What is the distribution and characteristics of tropical cyclones?

Tropical cyclones are classed as rotating storms that are large in scale and form over large bodies of warm water, and are mainly found in tropical oceans. Across the world, these tropical storms are known by different names. In the Americas, Hurricanes are formed in the North Atlantic and the Pacific coast of the USA. In South-East Asia, Typhoons form in the western North Pacific Ocean. In the Southern Indian Ocean and South Pacific Ocean surrounding northern Australia, you will find Cyclones. While particularly harmless when moving over the sea, as soon as these storms hit land, the effects can be devastating.

The distribution of tropical storms

What hazards do tropical storms bring?

Cyclones can bring a range of hazards once they make landfall:

• Strong winds can damage homes and businesses, cut power and topple trees, making the recovery effort more hazardous.
• Storm surges can cause severe flooding due to a combination of very high tides and low air pressure. Flooding can reach far inland, cutting off communities.
• Intense rainfall can lead to localised flooding, with up to 1000mm falling during a cyclone. In 1967, there was a record of 2700mm of rain that fell during Typhoon Carla in mainland China. Still, the wettest record goes to the Pacific Island of La Reunion, which recorded a staggering 6400mm of rain in 1980 during Cyclone Hyacinthe.
• Landslides are also a significant hazard when the ground becomes saturated. 53 people were sadly killed when, in 2014, Typhoon Jangmi hit the Philippines, causing the ground to slump, triggering the deadly landslide.

Structure of a Tropical Cyclone

Annotated Cross Section of a Tropical Storm

• Ocean temperatures need to be at least 26.5°C.
• Cyclones tend to form in late summer as the oceans have warmed up. This is between June and November in the northern tropics, and April and November in the southern tropics. Much like rainfall, cyclones can have a seasonal distribution.
• To help cyclones form, the troposphere needs to have high winds, usually located 10-12km above the Earth’s surface. This allows the warm air to be drawn up from the ocean.
• The Earth’s rotation plays a key role. This is referred to as the Coriolis force. Cyclones are formed between 5° and 30° latitude, where the rotation is stronger.
• The central part of the tropical storm is known as the eye. The eye is usually between 30-50km across. It is an area of calm, with light winds and no rain. It contains descending air.
• Large cumulonimbus clouds surround the eye. These are caused by moist air condensing as it rises. Wind speeds average 160km per hour around the eye.

Key stages in a tropical cyclone formation

The formation of a tropical cyclone

1. Tropical storms rely on plenty of warm, moist air from the sea (26.5°C). This warm air rises, with more warm air rushing in to replace it. A draught is created above which draws in more warm air.
2. This updraught contains a large volume of water vapour. As this cools and condenses, large cumulonimbus clouds are formed. Energy is also released from the water vapour when condensation if formed, further powering the cyclone.
3. The Coriolis effect influences the rotation of tropical cyclones. The Earth’s rotation causes these winds to be deflected into a spin, resulting in all tropical cyclones in the Northern Hemisphere rotating counterclockwise and those in the Southern Hemisphere rotating clockwise. The rising currents of air eventually cool, with some of the currents descending to form the cloudless eye of the storm.
4. Once tropical storms move away from their source, the cyclone absorbs new heat and moisture from the oceans, growing larger in size. The prevailing wind influences the direction the cyclone travels, and this is called the track. Changes in wind direction can alter the cyclone’s path, sometimes causing it to curve or form a hook-shaped track.
5. Once the tropical storm makes landfall, the energy source of the ocean is no more. The storm reduces in intensity as the temperature falls and the air pressure rises. This reduces the rainfall and leads to calmer winds.