A cyclone is a large scale weather system in which air circulates around a region of relatively low atmospheric pressure. The word cyclone is used broadly for rotating low pressure systems, but its precise meaning depends on geographical and meteorological context. In the Northern Hemisphere, air around a mature low pressure cyclone generally circulates anticlockwise, whereas in the Southern Hemisphere it generally circulates clockwise, because the Coriolis effect deflects moving air in opposite directions in the two hemispheres.
Cyclones can range enormously in size and intensity, from relatively small tropical disturbances to enormous extratropical weather systems extending across thousands of kilometres. Their defining feature is organised atmospheric rotation around a low pressure centre, but the physical structure, energy source, cloud pattern, lifetime and associated hazards differ substantially between tropical and extratropical cyclones.
In everyday language, cyclone is sometimes used as a synonym for a hurricane or typhoon, although those terms describe particular categories of tropical cyclone or particular regional naming conventions. Cyclones are therefore best understood as a broad class of rotating atmospheric low pressure systems rather than as one single type of storm.
Tropical cyclones are particularly intense cyclones that develop over sufficiently warm tropical or subtropical oceans when atmospheric and oceanic conditions allow a rotating disturbance to organise and strengthen. They are called hurricanes in the Atlantic Ocean and northeastern Pacific Ocean, typhoons in the northwestern Pacific Ocean, and tropical cyclones in many other regions. A tropical cyclone usually has a warm core, meaning that its central atmospheric column is warmer than the surrounding environment at comparable levels.
It develops around a central area of very low pressure and can contain powerful thunderstorms, spiral rainbands, exceptionally strong winds and, in mature systems, an eye surrounded by an eyewall. Tropical cyclones obtain much of their energy indirectly from warm ocean water through evaporation, condensation and the release of latent heat. They generally require sufficiently warm sea surface temperatures, adequate atmospheric moisture, a pre existing disturbance, sufficient Coriolis influence and relatively weak vertical wind shear. They do not form directly on the equator because the Coriolis effect there is too weak to provide the rotation required for a tropical cyclone to organise efficiently.
Extratropical cyclones are another major category and are especially important across the middle latitudes, including much of Europe. Unlike tropical cyclones, they are commonly associated with strong horizontal temperature contrasts between different air masses and derive much of their energy from atmospheric baroclinic processes. A typical extratropical cyclone develops along a weather front where relatively warm and cold air masses interact.
Rising air, upper atmospheric dynamics, jet stream structure and temperature gradients can cause pressure to fall and the circulation to intensify. Such systems can produce extensive areas of cloud, prolonged rain or snow, strong winds, thunderstorms and rapidly changing weather conditions. Some extratropical cyclones undergo explosive cyclogenesis, sometimes called a weather bomb when the pressure falls exceptionally rapidly according to a specified meteorological criterion. Extratropical cyclones can become very large and can influence weather over continental regions for several days. A tropical cyclone can also transition into an extratropical cyclone when it moves into an environment with cooler water, stronger vertical wind shear or substantial temperature contrasts, changing its energy source and structure.
The central pressure of a cyclone is an important indicator of its organisation and potential intensity, but pressure alone does not completely determine the danger posed by a storm. Atmospheric pressure is the force exerted by the weight of air above a particular location, and pressure decreases towards the centre of a typical cyclone. The pressure gradient between the low pressure centre and surrounding higher pressure produces a pressure gradient force that drives air towards lower pressure.
The Coriolis effect then deflects moving air, while friction near the surface modifies the circulation and allows air to cross pressure contours towards the centre. The resulting balance among pressure gradient force, Coriolis force, centrifugal effects and friction produces the characteristic rotating structure. In the free atmosphere, large scale cyclone circulation can approach a geostrophic or gradient wind balance, while near the surface friction reduces wind speed and changes its direction. These physical processes explain why a cyclone is not simply a giant spinning column of air but a dynamically evolving three dimensional atmospheric system.
Cyclones are important because they transport heat, moisture and momentum through the atmosphere and strongly influence weather and climate. Tropical cyclones can produce destructive winds, torrential rainfall, coastal flooding, storm surges, landslides and, occasionally, tornadoes. The most dangerous impacts may not necessarily come from the strongest winds.
A relatively moderate tropical cyclone can cause catastrophic flooding if it moves slowly over a region already saturated by rain, while a powerful storm arriving at high tide can produce severe coastal inundation through storm surge. Extratropical cyclones can generate widespread damaging winds, heavy precipitation, blizzards, coastal flooding and dangerous marine conditions. Their fronts can also organise thunderstorms and other severe weather. Cyclones can therefore be both destructive and meteorologically beneficial in some circumstances because they redistribute atmospheric energy and water. Their impacts depend on intensity, size, movement, duration, rainfall, terrain, population exposure, infrastructure vulnerability and the effectiveness of forecasting and warning systems.
The development of a cyclone involves feedbacks that can amplify an initially weak disturbance. In a tropical system, thunderstorms release latent heat when water vapour condenses into liquid water or ice. This heating can warm the atmospheric column, lower central pressure and encourage further convergence of moist air near the surface. Increased convergence supplies additional moisture to convection, allowing the circulation to strengthen when environmental conditions remain favourable.
The ocean also responds to the storm, and intense mixing can bring colder subsurface water towards the surface, potentially reducing the energy supply. Dry air intrusion, strong vertical wind shear, interaction with land and cooler ocean temperatures can disrupt the organisation of a tropical cyclone. Extratropical cyclone development depends strongly on the distribution of temperature and pressure, upper level waves, jet streams and interactions between air masses. Cyclogenesis is therefore a dynamic process involving feedbacks between different parts of the atmosphere and, for tropical systems, the ocean as well.
The word cyclone comes through scientific terminology derived from the Greek word kyklos, meaning circle or wheel, reflecting the circular or spiral motion associated with these systems. Meteorologists distinguish cyclones from anticyclones, which are large scale high pressure systems with generally opposite circulation patterns. In the Northern Hemisphere, anticyclones normally circulate clockwise, while in the Southern Hemisphere they normally circulate anticlockwise. Cyclones can also be classified by structure, origin, latitude, thermal characteristics and energy source.
Tropical cyclones, subtropical cyclones, extratropical cyclones and polar lows are examples of different systems that share organised low pressure circulation but have different physical mechanisms. The term cyclone should therefore not automatically be interpreted as meaning hurricane strength winds. Some cyclones are relatively weak and produce ordinary unsettled weather, while others become exceptionally powerful. Similarly, not every rotating storm system is technically a tropical cyclone, and regional naming conventions can create apparent differences in terminology even when related physical phenomena are being described.
Modern cyclone science combines observations from weather stations, radiosondes, aircraft, ships, radar, satellites, ocean buoys and numerical weather prediction models. Satellite imagery is particularly important over oceans where conventional observations are sparse, allowing meteorologists to monitor cloud structure, convection and storm organisation. Radar can provide detailed information about precipitation structure and, in some regions, wind circulation. Numerical models use equations describing atmospheric motion, thermodynamics, radiation, moisture and other processes to simulate cyclone development and movement.
Forecasting remains uncertain because small differences in initial atmospheric and oceanic conditions can influence subsequent evolution, and because cyclone intensity depends on complex interactions occurring across many spatial and temporal scales. Forecast centres therefore issue track forecasts, intensity estimates, watches, warnings and probabilistic information. Understanding cyclones requires distinguishing the physical definition of a rotating low pressure system from the particular hazards associated with individual storms.
