Types of Cyclones

Extratropical cyclones are also called wave cyclone or mid-latitude cyclone or temperate cyclones. The systems developing in the mid and high latitude, beyond the tropics are called the middle latitude or extra tropical cyclones.

To understand and formation of cyclone formation, we have to understand several terms related to it.

Cyclogenesis

It is an umbrella term to identify several different processes that result in a cyclone.

Air Masses

When the air remains over a homogenous area for a sufficiently longer time, it acquires the characteristics of the area. The homogenous regions can be the vast ocean surface or vast plains. The air with distinctive characteristics in terms of temperature and humidity is called an air mass.

It is defined as a large body of air having little horizontal variation in temperature and moisture. The homogenous surfaces, over which air masses form, are called the source regions.

The air masses are classified according to the source regions. There are five major source regions. These are:

• Warm tropical and subtropical oceans;
• The subtropical hot deserts;
• The relatively cold high latitude oceans;
• The very cold snow covered continents in high latitudes;
• Permanently ice covered continents in the Arctic and Antarctica.

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. There are four types of fronts:

• Stationary: When the front remains stationary, it is called a stationary front.
• Warm: When the warm air mass moves towards the cold air mass, the contact zone is a warm front.
• Cold: When the cold air moves towards the warm air mass, its contact zone is called the cold front
• Occluded: When an air mass is fully lifted above the land surface, it is called the occluded front.

The fronts occur in middle latitudes and are characterized by steep gradient in temperature and pressure. They bring abrupt changes in temperature and cause the air to rise to form clouds and cause precipitation.

Formation of Extratropical Cyclones

• Extratropical 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 move 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 plan and cross section of a well-developed cyclone is given in Figure 2.
• 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.

• Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas.
• It brings large-scale destruction caused by violent winds, very heavy rainfall and storm surges.
• The winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Terminology

It varies location to location. They are known as

• Cyclones in the Indian Ocean
• Hurricanes in the Atlantic
• Typhoons in the Western Pacific and South China Sea, and,
• Willy-willies in the Western Australia.

Naming Of Cyclones

• Adopting names for cyclones makes it easier for people to remember, as opposed to numbers and technical terms.
• Cyclones that form in every ocean basin across the world are named by the regional specialized meteorological centers (RSMCs) and Tropical Cyclone Warning Centres (TCWCs).
• There are six RSMCs in the world, including the India Meteorological Department (IMD), and five TCWCs.
• The IMD is mandated to naming of cyclones as well as issuing advisories to 13 countries in the region on the development of cyclones and storms, namely Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka, Thailand, Iran, Qatar, Saudi Arabia, United Arab Emirates and Yemen.

Conditions favorable for the formation

• Large sea surface with temperature higher than 27° C;
• Presence of the Coriolis force;
• Small variations in the vertical wind speed;
• pre-existing weak low-pressure area or low-level-cyclonic circulation;
• Upper divergence above the sea level system.

Characteristics

• The energy that intensifies the storm comes from the condensation process in the towering cumulonimbus clouds, surrounding the centre 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.
• The cyclones, which cross 20^o N latitude generally, re-curve and they are more destructive.
• A mature tropical cyclone is characterized by the strong spirally circulating wind around the centre, called the eye. The diameter of the circulating system can vary between 150 and 250 km. The eye is a region of calm with subsiding air.
• Around the eye is the eye wall, where there is a strong spiraling ascent of air to greater height reaching the tropopause. The wind reaches maximum velocity in this region, reaching as high as 250 km per hour. Torrential rain occurs here.

From the eye wall rain bands may radiate and trains of cumulus and cumulonimbus clouds may drift into the outer region.