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Hurricane / Tropical Cyclone

Tropical cyclones have very low atmospheric pressures in the calm, clear centre (the eye) of a circular structure of rain, cloud, and very high winds. In the Atlantic and Caribbean they are called hurricanes; in the Pacific they are known as typhoons. Because of the Earth's rotation, tropical cyclones rotate clockwise in the Southern Hemisphere and counterclockwise in the Northern. They may be 50-500 mi (80-800 km) in diameter, and sustained winds in excess of 100 mph (160 kph) are common. In the eye, however, the winds drop abruptly to light breezes or even complete calm. The lowest sea-level pressures on Earth occur in or near the eye.

Mechanics of Tropical Cyclones

Structurally, a tropical cyclone is a large, rotating system of clouds, wind, and thunderstorms. Its primary energy source is the release of the heat of condensation from water vapor condensing at high altitudes, the heat ultimately derived from the sun. Therefore, a tropical cyclone can be thought of as a giant vertical heat engine supported by mechanics driven by physical forces such as the rotation and gravity of the Earth. Condensation leads to higher wind speeds, as a tiny fraction of the released energy is converted into mechanical energy, the faster winds and lower pressure associated with them in turn cause increased surface evaporation. Much of the released energy drives updrafts that increase the height of the storm clouds, speeding up condensation. This gives rise to factors that provide the system with enough energy to be self-sufficient and cause a positive feedback loop where it can draw more energy as long as the source of heat, warm water, remains. Factors such as a continued lack of equilibrium in air mass distribution would also give supporting energy to the cyclone. The rotation of the Earth causes the system to spin, an effect known as the Coriolis effect, giving it a cyclonic characteristic and affecting the trajectory of the storm.

The factors to form a tropical cyclone include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist and allow it to create a feedback loop by maximizing the energy intake possible, for example, such as high winds to increase the rate of evaporation, they can combine to produce the violent winds, incredible waves, torrential rains, and floods associated with this phenomenon.

Condensation as a driving force is what primarily distinguishes tropical cyclones from other meteorological phenomena. Because this is strongest in a tropical climate, this defines the initial domain of the tropical cyclone. By contrast, mid-latitude cyclones draw their energy mostly from pre-existing horizontal temperature gradients in the atmosphere. In order to continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the atmospheric moisture needed. The evaporation of this moisture is accelerated by the high winds and reduced atmospheric pressure in the storm, resulting in a positive feedback loop. As a result, when a tropical cyclone passes over land, its strength diminishes rapidly.

Ozone levels give a clue that a storm will develop before other methods. The early spin of a tropical cyclone is weak and sometimes covered by clouds, and not easily detected by satellites that provide pictures of clouds. However, instruments such as the Total Ozone Mapping Spectrometer can identify ozone amounts that are closely related to the formation, intensification, and movement of a cyclone. As a result, ozone levels turn out to be very helpful in determining the location of the eye. Concentrations of naturally-occurring ozone are highest in the stratosphere. Air nearer to the ocean surface is less rich in ozone. Surrounding the eye is a ring of powerful thunderstorms that are sucking up warm, moist air from the ocean surface and hurling it miles into the atmosphere, sometimes all the way to the lower stratosphere. This ozone-poor air replaces the ozone-rich air, causing ozone concentrations to drop. The process reverses in the eye itself: high-altitude air sinks down to the surface, infusing the entire column of atmosphere with ozone. Dropping ozone levels around the eye may turn out to be a strong sign that a storm is strengthening.

The passage of a tropical cyclone over the ocean can cause the upper ocean to cool substantially, which can influence subsequent cyclone development. Tropical cyclones cool the ocean by acting like "heat engines" that transfer heat from the ocean surface to the atmosphere through evaporation. Cooling is also caused by upwelling of cold water from below. Additional cooling may come from cold water from raindrops that remain on the ocean surface for a time. Cloud cover may also play a role in cooling the ocean by shielding the ocean surface from direct sunlight before and slightly after the storm passage. All these effects can combine to produce a dramatic drop in sea surface temperature over a large area in just a few days.

Scientists at the National Center for Atmospheric Research estimate that a hurricane releases heat energy at the rate of 50 to 200 trillion joules per day. For comparison, this rate of energy release is equivalent to exploding a 10-megaton nuclear bomb every 20 minutes or 200 times the world-wide electrical generating capacity.

While the most obvious motion of clouds is toward the center, tropical cyclones also develop an upper-level (high-altitude) outward flow of clouds. These originate from air that has released its moisture and is expelled at high altitude through the "chimney" of the storm engine.[citation needed] This outflow produces high, thin cirrus clouds that spiral away from the center. The high cirrus clouds may be the first signs of an approaching hurricane.

