Read more: Instant Expert: Extreme weather
Warm ocean waters provide the power that drives the world鈥檚 most fearsome storms. Hurricanes can be more than 2000 kilometres wide, generate storm surges of over 10 metres and deposit more than a metre of rain in a day
Typhoon or hurricane?
A tropical cyclone starts life as a tropical depression 鈥� an organised, spinning storm system with wind speeds of less than 63 kilometres per hour. When the winds grow faster, the system is given a name and classified as a tropical storm. When the winds reach 119 km/h, a ring of intense thunderstorms called the 鈥渆yewall鈥� forms around the storm鈥檚 centre. Within the eyewall is the 鈥渆ye鈥� of the storm, a clear, calm region of sinking air.
Advertisement
Once the sustained winds exceed 119 km/h, the storm is classified as a hurricane if it is in the Atlantic or eastern Pacific, or as a typhoon in the western Pacific. In the Indian Ocean or in the southern hemisphere it is simply called a cyclone or tropical cyclone. There is no meteorological difference between these differently named storms.
Very warm water extending to a depth of 50 metres or more can help fuel rapid intensification of a tropical cyclone to 鈥渕ajor鈥� hurricane status, with winds of 178 km/h or more 鈥� the most fearsome and destructive type of storm on the planet.
Traditionally, hurricanes are ranked from 1 to 5 on the Saffir-Simpson scale, based on the maximum sustained wind speed. However, this scale can be misleading. A weak storm that covers a huge area of the sea can generate a larger storm surge than a smaller but more intense hurricane with a higher Saffir-Simpson rating. To give a better idea of the storm surge potential, the experimental Integrated Kinetic Energy scale has been developed. It is a measure of both wind speed and the area over which high winds extend.
Monsoon depressions
Monsoons operate via the same principle as the familiar summer afternoon sea breeze, but on a grand scale. In summer, the land gets hotter than the sea: that鈥檚 because on land, the sun鈥檚 heat is concentrated close to the surface, while at sea wind and turbulence mix warm water at the surface with cooler water lower down. Also, the molecular properties of water mean it takes more energy to raise its temperature than it does to heat the soil and rock that make up dry land.
As a result, a low-pressure region of rising air develops over land areas. Moisture-laden ocean winds blow towards this region and are drawn upwards when they reach land. The rising air expands and cools, releasing its moisture as some of the heaviest rains on Earth 鈥� the monsoon.
Each summer, monsoons affect every continent except Antarctica and are responsible for life-giving rains that sustain the lives of billions of people. In India, home to 1.1 billion people, the monsoon provides 80 per cent of the annual rainfall. Monsoons have their dark side too, though: hundreds of people in India and surrounding nations die each year in floods and landslides triggered by the heavy rains.
The most deadly flooding events usually come from monsoon depressions, also known as monsoon lows. A monsoon depression is similar to, but larger than, a tropical depression. Both are spinning storms hundreds of kilometres in diameter with sustained winds of 50 to 55 kilometres per hour, nearly calm winds at their centre and very heavy rains. Each summer, around seven monsoon depressions form over the Bay of Bengal and track westwards across India. This year, two major monsoon depressions crossed over India into Pakistan in July and August, bringing heavy rains and the worst flooding in Pakistan鈥檚 history.
Tropical cyclones
The word 鈥渃yclone鈥� can be used to describe any rotating storm system. This includes hurricanes, tornadoes and the ordinary low-pressure systems that develop in the mid-latitudes.
Tropical cyclones, though, form only over warm ocean waters of at least 26 掳C, and unlike storm systems over land, derive their energy exclusively from latent heat. Hurricanes, typhoons, tropical storms and tropical depressions are all examples of tropical cyclones.
Like tornadoes, tropical cyclones need a particular set of ingredients in order to form, starting with warm ocean water. In addition, vertical wind shear must be very low: in other words, the difference in wind speed between the surface and the top of the troposphere must be less than around 10 metres per second. Any faster and the shear tilts and stretches the core of a developing tropical cyclone, carrying away its heat and moisture.
Strong upper-level winds associated with the jet stream or upper-level low-pressure systems are the most common source of wind shear. The more equator-ward position of the subtropical jet stream in winter and spring is the main reason why hurricanes and typhoons rarely form in the Caribbean Sea or western Pacific in those seasons, even though ocean temperatures are warm enough year-round to support such storms. Tropical cyclones also need high moisture through a deep layer of the atmosphere. Dry air from Africa or North America often disrupts a hurricane in the process of forming.
Finally, a tropical cyclone needs something to get it spinning. In the Atlantic this is usually provided by disorganised areas of low pressure called African easterly waves, which emerge from the coast of Africa and move westwards towards the Caribbean.
Hurricanes get more spin from the effect of Earth鈥檚 rotation. Since the amount of vertical spin of the atmosphere due to the Coriolis force is zero at the equator and maximum at the poles, tropical cyclones generally cannot form within about 5 degrees of latitude from the equator. They then tend to expand as they move polewards, due to the increasing amount of vertical spin.
鈥淎 weak hurricane can generate a bigger storm surge than a strong one if it covers a bigger area鈥�
