Weather Hazards: Tropical Cyclones Transcript Reporter: The cyclone has made landfall as a Category 4, and continues its path of destruction across towns and communities along the east coast. Monster waves caused by the storm surge are lashing the coast. Torrential rain is falling, and destructive winds of 240 kilometres an hour have been recorded. Flying debris is a threat to human life, and power lines are down. Hundreds of thousands of homes are without electricity. There's still many hours to go, and we urge people to remain inside their homes and stay sheltered. Seek higher ground if flooding occurs. Narrator: Thirty percent of the world's population live in areas that are affected by tropical cyclones. The better we understand them, the better we can take action to protect people and property. So let's look at what qualifies as a tropical cyclone, how they're formed, and the conditions required to bring them about. ‘Tropical cyclones’ is the generic term used by meteorologists to describe a rotating, organised system, strong winds, thick clouds, intense rain and thunderstorms that form and develop as part of a low pressure system over tropical or sub-tropical waters. Once its winds sustain 118 kilometres an hour, it is officially classed as either a cyclone, hurricane, or typhoon. The name differs according to the part of the world in which they occur, but they all refer to tropical cyclones. In the regions of the South Pacific and Indian Ocean, the term 'cyclone' or 'tropical cyclone' is used. In the Atlantic and northeast Pacific, they're called 'hurricanes', while in the northwest Pacific they're known as 'typhoons'. We'll use the term 'tropical cyclone', the general, global term used for such storms. Tropical cyclones occur every year, and many may develop in any one warm season. Tropical cyclones typically form in late summer when the oceans are warmed. This is mid-July to September in the northern hemisphere, and midJanuary to March in the southern hemisphere. To differentiate them, in most regions they are given alternating male and female names which follow an alphabetical list. This makes communication between meteorologists and the public more straightforward when issuing forecasts, watches, and warnings. The exception is the western north Pacific and north Indian Oceans, where they are named after animals, flowers, trees, foods, or descriptive adjectives. Tropical cyclones form in the world's warmest areas, the tropics, at latitudes between about 5 and 30 degrees. It can take hours or even days for a tropical cyclone to develop. Conditions have to be favourable. So what are tropical cyclones and how do they form? 1 17GEO01C© ClickView Pty Limited 2017. Weather Hazards: Tropical Cyclones Transcript 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 ten kilometres up in the troposphere. More warm air rushes in and rises, drawn by the draught above. The rising draughts 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 vapour, 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 kilometres 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 centre 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 a 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, anti-clockwise 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. 2 17GEO01C© ClickView Pty Limited 2017. Weather Hazards: Tropical Cyclones Transcript Once 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. Tropical cyclones do not form near the equator where the Coriolis effect is weak, but between 5 and 30 degrees north and south of the equator. Satellite photography is used to identify where a cyclone originates, which then helps weather forecasters to plot where it will go next. Dissipation is the result of the tropical cyclone moving over land or cool water. When it reaches land it loses its energy source, the warm ocean water. Air pressure rises as the temperature falls, and winds drop as the land surface interferes with the air flow. Rainfall decreases, and eventually it weakens to become a tropical storm. Tropical cyclones can be an incredibly destructive and deadly force of nature. Understanding their life cycles is critical to minimising the loss of life and property they cause, and for mounting effective disaster responses. 3 17GEO01C© ClickView Pty Limited 2017.