Tropical cyclones are storms that are born in tropical oceans and depend on warm water for their source of energy. They originate between 4° and 22° S of the equator, and between 4° and 35° N of the equator. During the northern hemisphere’s summer (July-September), most of the energy is delivered to the sub-tropical region of the northern hemisphere. The result is the development of large cyclonic (counterclockwise rotating) cells that are termed hurricanes if they form near North America and typhoons if they are formed in the western Pacific. Tropical cyclones do not, however, occur in the equatorial zone. More than two-thirds of tropical cyclones occur in the northern hemisphere and occur between May and November, when the ocean is at its warmest, with a peak in August and September. In the southern hemisphere, they occur between December and April, with peaks in January and February, where they are typically called cyclones.
We generally associate tropical cyclones with the damage they do when they come in contact with land, but they spend most of their “life” out in the ocean and are a phenomenon best explained in terms of heat transfer between the atmosphere and the ocean.
The Birth of a Cyclone
The generation of a tropical cyclone is dependent upon the transfer of thermal energy from the ocean to the atmosphere. When significant volumes of warm water vapor move upwards, low-pressure cells develop at the Earth's surface and large storms are generated. In the process, water vapor evaporated into the atmosphere will cool and condense. This is ultimately how storm clouds are produced. As the water begins to fall back to the surface, large scale convective cells are generated, and the air mass is forced to spin in response to the Coriolis Effect. This causes deflection of air masses to the right in the northern hemisphere and deflection to the left in the southern. For storms to sustain themselves, the upward movement of warm water vapor needs to be constantly resupplied or the storm will weaken and eventually fall apart. The video beelow explain how a hurricane forms and outlines its anatomy:
Video: Hurricanes 101 | National Geographic (2:57)
How a hurricane develops: Hurricane Facts
For a tropical cyclone to generate, the temperature of the upper 60 meters (~200 ft.) of the ocean water must be greater than 26°C (~79°F). In addition, certain atmospheric conditions are needed to drive the formation of convection cell described above. Horizontal shear winds prevent formation; conversely, in conditions with low wind shear, heat and moisture are retained to allow continued development. The building of the tall cumulonimbus clouds associated with a tropical cyclone depends on the deep convection currents extending into the troposphere. The initial anomaly is called a tropical wave, and then with progressive intensification, it is called a disturbance, an investigation (where it is given a number), then a depression, tropical storm, then a hurricane. This is explained in the following video:
Video: The Basics of Hurricanes and Tropical Weather Forecasting (8:07)
Anatomy of a Hurricane
There are distinct stages of formation that vary greatly from storm to storm. As the pressure drops in the center of the storm, the winds that rotate around the center increase. Narrow cloud bands form, spiraling inwards. The center of the storm, or the eye, is where the lowest pressure is found. The eye wall at the edge of the eye has some of the strongest winds and intense thunderstorms, but the eye itself is characterized by calm. The hurricane-force winds may extend out from the eye for 300 km (~186 miles). The strongest winds occur towards the right-hand side of the center in the direction of the cyclone’s path, so a storm moving north will have the strongest winds in the northeastern quadrant.
Hurricane Tracks and Forecasting
Hurricane movement is guided by surface winds. As we will see in the examples given, the Atlantic hurricanes travel west or northwest across the Atlantic and then recurve to the north and then northeast. This curvature is dictated by the Coriolis effect, which is caused by the Earth’s rotation. This pattern also occurs in the northern Pacific. Although these are predictable patterns, each storm has a unique path and there is great variation depending upon the atmospheric conditions near the storm, which act as steering forces. As we will see later wind movements are a function of atmospheric pressure, with storms basically moving around high pressure systems and towards low pressure systems.
Nevertheless, the patterns made by the historic tracks seen in the animation below show how the storms generally follow certain rules. This predictability aids in forecasting the paths using satellite data and numerical modeling.
Video: Hurricane Tracks Animation and Cumulative Map (0:14) (Video is not narrated.)
The cyclone’s interaction with the ocean’s surface has an effect of reducing the surface temperatures of the ocean. Once the storm approaches land, it encounters shallower water and begins interacting with coastal features. Friction and loss of the warm water “fuel” remove energy from the system, and it will dissipate once over land. It is at the ocean-land interface that the storm surge, which builds with the storm in the ocean, creates a tremendous hazard for those living on the coast.