How Tropical Cyclones Form and Move

Tropical cyclones usually form over ocean water warmer than about 26.5 degrees Celsius across a deep enough layer. Warm water increases evaporation, adding water vapour to the air. When that moist air rises and cools, the vapour condenses into cloud droplets.

Condensation releases latent heat into the atmosphere. This warms the surrounding air, making it more buoyant and encouraging further rising. Surface pressure falls beneath the rising air, so more warm, moist air flows inward. The process can strengthen itself while the storm remains over a suitable ocean.

A cyclone does not form from warm water alone. The atmosphere must also be unstable enough for air to rise, and the air needs plenty of moisture. Strong vertical wind shear can tear the growing system apart by pushing its upper clouds away from the centre.

Air moving towards low pressure is deflected by Earth's rotation through the Coriolis effect. This helps the developing storm rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Cyclones rarely form very close to the equator because the Coriolis effect is too weak there.

As the circulation strengthens, bands of thunderstorms curve around the centre. In a mature cyclone, the eye may become a relatively calm region of sinking air. Around it sits the eyewall, where some of the strongest winds and heaviest rain occur.

The pressure difference between the centre and surrounding air helps drive the wind. A lower central pressure often supports stronger winds, although storm size and structure also matter. Meteorologists use satellites, aircraft, radar, ocean buoys and computer models to monitor these changing features.

Tropical cyclones are carried by surrounding winds, especially broad high-pressure systems and upper-level airflow. Their tracks can curve as these patterns change. Forecasting improves as new observations arrive, but small differences in the atmosphere can still shift a projected path.

Storms usually weaken over land, cool water or strong wind shear because they lose their supply of warm moist air or their structure becomes disrupted. Hazards can continue after wind speeds fall, especially where slow movement produces prolonged rain.

Follow these ingredients when reading a storm map:

• •Warm deep ocean water provides heat and moisture.
• •Rising air lowers surface pressure.
• •Condensation releases energy into the storm.
• •Earth's rotation helps organise circulation.
• •Surrounding winds steer the storm's track.

Strong winds are the most famous feature of a tropical cyclone, but coastal flooding is often more deadly. A storm surge is a rise in sea level driven mainly by winds piling water towards the shore. The low pressure at the centre also lets the sea bulge upward slightly. A gently sloping, shallow seabed allows water to build higher as it approaches land, and if the surge arrives at high tide, the flooding can reach far inland.

This is why the category of a storm does not tell the whole story. Common scales, such as the one used for Atlantic hurricanes, rate storms by sustained wind speed alone. A large, slow storm with weaker winds can push a bigger surge or drop enormous amounts of rain over the same area for days. Forecasters therefore issue separate warnings for surge, rainfall and wind, and evacuation orders often focus on low coastal zones rather than on wind strength.

Forecasters name tropical cyclones because names are easier to remember and communicate than positions or numbers, especially when several storms are active at once. A warning about a named storm is less likely to be confused with another system in the same ocean. The World Meteorological Organization coordinates separate naming arrangements for different ocean regions. For Atlantic hurricanes, six alphabetical lists are used in rotation, so a list returns every six years, with names alternating between male and female.

When a storm is especially deadly or destructive, its name is usually retired and replaced, so that it is not reused for a future storm and does not cause distress to the people affected. Katrina, which devastated New Orleans in 2005, is one example. Not every region uses personal names. In the western North Pacific, the countries and territories that share forecasting duties each contribute names, and many are words for animals, flowers, places or objects, so the list reflects several languages and cultures rather than a single tradition.

A popular claim says water swirls down a plughole one way in the Northern Hemisphere and the other way in the Southern Hemisphere, just as cyclones do. In ordinary sinks and baths this is not true. The Coriolis effect depends on how far and for how long something moves across the rotating Earth. A cyclone spans hundreds of kilometres and lasts for days, giving the tiny deflection time to build into an organised spin. Water in a basin travels less than a metre in a few seconds, so the deflection is far weaker than the effects of the basin's shape and leftover currents from filling it.

Scientists have shown that the effect can appear under very carefully controlled conditions. In the early 1960s, an engineer at the Massachusetts Institute of Technology filled a large round tank, left the water to settle for about a day, then drained it slowly and saw a consistent anticlockwise swirl. Ordinary household drains give no such guarantee. Demonstrations for tourists near the equator, in which water swirls one way a few metres north of a painted line and the other way a few metres south, depend on how the bowl is moved or the water is poured, not on Earth's rotation.

The force of wind on a wall or roof rises with the square of the wind speed, so doubling the speed roughly quadruples the push. The categories used for Atlantic hurricanes start at 119 kilometres per hour for category 1 and reach 252 kilometres per hour for category 5. That is roughly twice the speed, which means about four times the pressure on every exposed surface. Damage grows faster still, because once a roof edge or window fails, wind can get inside a building and push outwards as well. Each step up the scale therefore represents far more than a modest increase.

A storm's own motion also makes one side more dangerous than the other. In the Northern Hemisphere, where the winds circle anticlockwise, the forward movement of the storm adds to the wind speed on the right-hand side of its track and subtracts from it on the left. Picture a storm with winds of 150 kilometres per hour circling its centre while the whole system moves at 20 kilometres per hour. On the right the combined wind can approach 170, and on the left it may be nearer 130. In the Southern Hemisphere the pattern is reversed, so the left side is usually worse.

Tropical cyclones form when warm ocean water, moist rising air, low wind shear and planetary rotation combine. They strengthen by drawing energy from condensation and move with larger atmospheric currents. Track the energy source, rotation and steering winds separately, and the behaviour of these complex storms becomes easier to explain.