There are three basic stages in the life of a tropical cyclone, its origin or source, the mature stage, and the dissipation stage where it dies out. These occur in a continuous process, not as separate and distinct stages. Each stage may occur more than once during the life cycle as the strength of the cyclone rises and falls.
It may reach land, weaken, then go back out to sea where it strengthens once more. The formation of a cyclone depends upon the following conditions coinciding. A large, still and warm ocean area with a surface temperature that exceeds 26.5 degrees Celsius over an extended period. This allows a body of warm air to develop above the ocean's surface. Low altitude winds are also needed to form a tropical cyclone.
As air warms over the ocean it expands, becomes lighter and rises. Other local winds blow in to replace the air that has risen, then this air is also warmed and rises. The rising air contains huge amounts of moisture evaporated from the ocean's surface.
As it rises it cools, condensing to form huge clouds about 10 kilometres up. in the troposphere. More warm air rushes in and rises, drawn by the draft above.
The rising drafts of air carry moisture high into the atmosphere, so that these clouds eventually become very thick and heavy. Condensation then releases the latent heat energy stored in the water vapor, providing the cyclone with more power. This creates a self-sustaining heat cycle.
Drawn further upwards by the new release of energy, the clouds can grow to 12 to 15 kilometers high. The force created by the Earth's rotation on a tilted axis, the Coriolis effect, causes rising currents of air to spiral around the center of the tropical cyclone. It is at this stage that the cyclone matures and the eye of the storm is created. As the air rises and cools, some of this dense air descends to form the clear, still eye as the cyclone rages around it. The eye wall, where the wind is strongest, behaves like a whirling cylinder.
Cyclones rotate clockwise in the southern hemisphere, anticlockwise in the northern. The lowest air pressure in a tropical cyclone is always found at the centre, and is typically 950 millibars or less. The average air pressure at the Earth's surface is about 1,010 millibars.
Tropical cyclones have significantly lower air pressure than the air that surrounds them. The bigger the pressure difference, the stronger the wind force. One of the lowest air pressures ever recorded was 877 millibars for Typhoon Ida, which hit the Philippines in 1958, where winds reached 300 kilometres an hour. Transforms formed. the cyclone's movement or track follows a pathway away from its source, driven by global wind circulation.
As warm ocean waters feed it heat and moisture, the cyclone continues to enlarge.