Physical Structure of Tropical Cyclone

• Surface low: All tropical cyclones rotate around an area of low atmospheric pressure near the Earth's surface. The pressures recorded at the centers of tropical cyclones are among the lowest that occur on Earth's surface at sea level.
• Warm core: Tropical cyclones are characterized and driven by the release of large amounts of latent heat of condensation as moist air is carried upwards and its water vapor condenses. This heat is distributed vertically, around the center of the storm. Thus, at any given altitude (except close to the surface where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.
• Central Dense Overcast (CDO): The Central Dense Overcast is a dense shield of very intense thunderstorm activity that make up the inner portion of the hurricane. This contains the eye wall, and the eye itself. The classic hurricane contains a symmetrical CDO, which means that it is perfectly circular and round on all sides.
• Eye: A strong tropical cyclone will harbor an area of sinking air at the center of circulation. Weather in the eye is normally calm and free of clouds (however, the sea may be extremely violent). Eyes are home to the coldest temperatures of the storm at the surface, and the warmest temperatures at the upper levels. The eye is normally circular in shape, and may range in size from 3 km to 320 km (2 miles to 200 miles) in diameter. In weaker cyclones, the CDO covers the circulation center, resulting in no visible eye.
• Eyewall: A band around the eye of greatest wind speed, where clouds reach highest and precipitation is heaviest. The heaviest wind damage occurs where a hurricane's eyewall passes over land.
• Rainbands: Bands of showers and thunderstorms which spiral cyclonically toward the storm center. High wind gusts and heavy downpours often occur in individual rainbands, with relatively calm weather between bands. Tornadoes often form in the rainbands of landfalling tropical cyclones. Annular hurricanes are distinctive for their lack of rainbands.
• Outflow: The upper levels of a tropical cyclone feature winds headed away from the center of the storm with an anticyclonic rotation. Winds at the surface are strongly cyclonic, weaken with height, and eventually reverse themselves. Tropical cyclones owe this unique characteristic to the warm core at the center of the storm.

Hurricane / Tropical Cyclone Links

National Hurricane Center
The TPC mission is to save lives, mitigate property loss, and improve economic efficiency by issuing the best watches, warnings, forecasts and analyses of hazardous tropical weather, and by increasing understanding of these hazards.

The Hurricane Hunters
Fly into the eye of a hurricane with the Air Force Reserve Hurricane Hunters.

Hurricane & Storm Tracking for the Atlantic & Pacific Oceans
The StormTrack system receives weather data from the US National Weather Service via satellite. The system creates an entry for each tropical depression, storm, or hurricane when the National Weather Service begins issuing advisories.

Canadian Hurricane Centre
ocated in downtown Dartmouth, Nova Scotia; the hurricane centre specializes in providing information to Canadians on storms of tropical origin that affect Canada or it's territorial waters.

Hays' Tropical Weather Pages
Hurricanes, Typhoons, Satellite Imagery and Forecasts.

What are Hurricanes?
Most people associate twisters with tornadoes, but in fact tropical twisters come from hurricanes. Hurricanes are what scientists call "strong Tropical Cyclones".

Hurricane FAQ
Attempts to address various questions regarding hurricanes, typhoons and tropical cyclones that have been posed to us as hurricane researchers over the years.

EO Library: Hurricanes: The Greatest Storms on Earth
Few things in nature can compare to the destructive force of a hurricane. Called the greatest storm on Earth, a hurricane is capable of annihilating coastal areas with sustained winds of 155 mph or higher and intense areas...

Hurricanes: Nature's Greatest Storms
a severe tropical storm that forms in the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E. Hurricanes need warm tropical oceans...

Hurricanes and Societal Impacts: Are We Prepared?
So what can society do? Hurricane-dissipating technology is the stuff of science fiction, so scientists strive to improve forecasting and early warning systems. With that knowledge public officials must make decisions about building codes...

Hurricane Headquarters: South Florida Sun-Sentinel
In-depth coverage of hurricane season, including updated hurricane coordinates, hurricane history, hurricane satellite maps, hurricane pictures and hurricane preparation tips.

Outer Banks Hurricane Information
Come to this page to find current conditions on the Outer Banks whenever there is a threatening tropical storm or hurricane in the Atlantic approaching the U.S. coastline.

Atlantic basin hurricanes: Indices of climatic changes
There is some evidence from model simulations and empirical considerations that the frequency per year, intensity and area of occurrence of tropical disturbances may increase [in a doubled carbon dioxide world], though it is not yet compelling."