Khushboo Malhotra
Powerful tropical cyclones occur all around the world, but their names vary depending on where they crop up. The annual tropical cyclone season has been a busy one for hurricanes and typhoons. So, visualising their characteristics and tendencies will show the difference between various storm systems and how they form.
Gusts of strong winds lashed Vietnam, Thailand, and the Philippines earlier in September 2022. This Typhoon Noru, or Super Typhoon Karding in the Philippines caused widespread agricultural mayhem. The two most destructive storms pummelled Western Cuba and the Southeast of the United States. Likewise, the aftermath of the two hurricanes – Hurricane Ian and Hurricane Fiona – drastically hit parts of the Caribbean and Eastern Canada. Millions were then forced to evacuate their homes.
So why do we call one storm a hurricane and the other a typhoon? And while we’re at it, what exactly is a cyclone?
What are tropical cyclones?
A tropical cyclone is a generic term used by meteorologists to describe “a rotating, organized system of clouds and thunderstorms that originates over tropical or subtropical waters, and has a closed low-level circulation,” according to the National Oceanic and Atmospheric Administration (NOAA) of the US.
Tropical cyclones rotate counterclockwise in the Northern Hemisphere and are classified according to their maximum wind speed intensity and the structure of the system:
- Tropical Depression: maximum sustained winds of 38 mph (33 knots or fewer)
- Tropical Storm: maximum sustained winds of 39 to 73 mph (34 to 63 knots)
- Hurricane: maximum sustained winds of 74 mph (64 knots) or higher
- Major Hurricane: maximum sustained winds of 111 mph (96 knots) or higher, corresponding to category 3, 4, or 5 on the Saffir-Simpson Hurricane Wind Scale
Hurricanes, typhoons, and cyclones are one of mother nature’s most catastrophic natural disasters. These natural calamities produce cataclysmic winds, storm surge flooding, and heavy rainfall that can bring a variety of life-threatening hazards including: inland flooding, tornadoes, and rip currents that alter property, livelihood, infrastructure, development, non-moving objects, wildlife, forests, crops, and land.
They are all essentially the same type of circular storms that form over warm waters with very low air pressure at the centre and winds exceeding 74 mph. But these terms are not synonymous, as they have different names based on their location (tropical cyclone basins).
- Hurricanes: Occur in the North Atlantic, Eastern North Pacific and South Pacific Ocean, often affecting the US east coast and Caribbean.
- Typhoons: Occur in the Western North Pacific Ocean and usually affect Asia, frequently hitting the Philippines and Japan.
- Cyclones: Occur in the South Pacific and the Indian Ocean, often knocking countries from Australia all the way to Mozambique. In the Bay of Bengal, the Arabian Sea, and the Western South Indian Ocean, the name is “cyclone”. In the eastern part of the Southern Indian Ocean and North-West Australia, it is “willy-willy.”
How are they formed?
Tropical cyclones are warm-core low-pressure systems associated with a spiral inflow of mass at the bottom level and a spiral outflow at the top level. As a result of large-scale atmospheric disturbances, they form over mass bodies of relatively warm water, gaining energy through water vapour. This is abundant in oceans and seas.
“heat releases, warming the atmosphere and making the air lighter”
As NOAA described, water vapour rises into the atmosphere, and as it rises it cools to condensation, ultimately condensing into liquid droplets that form clouds and rain. During the condensation process, heat releases, warming the atmosphere and makes the air lighter, continuing to rise into the atmosphere. More air moves in near the surface to take its place, which is the strong wind we feel from these storms.
This replenishing of moisture-bearing air after rain can produce an exceptionally intense downpour that lasts for several hours or days up to 40 km (25 mi) from the shoreline, exceeding the amount of water that the local atmosphere can hold. This can whisk up enormous waves, which flood extensive areas, including towns and cities.
Tropical cyclones are typically strongest when over or near water. Costal regions are therefore more vulnerable, weakening rapidly over inland regions. Once the eye of the storm moves over land, it diminishes because the storm lacks the moisture and heat sources that the ocean provided.
For these disturbances to grow into a tropical cyclone, the following environmental conditions must happen:
- Warm ocean waters with sea surface temperatures need to be at least 27°C (naturally near the equator), throughout a depth of about 150 ft.
- Atmospheric instability, driven by differences in temperature, encourages the formation of massive vertical cumulus clouds.
- Moist air near the mid-level of the troposphere (16,000 ft).
- Must be at least 300 miles north or south of the equator for it to spin.
- Little change in wind speed or direction with height.
The aftermath of tropical cyclones on human populations is undoubtedly devastating. But, they do play a role in relieving drought conditions. Instead the tropics carry heat and energy away from the tropics and transport it towards temperate latitudes. This plays a significant role in regulating global climate.
How are they named?
“Typhoon Haiyan or Hurricane Katrina are now retired.”
The World Meteorological Organization (WMO) maintains rotating lists of names appropriate for each tropical cyclone around the world. Once a storm escalates to tropical storm-level winds, they gave it a name.
There can be over one cyclone at a time. So, weather forecasters give each tropical cyclone a unique name to avoid confusion. Countries in the regions of hurricanes, typhoons, and cyclones send suggestions for the list to the global met authority. WMO reuses these names every six years, although they may retire if used to name a particularly deadly or costly storm. For instance, the names of the deadliest storms like Typhoon Haiyan or Hurricane Katrina are now retired.
The history and cultural junctions have significantly influenced tropical cyclone terminology. To learn more about the practise of naming tropical cyclones and its historical context, click here.
A season for every storm: When do they occur?
Besides having different names, hurricanes, typhoons, and cyclones also have different seasons.
The Atlantic hurricane season runs from 1 June to 30 November, and over 95% of tropical cyclone activity occurs during this period. Typhoons in the Northwest Pacific Ocean are most common from May to October, although they can form year-round. The North Indian Ocean cyclone season has no official bounds. But, cyclones usually form between April and December, with their peak from May to November. And in the South Pacific, it’s cyclone season between November and April. The season begins two weeks later and concludes at the same time in the Southern Indian Ocean. The Island Republics of Mauritius and the Seychelles are the exceptions. There, the season is lasts until May 15.
Featured image courtesy of JD Designs on Unsplash. Image license can be found here. No changes were made to this image.
Khushboo Malhotra
Khushboo is a Twitter Editor at Empoword Journalism. She is an Award-nominated lifestyle and pop culture journalist, writer, and budding entrepreneur aiming to break down the boundaries of cultural stigma and shame attached to mental health and sexual health and bring marginalised topics to light. Send your stories, tips, and press announcements to khushboomalhotrafreelance@gmail.com