Tropical cyclones. The Admiralty Sailing Directions (Pilot Books) describe the approach of a cyclone “the atmosphere becomes oppressive, ship board sounds are amplified and there is a feeling of impending doom.” Tropical cyclones are compact, circular storms, generally some 320 km (200 miles) in diameter, whose winds swirl around a central region of low atmospheric pressure. The winds are driven by this low-pressure core and by the rotation of the Earth, which deflects the path of the wind through a phenomenon known as the Coriolis force. The wind field of a tropical cyclone may be divided into three regions. First is a ring-shaped outer region, typically having an outer radius of about 160 km (100 miles) and an inner radius of about 30 to 50 km (20 to 30 miles). In this region the winds increase uniformly in speed toward the centre. Wind speeds attain their maximum value at the second region, the Eyewall, which is typically 15 to 30 km (10 to 20 miles) from the centre of the storm. The Eyewall in turn surrounds the interior region, called the Eye, where wind speeds decrease rapidly and the air is often calm. These main structural regions are described in greater detail below.
The Eyewall. The most dangerous and destructive part of a tropical cyclone is the Eyewall. Here winds are strongest, rainfall is heaviest, and deep convective clouds rise from close to the Earth’s surface to a height of 15,000 metres (49,000 feet). Friction at the surface, in addition to lowering wind speeds, causes the wind to turn inward toward the area of lowest pressure. Air flowing into the low-pressure eye cools by expansion and in turn extracts heat and vapour from the sea surface. Updrafts are vital to the existence of the towering convective clouds embedded in the Eyewall. Much of the heavy rainfall associated with tropical cyclones comes from these clouds.
The Eye. A characteristic feature of tropical cyclones is the Eye, a central region of clear skies, warm temperatures, and low pressure. Typically, pressure at the surface of the Earth is about 1,000mb. At the centre of a tropical cyclone, however, it is typically around 960mb. Horizontal winds within the eye, on the other hand, are light. In addition, there is a weak sinking motionas air is pulled into the eyewall at the surface. As the air subsides, it compresses slightly and warms, so that temperatures at the centre of a tropical cyclone are some 5.5 °C (10 °F) higher than in other regions of the storm. Because warmer air can hold more moisture before condensation occurs, the eye of the cyclone is generally free of clouds.
Rainbands. In addition to deep convective cells (compact regions of vertical air movement) surrounding the eye, there are often secondary cells arranged in bands around the centre. These bands, commonly called rainbands, spiral into the centre of the storm. The rotating cloud bands often are associated with an apparent wobbling of storm track.
Conditions for formation of a cyclone.There are six conditions favourable for this process to take place. The conditions are listed first below:
1. 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.
2. A preexisting atmospheric circulation must be located near the surface warm layer.
3. The atmosphere must cool quickly enough with height to support the formation of deep convective clouds.
4. The middle atmosphere must be relatively humid at a height of about 5,000 metres (16,000 feet) above the surface.
5. The developing system must be at least 500 km (300 miles) away from the Equator.
6. 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).