How do tropical storms form?

Tropical storms occur between 5* and 30* north and south of the equator. Find out how they form below.

How do tropical storms form?

What are the conditions which cause tropical storms?

Conditions which cause tropical storms	Why does this contribute to their formation?
Low latitude Between 5-30 degrees north and south of the Equator	Temperatures are higher here than at the Poles so the sea and air are heated more quickly, to higher temperatures. Air pressure is low, and air rises.
Originate in oceans with temperatures above 26.5°C to a depth of 60-70m	Provides heat and moisture so warm air rises rapidly.
Between summer and autumn	Typically the warmest seasons to encourage warmer air to rise rapidly, on account of low pressure.
Low wind shear	Wind is constant and doesn’t vary so clouds rise to high altitudes without being torn apart.

The Formation of a Tropical Storm

Tropical Storms start between 5º and 30º north and south of the equator, where surface sea temperatures reach at least 26.5ºC, which is about the temperature of a heated swimming pool, and why we don’t get tropical storms in the UK.
Air is heated above the surface of these warm tropical oceans. The warm air rises rapidly under low-pressure conditions.
The rising air draws up more airing large volumes of moisture from the ocean, causing strong winds.
The Coriolis effect (spinning movement of the Earth) causes the air to spin upwards around a calm central eye of the storm.
As the air rises, it cools and condenses to form large, towering cumulonimbus clouds, which generate torrential rainfall. Heat is given off when the air cools and powers the tropical storm.
Cool air sinks into the eye. Therefore, there is no cloud, so it is drier, clear and much calmer.
The tropical storm travels across the ocean by the prevailing wind.
When the tropical storm meets land, it is no longer fuelled by the source of the moisture and heat from the ocean, so it loses power and weakens.

The diagram below summarises the formation of a tropical storm.

What is the structure of a Tropical Storm?

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.

The structure of a hurricane in the northern hemisphere

The weather system generates heat which powers the storm, causing wind speeds to increase. This causes the tropical storm to sustain itself. Tropical storms rely on plenty of warm, moist air from the sea – this is why they die out over land.

Annotated Cross Section of a Tropical Storm

Why Do Tropical Cyclones Spin?

The rotation of tropical cyclones is a defining feature, giving them their distinctive spiral shape. The strong winds within these storms are driven by air moving from high to low-pressure areas. In the case of tropical cyclones, this movement is influenced by the Coriolis effect. 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.

Case Studies
Below is a list of Tropical Storms, each with a link to a site containing information about it:

1. Hurricane Katrina – America’s costliest hurricane.

2. Hurricane Andrew – 1992 America’s second-costliest hurricane

3. Hurricane Mitch – 1998

4. Indian ‘Super Cyclone’ 1999 Orissa cyclone – The strongest and deadliest cyclone in the region since the Bangladesh cyclone of April 1991

5. Cyclone Eline – 2000 The cause of the major disaster in Mozambique

For current hurricanes, visit the US National Hurricane Centre website.