Tropical Cyclones Yaas

Definition of a Cyclone

• Cyclone refers to any spinning storm that rotates around a low-pressure center. The low-pressure centre is also referred to as the 'eye' of the storm, which is well known for being eerily calm compared with the areas under the spinning 'arms' of the storm.
• You could say that the eye is watching what's going on down below, so it needs a clear path, but the arms are where all the action happens because this is where the storm is throwing out all of its rain and wind.

How are cyclones formed?

• To form a cyclone, warm, moist air over the ocean rises upward from near the surface. As this air moves up and away from the ocean surface, it leaves is less air near the surface. So basically as the warm air rises, it causes an area of lower air pressure below.
• Air from surrounding areas with higher air pressure pushes in to the low pressure area. Then this new “cool” air becomes warm and moist and rises, too. And the cycle continues.
• As the warmed, moist air rises and cools the water in the air forms clouds. The whole system of clouds and wind spins and grows, fed by the ocean’s heat and water evaporating from the ocean surface.
• As the storm system rotates faster and faster, an eye forms in the centre. It is very calm and clear in the eye, with very low air pressure. Higher pressure air from above flows down into the eye.
• Tropical cyclones usually weaken when they hit land, because they are no longer being “fed” by the energy from the warm ocean waters. However, they often move far inland, dumping many centimetres of rain and causing lots of wind damage before they die out completely.

Tropical cyclone

• Tropical cyclone is an intense circular storm that originates over warm tropical oceans and is characterized by low atmospheric pressure, high winds, and heavy rain.
• Drawing energy from the sea surface and maintaining its strength as long as it remains over warm water, a tropical cyclone generates winds that exceed 119 km (74 miles) per hour. In extreme cases winds may exceed 240 km (150 miles) per hour, and gusts may surpass 320 km (200 miles) per hour.
• Accompanying these strong winds are torrential rains and a devastating phenomenon known as the storm surge, an elevation of the sea surface that can reach 6 metres (20 feet) above normal levels.
• Such a combination of high winds and water makes cyclones a serious hazard for coastal areas in tropical and subtropical areas of the world. Every year during the late summer months (July–September in the Northern Hemisphere and January–March in the Southern Hemisphere), cyclones strike regions as far apart as the Gulf Coast of North America, northwestern Australia, and eastern India and Bangladesh.

Different names of tropical cyclones

• Tropical cyclones are known by various names in different parts of the world. In the North Atlantic Ocean and the eastern North Pacific they are called hurricanes, and in the western North Pacific around the Philippines, Japan, and China the storms are referred to as typhoons.
• In the western South Pacific and Indian Ocean they are variously referred to as severe tropical cyclones, tropical cyclones, or simply cyclones.
• All these different names refer to the same type of storm.

Conditions for formation of tropical cyclones

• The temperature of the surface layer of ocean water must be 26.5 °C (80 °F) or warmer, and this warm layer must be at least 50 metres (150 feet) deep.
• A preexisting atmospheric circulation must be located near the surface warm layer.
• The atmosphere must cool quickly enough with height to support the formation of deep convective clouds.
• The middle atmosphere must be relatively humid at a height of about 5,000 metres (16,000 feet) above the surface.
• The developing system must be at least 500 km (300 miles) away from the Equator.
• The wind speed must change slowly with height through the troposphere—no more than 10 metres (33 feet) per second between the surface and an altitude of about 10,000 metres (33,000 feet).

Types of Cyclones:

Tropical cyclones are what most people are familiar with because these are cyclones that occur over tropical ocean regions.

• Hurricanes and typhoons are actually types of tropical cyclones, but they have different names so that it's clear where that storm is occurring. Hurricanes are found in the Atlantic and Northeast Pacific, typhoons are found in the Northwest Pacific. If you hear 'tropical cyclone,' you should assume that it's occurring in the South Pacific or Indian Ocean, but for this lesson, we'll use it refer to all types of tropical ocean cyclones.
• We can also further describe tropical cyclones based on their wind speeds. They are called category 1, 2, 3, 4 or 5, increasing with intensity and wind speed as the number increases. A category 1 cyclone is the weakest, with wind speeds of 74-95 mph. A category 5 cyclone, on the other hand, is extremely dangerous and has the potential for major damage. Category 5 cyclones have wind speeds of 155 mph and above!
• Polar cyclones are cyclones that occur in polar regions like Greenland, Siberia and Antarctica. Unlike tropical cyclones, polar cyclones are usually stronger in winter months. As you can see, these storms really do prefer the colder weather! They also occur in areas that aren't very populated, so any damage they do is usually pretty minimal.
• A mesocyclone is when part of a thunderstorm cloud starts to spin, which may eventually lead to a tornado. 'Meso' means 'middle', so you can think of this as the mid-point between one type of storm and the other. Tornadoes all come from thunderstorm clouds, but not all thunderstorm clouds make tornadoes. In order for a tornado to occur, part of that cloud has to spin, and though you can't really see this happening, this is the intermediate, or 'meso' step from regular cloud to dangerous spinning cloud running along the ground.

Depending upon its location and strength, a tropical cyclone is referred by different names:

• Typhoons in Western North Pacific
• Willy-willies in Australia
• Baguio in Philippine Islands
• Hurricanes around North America
• Taifu in Japan
• Cyclone in the Indian Ocean

How are the cyclones named?

• If the speed of a cyclone is more than 34 nautical miles per hour then it becomes necessary to give it a special name. If the speed of the storm reaches or crosses 74 mph, it is then classified into a hurricane/cyclone/typhoon.
• The cyclones that are formed in any ocean basin around the world are named by the Regional Specialised Meteorological Centres (RSMCs) and Tropical Cyclone Warning Centres (TCWCs). There are a total of six RSMCs in the world, including the India Meteorological Department (IMD).
• The World Meteorological Organization (WMO) and the United Nations Economic and Social Commission for the Asia Pacific (ESCAP) have been naming cyclonic storms since 2000.
• The India Meteorological Department (IMD) names the cyclones developing over the north Indian Ocean, including the Bay of Bengal and the Arabian Sea. It also issues advisories to 12 other nations in the region on the development of cyclones and storms.
• In 2000, a group of nations called WMO/ESCAP-- Bangladesh, India, the Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand-- decided to name cyclones in the region.
• In 2018, five more countries were added-- Iran, Qatar, Saudi Arabia, United Arab Emirates and Yemen.
• After the aforementioned countries sent in suggestions, the WMO/ESCAP Panel on Tropical Cyclones (PTC) finalise the list.
• In April 2020, IMD released a list of 169 cyclone names. 13 suggestions were sent in by the aforementioned WMO/ESCAP member nations.

Why are cyclones named?

• The cyclones are named to help people identify them easily as it would be difficult to remember numbers and technical terms.
• Additionally, appending names makes it easier for the media, scientific community and the disaster management community to identify and report individual cyclones, disseminate warnings, increase community preparedness, and ward off confusion in areas that witness multiple cyclones.

Guidelines to name cyclones

The guidelines to name the cyclones are as follows:

1. The proposed name must be neutral to politics and political figures, religious beliefs, cultures and genders.
2. It must not hurt the sentiments of any group of people across the world.
3. It must not be rude and cruel in nature.
4. The name must be short, easy to pronounce and inoffensive to any member.
5. It must be of a maximum of eight letters and be given with its pronunciation and voice over.
6. The names of cyclones developing over the north Indian Ocean will not be repeated. Once used, it will cease to be used again.