What is a Cyclone

Cyclone seen from Space

A tropical cyclone is a low-pressure system which develops in the tropics and is sufficiently intense to produce sustained gale force winds of at least 63km/h. If the sustained wind reaches hurricane force of at least 118km/h the system is defined as a severe tropical cyclone. In other parts of the world they are called hurricanes or typhoons.

Tropical cyclones can cause significant phenomena which can adversely, and sometimes favourably impact on communities and the environment. The most common features are destructive winds and heavy rainfall that can lead to flooding. Storm surge, or coastal inundation by seawater, is a lesser known phenomenon but can be the most dangerous element of a cyclone. Though rare in Australia, tornadoes have been reported during cyclone events.

Severe wind

In tropical cyclones, wind gusts in excess of 90km/h can be expected around their centre, or eye, while in the most severe events, gusts can exceed 360km/h. Although the strongest winds are near the eye, damaging winds can extend hundreds of kilometres from the centre. The eye can have quite calm winds and cloud-free skies, but this lull is temporary and is followed by destructive winds from another direction. This is because, from above, the winds spiral around the eye in a clockwise direction (in the Southern Hemisphere). The effect of this on the ground is that winds on opposite sides of the eye blow in different directions.

Wind damage is mostly caused by the maximum gusts in the cyclone. For this reason, the well-known tropical cyclone severity categories used by the Bureau of Meteorology to communicate warnings are based on maximum gust strengths.

Heavy rainfall

Because tropical cyclones form over warm tropical oceans, they generally hold enormous amounts of moisture and can produce heavy rainfall over extensive areas. Rain can create severe impacts by causing floods and landslides and through the direct damage of materials by contact, such as being driven by wind into buildings. Direct damage is generally the result of wind damage to walls, windows or roofs, which allows water to penetrate buildings.

Rainfall can be associated with the tropical cyclone when it impacts on the coast or further inland as it weakens to become a tropical depression. Heavy rain from tropical cyclones or tropical depressions can often reach Australia's more southerly latitudes where the rainfall is a major source of water for the country's inland river and ground water systems. Flooding can wreak havoc over vast areas, inundating land, isolating communities and destroying infrastructure.

Storm surge

Potentially the most dangerous hazard associated with tropical cyclones which make landfall is storm surge. Storm surge has been responsible for more deaths than any other feature of tropical cyclones. Storm surge is a raised dome of water about 60 to 80 kilometres across and typically about two to five metres higher than the normal tide level. It is caused by a combination of strong winds driving water onshore and the lower atmospheric pressure in a tropical cyclone. In the southern hemisphere the onshore winds occur to the left of the tropical cyclone's path. In Australia, this is the east side on the north west and north coasts and the south side on the east coast.

The largest surge usually extends between 30 and 60 kilometres from the crossing point of the tropical cyclone centre, or eye. Its influence also depends on the local topography of the seafloor and the angle at which the cyclone crosses the coast. If the surge occurs at the same time as a high astronomical tide the area inundated can be extensive, particularly along low-lying coastlines.

Interesting fact: Tropical cyclone Tracy is the smallest recorded cyclone, with a radius of gales extending less than 50 kilometres. The largest tropical cyclone on record is Typhoon Tip which had gales over a radius 1100 kilometres in the north-west Pacific Ocean.

What is a Flood?

A simple definition of flooding is water where it is not wanted. Another, more comprehensive definition of a flood is:

A general and temporary condition of partial or complete inundation of normally dry land areas from overflow of inland or tidal waters from the unusual and rapid accumulation or runoff of surface waters from any source.

In November 2011, the Australian Government introduced a standard definition of flood for certain insurance policies. The announcement was part of the Government’s response to the recommendations in the Natural Disaster Insurance Review report.

The standard definition will apply when an insurer offers flood cover for a home building, home contents, small business or strata title insurance policy. For this purpose a flood is defined as:

The covering of normally dry land by water that has escaped or been released from the normal confines of: any lake, or any river, creek or other natural watercourse, whether or not altered or modified; or any reservoir, canal, or dam.

Floods can have both positive and negative impacts. They can bring welcome relief for people and ecosystems suffering from prolonged drought, but also are estimated to be the most costly natural disaster in Australia.

Every year in Australia, floods cause millions of dollars damage to buildings and critical infrastructure, such as roads and railways as well as to agricultural land and crops. They also disrupt business and can affect the health of communities. Between 1967 and 2005, the average direct annual cost of flooding has been estimated at A$377 million (Bureau of Infrastructure, Transport and Regional Economics analysis of the Emergency Management Australia database).

The losses due to flooding vary widely from year to year and are dependent on a number of factors such as the severity of a flood and its location. The most costly year for floods was 1974, when events resulted in a total cost of A$2.9 billion (Bureau of Transport and Regional Economics, 2001).

What is a Tsunami?

