Tropical cyclones are like engines that require warm, moist air as fuel. So the first ingredient needed for a tropical cyclone is warm ocean water. That is why tropical cyclones form only in tropical regions where the ocean is at least 80 degrees F for at least the top 50 meters about feet below the surface. The second ingredient for a tropical cyclone is wind.
In the case of hurricanes that form in the Atlantic Ocean, the wind blowing westward across the Atlantic from Africa provides the necessary ingredient.
As the wind passes over the ocean's surface, water evaporates turns into water vapor and rises. As it rises, the water vapor cools, and condenses back into large water droplets, forming large cumulonimbus clouds.
These clouds are just the beginning. Credit: NOAA. Meteorologists have divided the development of a tropical cyclone into four stages: Tropical disturbance, tropical depression, tropical storm, and full-fledged tropical cyclone. When the water vapor from the warm ocean condenses to form clouds, it releases its heat to the air. The warmed air rises and is pulled into the column of clouds. Evaporation and condensation continue, building the cloud columns higher and larger. 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. These satellites, built by NASA and operated by the National Oceanic and Atmospheric Administration NOAA , save lives by helping weather forecasters predict and warn people where and when these severe storms will hit land. Our World: What is a Hurricane? Here is a movie of Hurricane Katrina, which struck the coast of Louisiana, Alabama and Mississippi on August 29, , as a Category 3.
This movie was made from images taken by the GOES weather satellite. In the movie you can see the storm starting to form in the Atlantic on August 24 and becoming more and more organized as it moves over the warm waters of the Gulf of Mexico. How Do Hurricanes Form? Hurricane Fran. Image made from GOES satellite data. Click to view movie.
Many tropical cyclones eventually drift far enough from the equator to move into areas dominated by westerly winds found in the middle latitudes. These winds tend to reverse the direction of the tropical cyclone to an eastward path. As the tropical cyclone moves polewards it picks up forward speed and may reach 30 m. An average tropical cyclone can travel about to miles a day, or about 3, miles before it dies out.
Tropical cyclones also occur in various parts of the Pacific Ocean, and can affect coastal regions of Mexico, south-east Asia, north-east Australia and the south Pacific islands. In the northern hemisphere most tropical cyclones occur between June and November with a peak in September. However, in the western North Pacific it is not unusual to have the occasional tropical cyclone outside of this period. In the southern hemisphere the season usually lasts from November to April.
Tropical cyclones are occasionally observed in the South Atlantic, but this is a very rare occurrence. The map above shows the areas of the world affected by tropical cyclones. Past tropical cyclones. Other phenomena which can be just as damaging than the wind frequently accompany tropical cyclones:. These phenomena can cause major destruction, especially when the tropical cyclone's path takes it over land.
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.
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