important system—anticyclones. Unlike cyclones, which are associated with violent weather, anticyclones are often linked with calm, stable conditions. That is why they are sometimes called “weatherless phenomena.”

Let us understand their nature:

General Characteristics

• Definition:
  An anticyclone is a wind system where air pressure is highest at the centre and decreases outward.
  • In the Northern Hemisphere, winds blow outward in a clockwise direction.
  • In the Southern Hemisphere, winds blow outward in an anticlockwise direction.
• Location:
  They are most common in the subtropical high-pressure belts but are practically absent in the equatorial region.
• Weather association:
  Anticyclones are generally linked with rainless, clear, and stable weather, because air descends from above at the centre, preventing cloud formation.

Key Properties

1. Shape & Pressure:
   Usually circular (sometimes V-shaped).
   • Maximum pressure at the centre.
   • Pressure falls outward, usually by 10–20 mb, sometimes up to 35 mb.
2. Size:
   Much larger in area than temperate cyclones.
3. Movement:
   They often appear after cyclones but are slow-moving and unpredictable.
   • Average speed: 30–50 km/h.
   • Sometimes they remain stationary over a place for days.
4. Wind circulation:
   Outward spiraling winds—clockwise in north, anticlockwise in south.
5. Vertical motion:
   Air descends at the centre → creates stability, hence clear sky and no rainfall.
6. Temperature conditions:
   • Vary with air mass origin.
   • In summer: associated with warm air masses → hot conditions.
   • In winter: associated with polar air masses → very cold conditions.
7. No Fronts:
   Unlike cyclones, anticyclones do not have fronts.

Wind System in Anticyclones

• The pressure gradient is weak → wind system is not fully developed.
• Winds diverge outward from the high-pressure centre.
• In an advancing cold anticyclone:
  • Front portion → Westerly winds.
  • Rear portion → Easterly winds (sluggish compared to front).
• The centre usually experiences a light breeze.
• In warm anticyclones, wind circulation is even weaker.

Shape and Size

• Generally circular, near-circular, or wedge-shaped.
• Extremely large in size:
  • Diameter can extend up to 9,000 km.
  • A single temperate anticyclone may cover half of the USA.

Temperature

• Depends on origin of air mass:
  1. Cold Anticyclones → Descend from polar cold air mass.
     • Associated with extremely low temperature.
     • Cause cold waves in winter.
     • In summer, they can bring pleasant conditions.
  2. Warm Anticyclones → Formed from warm tropical air mass.
     • Associated with high temperature.
     • Cause heat waves in subtropical regions during summer.

In Summary

👉 Anticyclones are high-pressure systems, larger and slower than cyclones, with divergent winds, descending air, and mostly clear, stable weather.
👉 Their effect depends on the type of air mass—cold anticyclones can bring extreme chill, while warm ones bring heat waves.

So, while cyclones are often dramatic and destructive, anticyclones are subtle yet powerful regulators of weather, silently shaping seasons and climate patterns.

Weather Conditions

• Clear and dry weather:
  At the centre of an anticyclone, air descends. As it sinks, it warms up at the dry adiabatic rate.
  • Warming reduces the normal lapse rate (i.e., fall of temperature with height).
  • The atmosphere becomes more stable → less vertical movement → less chance of cloud formation.
  • Result = dry, cloudless, rainless weather.
• Occasional exceptions:
  If an anticyclone passes over oceans, it may pick up moisture → causing light rains and moderate clouds.
• Indicators of arrival:
  • Clearing of existing clouds
  • Calm and clear weather
  • Decrease in wind velocity
• Regional impact:
  Anticyclones strongly influence the climate of Canada, USA, and North Eurasia, often shaping seasonal weather patterns.

Classification of Anticyclones

Over time, different scholars have classified anticyclones differently:

• Hanzilk (1909):
  • Warm anticyclones
  • Cold anticyclones
• Humphreys:
  • Mechanical or Dynamic anticyclones
  • Thermal or Heat anticyclones
  • Radiation anticyclones
• A.E.M. Gaddes:
  • Large anticyclones (covering whole continents)
  • Temporary anticyclones (250–300 km, marginal areas of continents)
  • Cyclone-originated anticyclones (caused by high pressure between two temperate cyclones)

Modern Simplified Classification (widely used):

1. Cold Anticyclones
   1. Warm Anticyclones
   1. Blocking Anticyclones

Let us understand each in detail.

(i) Cold Anticyclones

• Origin: Arctic regions.
• Movement: Move easterly and south-easterly.
• Size & structure:
  • Smaller than warm anticyclones.
  • Move more rapidly.
  • Very shallow in height → rarely more than 3,000 m thick.
• Thermal origin:
  • They are thermally induced, not due to descent of air.
  • Formed because of very low insolation in Arctic winters → extreme cold → high pressure at surface.
• Subtypes:
  • Temporary anticyclones → die out during transit, rarely reach tropics.
  • Semi-permanent anticyclones → travel longer distances and remain active.
• Tracks:
  • From northern Canada → affect Canada & USA.
  • From northern Siberia → move towards China, Japan, Alaska.
• Sometimes also affect North-West Europe (in rear portion of temperate cyclones).
  • But when they reach tropical regions, they die out due to rise in temperature.

(ii) Warm Anticyclones

• Origin: In subtropical high-pressure belts due to descending air and surface divergence.
• Nature:
  • Dynamically induced (unlike cold ones, which are thermal).
  • Very large in size, but sluggish in movement.
  • Sometimes become stationary for days or weeks.
• Weather:
  • Light winds
  • Clear sky
  • Stable, rainless weather
• Influence: Particularly important for Southeastern USA and Western Europe weather.

(iii) Blocking Anticyclones

• Definition: These anticyclones act like a “block” in the upper tropospheric air circulation, obstructing the normal west-to-east flow of westerlies.
• Location:
  • Common over Northwest Europe & adjoining Atlantic Ocean
  • Also found in North Pacific Ocean (140°–170° W longitude)
• Nature:
  • Similar to warm anticyclones in terms of pressure, wind, and weather
  • But they are smaller in size and move very slowly
• Importance: Blocking anticyclones often cause prolonged cold waves or heat waves, since they “lock” a particular weather condition over a region for an extended period.

In Summary

• Anticyclones mostly bring dry, calm, and clear weather, but sometimes give light rain if over oceans.
• Based on origin and characteristics, they are of three main types:
  1. Cold Anticyclones → Small, fast, thermally induced, shallow, move from Arctic regions.
  2. Warm Anticyclones → Large, sluggish, dynamically induced, common in subtropics.
  3. Blocking Anticyclones → Act as barriers in upper air circulation, prolonging certain weather patterns.

👉 Thus, while cyclones are dramatic and short-lived, anticyclones are slow, subtle, and long-lasting weather-makers that quietly control much of the climate we experience.