Tsunami (pron: 'soo-nar-me') is a Japanese word; 'tsu' meaning harbour and 'nami' meaning wave. The phenomenon is usually associated with earthquakes, landslides or volcanic eruptions in, or adjacent to oceans, and results in sudden movement of the water column. Until recently tsunami were called tidal waves, even though the event has nothing to do with tides.

A tsunami is different from a wind generated surface wave on the ocean. The passage of a tsunami involves the movement of water from the surface to the seafloor which means its speed is controlled by water depth. Consequently, as the wave approaches land and reaches increasingly shallow water it slows. However, the water column still in deeper water is moving slightly faster and catches up, resulting in the wave bunching up and becoming much higher. A tsunami is often a series of waves and the first may not necessarily be the largest.

When a tsunami travels over a long and gradual slope, it allows time for the tsunami to grow in wave height. This is called shoaling and typically occurs in shallow water less than 100m. Successive peaks can be anywhere from five to 90 minutes apart. In the open ocean, even the largest tsunami are relatively small with wave heights of less than one metre. The shoaling effect can increase this wave height to a degree such that the tsunami could potentially reach an onshore height of up to 30 metres above sea level. However, depending on the nature of the tsunami and the nearshore surroundings, the tsunami may create only barely noticeable ripples.

Interesting fact: Tsunami can travel at speeds up to 950km/h in deep water which can be represented by the speed of a passenger jet.

What is a Volcano?

A volcano is a vent or chimney which transfers molten rock known as magma from depth to the Earth's surface. Magma erupting from a volcano is called lava and is the material which builds up the cone surrounding the vent.

A volcano is active if it is erupting lava, releasing gas or generates seismic activity. A volcano is dormant if it has not erupted for a long time but could erupt again in the future. Once a volcano has been dormant for more than 10 000 years, it is termed extinct.

The explosiveness of a volcanic eruption depends on how easily magma can flow and the amount of gas trapped within the magma. Large amounts of water and carbon dioxide are dissolved in magma causing it to behave in a similar way to gas expanding in fizzy drinks, which forms bubbles and escapes after opening.
As magma rises quickly through the Earth's crust, gas bubbles form and expand up to 1000 times their original size.

Volcanoes can be different in appearance with some featuring perfect cone shapes while others are deep depressions filled with water. The form of a volcano provides a clue to the type and size of its eruption which is controlled by the characteristics and composition of magma. The size, style and frequency of eruptions can differ greatly but all these elements correlated to the shape of a volcano. Three common volcanoes are:

Shield volcano

When magma is very hot and runny, gases can escape and eruptions are gentle with considerable amounts of magma reaching the surface to form lava flows. Shield volcanoes have a broad, flattened dome-like shape created by layers of runny lava flowing over its surface and cooling. Because the lava flows easily, it can move down gradual slopes over great distances from the volcanic vents. The lava flows are sufficiently slow for humans to outrun or outwalk them. This type of magma has a temperature between 800°C and 1200°C and is called basaltic magma.

Composite volcano (Strato)

Also known as strato-volcanoes, these volcanoes are characterised by an explosive eruption style. When magma is slightly cooler it is thick and sticky, or viscous, which makes it harder for gas bubbles to expand and escape. The resulting pressure causes the magma to foam and explode violently, blasting it into tiny pieces known as volcanic ash. These eruptions create steep sided cones. They can also create lava flows, hot ash clouds called pyroclastic flows and dangerous mudflows called lahars. This type of magma has a temperature between 800°C and 1000°C and is called andesitic magma.

Caldera volcano

These erupt so explosively that little material builds up near the vent. Eruptions partly or entirely empty the underlying magma chamber which leaves the region around the vent unsupported, causing it to sink or collapse under its own weight. The resulting basin-shaped depression is roughly circular and is usually several kilometres or more in diameter. The lava erupted from caldera volcanoes is very viscous and generally the coolest with temperatures ranging from 650°C to 800°C and is called rhyolitic magma. Although caldera volcanoes are rare, they are the most dangerous. Volcanic hazards from this type of eruption include widespread ash fall, large pyroclastic surges and tsunami from caldera collapse.

Volcanic hazards

Volcanic hazards include explosions, lava flows, bombs or ballistics, ash or tephra, pyroclastic flows, pyroclastic surges, mudflows or lahars, landslides, earthquakes, ground deformation, tsunami, air shocks, lightning, poisonous gas and glacial outburst flooding known as jökulhlaups. Each hazard has a different consequence, although not all occur in all eruptions or in association with all volcanoes .

Volcanic eruptions are measured using a simple descriptive index known as the Volcano Explosivity Index which ranges from zero to eight. The index combines the volume of material ejected with the height of an eruption column and the duration of the eruption.

Interesting fact: Volcanic ash clouds can damage aircraft engines but ash is not visible by radar, the main navigation aid for aircraft. There are nine Volcanic Ash Advisory Centres around the world which use satellites to help track volcanic ash clouds and provide warnings for aircraft.