Hurricane

The warm seas and hot sun of the tropics conjure up vivid images in the mind -- white beaches of coral sand, palm trees rocking in a mild sea breeze against a deep blue sky, sparkling waves lapping at the shore, or jungle fringes full of dancing butterflies and brightly-colored tropical birds. For those with an even more imaginative turn of mind, thoughts of canoes full of gaudily-painted natives, Spanish treasure galleons, and dashing buccaneers may spring to mind. Although all of these things are, or were part of, the tropical scene, the tropical oceans create some the world's mightiest storms, as well -- hurricanes.

Hurricanes, or typhoons as they are known when they occur in the Pacific Ocean (after the Japanese tai fun, or "great wind"), are part of the family of the largest cyclones on the planet (although both mid-latitude and polar cyclones are larger in area), and produce some of the most vigorous weather. Each year's hurricane season in the southern United States brings at least a few hurricanes rushing ashore, accompanied by battering winds and storm surges, and can cause both fatalities and millions of dollars worth of damage.

Besides these direct human consequences, hurricanes are part of the world's air circulation system. On a grand scale, hurricanes pick up tropical moisture and move it northward, cooling and drying the tropics and warming and moistening the temperate latitudes. Without this mechanism, heat and moisture would tend to remain more concentrated at the equator, making conditions there truly broiling, while the northern regions would be colder and drier. The hurricane, in short, helps to moderate climate all over the world by moving tropical air north.

The birth of hurricanes

Unlike most storm systems, hurricanes are not the result of frontal weather systems. They are, in fact, not associated with fronts at all. This sets them apart from extratropical cyclones, which are associated with a warm and cold front pair, and eventually an occluded front. It also means that their movement is not dependent on the motion of a frontal system -- it is governed by other forces.

Warm tropical seas are the breeding ground for hurricanes. With the sun directly overhead for much of the year, and never being further than 23 degrees from the zenith, solar energy is intense and heats both the atmosphere and the sea water. When the sea water has been heated to 80 degrees Fahrenheit or more to a depth of at least 600 feet below the surface, then the potential for forming a hurricane exists.

The hot, humid air over these warm oceanic waters naturally tends to rise and condense into clouds. Many of these clouds become strong thunderstorms as the heat at the surface pumps air upwards in massive, wet updrafts. The groundwork for a hurricane's creation is laid when these random thunderstorms occur in a large cluster, at least five degrees distant from the equator. The Coriolis effect bends the flow of winds that are more than 5 degrees north or south of the equator, and this curving wind path is needed to set up the large-scale rotation that will allow the thunderstorms to combine into a tropical depression, a tropical storm, and finally a full-scale hurricane.

If wind shear or other disruptive forces intrude, the potential hurricane will break apart and vanish as the thunderstorms that make it up rain themselves out and evaporate into the atmosphere. But if conditions continue to be favorable, more and more storms crop up, organize into bands, and begin to rotate around the cyclonic center, accompanied by sustained winds of up to 40 miles per hour. This is a tropical depression. It is still possible at this point for the depression to break up and fail to produce a hurricane, but the more it strengthens, the less likely this becomes. A good source of the winds needed to create hurricanes are ripples or waves in the trade winds, which are known as tropical waves. Other mechanisms can also provide the focus and rotation needed to start the genesis of a hurricane.

The rotation continues and intensifies, fueled by heat and moisture from the tropical sea. The clouds thicken and organize, blending into a single tropical storm. When the sustained winds reach 70 miles per hour or more, a hurricane has formed (or, if in the Pacific Ocean, a typhoon).

Interestingly, there are areas of the tropical ocean which never get warm enough to produce hurricanes, because of massive cold currents which flow through the seas at these points. These two zones are both in the southern hemisphere -- one is the entire span of ocean between Africa and South America, and the other extends from South America to within a thousand miles or so of Australia. Cool water can't provide the energy, moisture, and lift that are needed to generate hurricanes, which is why these storms don't begin further north and why they eventually break apart and die when they move out of the tropics.

However, there is plenty of tropical water where hurricanes and typhoons can form, and every year, the world witnesses the formation of a number of hurricanes -- which happen mainly from June through November in the Atlantic, but which have a longer season in the Pacific and Indian Oceans. Australia's typhoon season is very nearly a mirror image of the Atlantic hurricane season, with storms occurring mainly from November to April -- the time when Atlantic hurricane formation typically doesn't occur.

The characteristics of a hurricane

A hurricane is a huge, rotating, circular cyclonic storm made up of bands of clouds and rain. A typical hurricane is about 800 miles wide, although large specimens can be 1,100 miles across and midget cyclones can be less than 250 miles wide. All hurricanes have an eye -- a central area of clear air from 2 to 200 miles wide. This is a place where a large central downdraft occurs. The eyewall of clouds surrounding the eye contains a vast spiral updraft which pumps hot, wet air upwards into the storm. This updraft also heats the hurricane, making it warmer than the surrounding air mass.

The strongest winds and heaviest rainfall also occur at the eyewall, although the whole system is filled with bands of thunderstorms and produces rain from nearly every point outside the eye.

From formation to landfall

Since hurricanes aren't formed by frontal systems, they also don't move along with a weather front, and so other factors steer these huge, rotating wheels of rain, cloud, and wind across the planet's surface. The large-scale wind flows of the Earth's atmosphere are what determines their track, along with the Coriolis effect (which tends to twist the path of a hurricane towards the north or south pole, depending on the hemisphere) and the influence of nearby high and low pressure systems.

Because so many factors contribute to steering hurricanes, their exact track is hard to predict and some may appear to move erratically, even if there are very good reasons, meteorologicaly speaking, for them to move along the path they do.

Some hurricanes dissipate without ever making landfall. However, some also reach land each year, with dramatic and destructive results. The storm surge created by the force of the hurricane winds, and the atmospheric pressure differences of the hurricane, is one of the deadliest parts of the storm. It can be up to 25 feet high, and causes immense damage along the shoreline, as well as drowning many people if the shore is low. It can also create conditions that are likely to produce disease in the storm's aftermath if it causes filthy water to pool up, polluting drinking water. The huge winds of a hurricane are also destructive, and the rainfall of the storm's rain bands is torrential, potentially leading to flooding. Hurricanes can also result in tornados.

The landfall of a hurricane is a devastating and massive event, but it is also the swansong of the mighty storm unless it somehow manages to move out over warm tropical waters again. The land is far too dry to sustain the continual flow of heat and moisture a hurricane needs to survive. Within hours of making landfall, the hurricane begins to wither and collapse from lack of moisture, turning into a large but fairly ordinary storm. It may reorganize into an extratropical cyclone, or it may peter out altogether. If the hurricane moves over mountains, its demise is even swifter and more complete, though it may cause immense damage by triggering landslides with its dying rains.

If a hurricane doesn't make landfall, it may survive longer, but it will ultimately perish just the same. If it becomes stationary, it will churn the waters beneath it until the cold water from the depths lowers the temperature at the surface below the critical 80 degrees Fahrenheit, and the storm will weaken and break apart. (It is for this reason that two hurricanes almost never make landfall at the same point in succession -- a hurricane leaves a wake of cooler water as it travels, which 'kills' any hurricane that follows it too closely.) Or, it will move north into cooler waters, and fall apart as it loses moisture and tropical convection.

But whatever its ultimate fate, the hurricane has helped to moderate the climate -- cooling the tropics and warming the northern regions -- by moving a massive chunk of tropical heat and moisture north and blending it into the northern atmosphere. Dangerous as they may be to people who live along coastlines, hurricanes are a crucial part of making our planet liveable for humans and all other creatures as well.