Why does cyclones occur

Storms south of the equator spin clockwise. This difference is because of Earth's rotation on its axis. As the storm system rotates faster and faster, an eye forms in the center. It is very calm and clear in the eye, with very low air pressure. Higher pressure air from above flows down into the eye. If you could slice into a tropical cyclone, it would look something like this.

The small red arrows show warm, moist air rising from the ocean's surface, and forming clouds in bands around the eye. The blue arrows show how cool, dry air sinks in the eye and between the bands of clouds. The large red arrows show the rotation of the rising bands of clouds. When the winds in the rotating storm reach 39 mph, the storm is called a "tropical storm. Tropical cyclones usually weaken when they hit land, because they are no longer being "fed" by the energy from the warm ocean waters.

However, they often move far inland, dumping many inches of rain and causing lots of wind damage before they die out completely. The two GOES satellites keep their eyes on hurricanes from far above Earth's surface—22, miles above, to be exact! Learn more about this kind of orbit. However, a path over land also causes the destruction of the tropical cyclone itself. As it moves over land, its energy source is depleted and friction across the land surface distorts the air flow.

This leads to the eye filling with cloud and the tropical cyclone dies. Due to advances in numerical weather prediction computer models, forecasting the initial development of tropical cyclones has improved greatly in recent years. There are certain areas over the oceans that are particularly favourable for tropical cyclone development, but it is usually certain characteristics in a cluster of thunderclouds that leads forecasters to recognise them as tropical depressions.

This is done by people at specialist tropical cyclone forecasting centres around the globe such as the National Hurricane Center in Miami, Florida who are constantly studying satellite images, instruments and other weather data to detect and track them through their life-cycle. Once detected, their track is forecast using a combination of numerical forecasting models, synoptic forecasting and statistical methods, which have been developed from the study of the behaviour of past storms.

Observations from ships at sea are always very useful, although, once the existence of a storm and its forecast track have been broadcast, ships tend to move out of their way! Reinforced aircraft, fitted with instruments, fly through and over tropical cyclones, and weather radar can locate storms within miles of the radar station.

In addition, tropical cyclones are tracked by satellites, which provide very useful information both to forecasters and numerical forecast models. However, the Met Office does produce tropical cyclone forecasts from its global model, which are disseminated to the regional centres. They then use these forecasts, along with other forms of guidance, to produce their forecasts.

Met Office forecasts of tropical cyclones show considerable skill and are among the best models available to agencies such as the National Hurricane Center. Once a tropical depression has been identified, a series of forecast advisories, which detail the expected track and likely strength of the tropical cyclone, are broadcast. When there are definite indications that a tropical cyclone is approaching land, watches and warnings along coastal regions are raised, which aim to give information to the local authorities of places likely to be in the tropical cyclone's path, so that they can make preparations to protect public safety.

Specially designed shelters in which people can ride out the worst of the weather may have been built in advance. Coastal regions immediately in a tropical cyclone's path are particularly dangerous and people are usually advised to move away from these. In the USA, this can involve the mass evacuation of a million or more people. Warnings are broadcast on TV and radio. NOAA Weather radio is available on a variety of frequencies and gives up-to-date information and advice.

There is also a lot of information on a variety of web sites, the most authoritative being those maintained by NOAA and the National Hurricane Center. Tropical cyclones warnings and guidance. Hurricanes are tropical features and require sea temperatures much higher than those around the UK, even in the summer. Hence, hurricanes cannot form at our latitudes.

However, we are sometimes affected by deep depressions that were originally tropical cyclones which have moved to higher latitudes, such as ex-Hurricane Ophelia in Intense mid-latitude depressions can produce near surface winds of hurricane strength, even those which do not originate from a tropical cyclone. The most widely publicised such depression occurred on 16 October , known as The Great Storm.

Some wind gusts were over m. On 13 February a deep depression passed just to the north of Scotland producing a gust of m. The reason that The Great Storm of October gained such publicity was that it occurred in a densely populated area causing loss of life and enormous damage to trees.

