Mumbai is witnessing an above normal February temperatures due to the prevailing North-Westerly winds in the city generated by an anticyclonic system above the Arabian Sea

• The IMD has forecast daily highs of between 36o to 37oCelsius until at least February end.
• A Large Diurnal Range: Mumbai is also witnessing a large diurnal range (highest and lowest temperatures in a 24-hour period) because of the anticyclone system resulting in hotter days and relatively cooler nights.
  • A large diurnal range is atypical for february and are generally seen in March.

About AntiCyclones Systems

• An anticyclone is a pressure system with a High Pressure central region than the surrounding parts.
• Wind Circulation: In anticyclone the winds circulate around the centre in a clockwise sense in the northern hemisphere and anticlockwise in the southern hemisphere.
• Formation: Anticyclones form when air converges in the upper troposphere, leading to a buildup of pressure at the surface.
  • Horizontal Divergence at Ground Level: The anticyclone is an area of horizontal divergence at surface level implying  a downward motion of the air accompanied by adiabatic heating. 
• Pressure Gradient: High pressure systems have small pressure gradients (ie the air pressure doesn’t change rapidly).  
• Effect on Weather: Small pressure gradients means gentler winds and as the air sinks, it warms up, leading to warm and dry weather. Anticyclones produce periods of settled and calm weather lasting many days or weeks.
• ‘Blocking Highs’: Anticyclones are much larger than depressions and often block the path of depressions, either slowing down the bad weather, or forcing it round the outside of the high pressure system thus called ‘Blocking Highs’.
• Examples:
  • Subtropical Highs: These are persistent anticyclones that occur in the subtropics and contribute to stable weather patterns. 
  • Polar Anticyclones: These are high-pressure systems that form over the poles and are strongest during the cold season.
• Types: 
  • Cold Anti-Cyclone: The air at the surface and in the lower layer of the troposphere is colder than the air in the adjacent region. 
    • Cold anti-cyclones are of limited vertical extent; they play a very important role in low level atmospheric circulation in winter.
    • The air in this type of anti-cyclone is denser than the surrounding air level.
    • Characteristics: Cloudless skies; Temperature drop, making the days cold and the nights even colder due to lack of cloud cover; Fog and frost forming at night
  • Warm Anti-cyclone: The air for the greater part of the troposphere is warmer than that of the surrounding region. 
    • These are the oceanic sub-tropical belts of high pressure systems which are characterized by subsiding air mass. 
    • Characteristics: Few or no clouds. Strong sunshine will make it hot; Light winds; Cooling of ground leading to morning mist; Warm moist air rising from the ground forming thunderstorms
• Impact on Weather
  • Anticyclones and Heat: As the descending air warms and inhibits cloud formation, allowing the sun’s rays to reach the surface, they can also be associated with heatwaves, especially in summer.
  • Anticyclones and Fog: In winter, anticyclonic weather can lead to the formation of fog, as the descending air can cool and allow moisture to condense near the ground
• Differences between Cyclones and Anticyclones