The Earth's weather is a consequence of its insolation received by the Sun. The atmospheric circulation can be viewed as a heat engine driven by the Sun's energy. It is the large-scale movement of air and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth.
The pattern of the movement of the planetary winds is called the general circulation of the atmosphere. This is divided into three parts-Primary winds, Secondary winds and Tertiary winds. The Primary winds (trade winds, westerlies and easterlies) are responsible for the majority of the planetary atmospheric circulations. Apart from these there are some seasonal and regional circulations as well which are called secondary circulations. Among these the phenomenon of cyclones and anticyclones are the two prominent ones.
Cyclones
In meteorology, the term ‘cyclone’ can be defined as the rapid inward circulation of air masses around a low-pressure center which is circling counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. Cyclonic rotation is associated with low-pressure systems, so that cyclone and low can be used interchangeably.
Depending upon the location and the characteristics cyclones are divided into 2 types- Tropical cyclones and Temperate cyclones.
Tropical cyclones
Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large scale destruction caused by violent winds, very heavy rainfall and storm surges. Tropical cyclones originate and intensify over warm tropical oceans. The conditions favourable for the formation and intensification of tropical storms are:
- Large sea surface with temperature higher than 27° C.
- Presence of the Coriolis force.
- Small variations in the vertical wind speed.
- A pre-existing weak low- pressure area or low-level-cyclonic circulation.
- Upper divergence above the sea level system.
The energy that intensifies the storm comes from the condensation process in the towering cumulonimbus clouds, surrounding the center of the storm. With continuous supply of moisture from the sea, the storm is further strengthened. The more time they spend over the seas, the stronger they become. On reaching the land the moisture supply is cut off and the storm dissipates. The place where a tropical cyclone crosses the coast is called the landfall of the cyclone.
Temperate Cyclone
Temperate cyclones are also known as Extra-tropical cyclones where the term “Extra-tropical” signifies that this type of cyclone generally occurs outside the tropics with a latitude range between 30° and 60°. In these regions there exists varying air masses which leads to the formation of Fronts. When two different air masses meet, the boundary zone between them is called a front. The process of formation of the fronts is known as frontogenesis.
Extra tropical cyclones form along the polar front. Initially, the front is stationary. In the northern hemisphere, warm air blows from the south and cold air from the north of the front. When the pressure drops along the front, the warm air moves northwards and the cold air moves towards south, causing an anticlockwise cyclonic circulation. The cyclonic circulation leads to a well-developed extra tropical cyclone, with a warm front and a cold front.
The warm air glides over the cold air and a sequence of clouds appear over the sky ahead of the warm front and cause precipitation. The cold front approaches the warm air from behind and pushes the warm air up. As a result, cumulus clouds develop along the cold front. The cold front moves faster than the warm front, ultimately overtaking the warm front. The warm air is completely lifted up and the front is occluded and the cyclone dissipates. Temperate cyclones are comparatively weaker than the tropical cyclones but extend up to hundreds of kilometers. Unlike Tropical cyclones, temperate cyclones can move both over land and sea and can last upto 15 days.
Anticyclones
An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure. It rotates clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere (opposite to a cyclone). Characteristics of surface-based anticyclones include clear skies as well as cooler and drier air. Fog can also form overnight within a region of higher pressure.
Due to climate change, tropical cyclones are likely to increase in intensity, causing increased rainfall, and have larger storm surges. Tropical cyclones may also intensify more rapidly, and occur at higher latitudes. These changes are driven by rising sea temperatures and increased maximum water vapour content of the atmosphere as the air heats up. The 2018 U.S. National Climate Change Assessment reported that "increases in greenhouse gasses and decrease in air pollution have contributed to increases in Atlantic hurricane activity since 1970".
Cyclones and anticyclones are very crucial components of the atmospheric circulation. They help in maintaining the heat budget of the earth by redistributing the heat especially in the temperate areas. Given the uncertainties due to climate change, an effective forecasting along with a reliable early warning system is the need of the hour for better disaster resilience.