The Great Storm of The impact of climate change - specifically global warming caused by the burning of fossil fuels - on tropical cyclone activity is the subject of ongoing debate and research in the scientific community. Recent catastrophic events, such as Typhoon Haiyan and Hurricanes Harvey, Irma and Maria, have given the debate a higher profile.

While some evidence has been presented in order to indicate a recent upturn in tropical cyclone activity in some parts of the world, others have argued that natural variations in tropical cyclone activity - on a regional, annual and inter-decadal scale - mask any signal from the impact of global warming, which remains relatively small.

Although there is no clear consensus on whether global warming is currently having any measurable impact on tropical cyclones, climate models indicate that there may be an increase in tropical cyclone intensity in the future, under continued global warming.

However, the models also indicate that tropical cyclone frequency will either remain unchanged or decrease. A number of leading tropical cyclone scientists have come together to issue a statement endorsed by the World Meteorological Organization and publish latest research on the status of understanding and research into tropical cyclones and climate change. WMO statement on tropical cyclones and climate change. Nature Geoscience article on tropical cyclones and climate change.

Tropical Cyclone Frequently Asked Questions. Tropical cyclone facts. However, tropical cyclones are essential features of the Earth's atmosp Introduction What is a tropical cyclone? How do tropical cyclones form? How does the tropical cyclone obtain its energy? Subject G4 Why do tropical cyclones occur primarily in the summer and autumn?

Contributed by Chris Landsea As described in Subject G1, the primary time of year for getting tropical cyclones is during the summer and autumn: July-October for the Northern Hemisphere and December-March for the Southern Hemisphere though there are differences from basin to basin.

While one would intuitively expect tropical cyclones to peak right at the time of maximum solar radiation late June for the tropical Northern Hemisphere and late December for the tropical Southern Hemisphere , it takes several more weeks for the oceans to reach their warmest temperatures. The atmospheric circulation in the tropics also reaches its most pronounced and favorable for tropical cyclones at the same time. This time lag of the tropical ocean and atmospheric circulation is analogous to the daily cycle of surface air temperatures - they are warmest in mid-afternoon, yet the sun's incident radiation peaks at noon.

Contributed by Chris Landsea Tropical cyclones - to a first approximation - can be thought of as being steered by the surrounding environmental flow throughout the depth of the troposphere from the surface to about 12 km or 8 mi.

Neil Frank, former director of the U. National Hurricane Center, used the analogy that the movement of hurricanes is like a leaf being steered by the currents in the stream, except that for a hurricane the stream has no set boundaries. This is because there exists an axis of high pressure called the subtropical ridge that extends east-west poleward of the storm.

On the equatorward side of the subtropical ridge, general easterly winds prevail. However, if the subtropical ridge is weak - often times due to a trough in the jet stream - the tropical cyclone may turn poleward and then recurve back toward the east.

On the poleward side of the subtropical ridge, westerly winds prevail thus steering the tropical cyclone back to the east. These westerly winds are the same ones that typically bring extratropical cyclones with their cold and warm fronts from west to east. Many times it is difficult to tell whether a trough will allow the tropical cyclone to recurve back out to sea for those folks on the eastern edges of continents or whether the tropical cyclone will continue straight ahead and make landfall.

For more non-technical information on the movement of tropical "Hurricanes: Their Nature and Impacts on Society". For a more detailed, technical summary on the controls on tropical cyclone motion, see Elsberry's chapter in "Global Perspectives on Tropical Cyclones".

Contributed by Chris Landsea What never?? Well, hardly ever. But this still leaves the question of why hurricanes are so rare in the South Atlantic. Though many people might speculate that the sea surface temperatures are too cold, the primary reasons that the South Atlantic Ocean gets few tropical cyclones are that the tropospheric near surface to mb vertical wind shear is much too strong and there is typically no inter-tropical convergence zone ITCZ over the ocean Gray Without an ITCZ to provide synoptic vorticity and convergence i.

This storm lasted about five days and drifted toward the west-southwest into the central South Atlantic. So far, there has not been a systematic study as to the conditions that accompanied this rare event. Penn State University write up on the South Atlantic hurricane.

Subject G7 How much lightning occurs in tropical cyclones?