Severe Weather

Tropical Cyclones and Hurricanes

Tropical Weather

In the broad belt around the earth known as the tropics -- the region 23½° north and south of the equator -- the weather is much different from the middle latitudes. The noon sun is always high in the sky, and so diurnal and seasonal changes in temperature are small. The daily heating of the surface and high humidity favor the development of cumulus clouds and thunderstorms.

Winds in the tropics generally blow from the east, northeast, or southeast -- the trade winds. Occasionally, the normal winds will be disturbed by a weak trough of low pressure called a tropical wave, or easterly wave. Occasionally, a tropical wave will intensify and grow into a hurricane.

Anatomy of a Hurricane

Figure 1: Hurricane Elena as photographed from the space shuttle Discovery during September, 1985.

A hurricane is an intense storm of tropical origin, with sustained winds exceeding 64 knots (74 mi/hr), which forms over the warm northern Atlantic and eastern North Pacific oceans.

This same storm goes by other names in the various parts of the world. In the western North Pacific, its is called a typhoon in the Philippines a baguio, and in India and Australia a cyclone.

To the left there is a photo of Hurricane Elena. The storm is approximately 500 km (310 mi) in diameter, which is about average for hurricanes. The area of broken clouds in the center is its eye. Elena's eye is almost 40 km (25 mi) wide. Within the eye, winds are light and clouds are mostly broken. The surface pressure is very low, nearly 955 mb.

Notice that the clouds align themselves into spiraling bands (called spiral rain bands) that swirl in toward the storm's center, where they wrap themselves around the eye. Surface winds increase in speed as they blow counterclockwise and inward toward this center.

Adjacent to the eye is the eye wall, a ring of intense thunderstorms that whirl around the storm's center and extend upward to almost 15 km (49,000 ft) above sea level. Within the eye wall we find the heaviest precipitation and the strongest winds.

Surface atmospheric pressure in the center of a hurricane tends to be extreme low. The lowest pressure reading ever recorded for a hurricane (typhoon Tip, 1979) is 870 millibars (mb). However, most storms have an average pressure of 950 millibars.

To be classified as a hurricane, sustained wind speeds must be greater than 118 kilometers per hour at the storm's center.

Figure 2: Relationship between surface pressure and wind speed for a number of tropical low pressure systems. Tropical low pressure systems are classified as hurricanes when their pressure is 980 millibars or lower, and sustained wind speeds are greater than 118 kilometers per hour.

Wind speed in a hurricane is directly related to the surface pressure of the storm. The following graph shows the relationship between surface pressure and sustained wind speed for a number of tropical low pressure systems.

Hurricane Formation and Dissipation

Hurricanes form over tropical waters where the winds are light, the humidity is high in a deep layer, and the surface water temperature is warm, typically 26.5°C (80°F) or greater, over a vast area. Moreover, the warm surface water must extend downward to a depth about 200 m (600 ft) before hurricane formation is possible.

Hurricanes are powered by the latent heat energy released from condensation. To form and develop they must be supplied with a constant supply of warm humid air for this process. Surface air with enough energy to generate a hurricane only exists over oceans with a temperature greater than 26.5 degrees Celsius.

Also, hurricanes do not develop in the region 5 degrees either side of the equator. Within this region Coriolis force is negligible. Coriolis force is required for the initiation of cyclonic flow.

Therefore, hurricanes occur over most tropical oceans, but do not generally occur over the South Atlantic and Southeastern Pacific Ocean.

Figure 3: Typical areas where hurricanes begin their development and the common paths of storm movement.

Hurricanes derive their energy from the warm tropical water and from the latent heat released as water vapor condenses into clouds.

Hurricanes grow strongest as long as the air aloft moves outward, away from the storm center more quickly than the surface air moves in toward the center.

Hurricanes dissipate rapidly when they move over colder water or over a large landmass.

Very nice animations of satellite imagery can be found at the GOES web site.

Hurricane Movement

Figure 3 shows where most hurricanes are born and the general direction in which they move. Notice that they form over tropical oceans, except in the South Atlantic and in the eastern South Pacific. The surface temperatures in these areas are too cold for their development.

Hurricanes that form over the North Pacific and North Atlantic are steered by easterly winds and move west of northwestward at about 10 knots. Gradually, they swing poleward, and when they move far enough north, they get become caught in the westerly flow, which curves them to the north or northeast.

Destruction and Warning

When a hurricane is approaching from the east, its highest winds are usually on its north (poleward) side. The reason for this phenomenon is that the winds that push the storm along add to the winds on the south (equator) side.

As the winds blows over the open water, the water beneath is set in motion. When the hurricane approaches land, there is a net transport of water toward the shore. Here, the water piles up and rapidly inundates the region.

The high winds of a hurricane also generate large waves, sometimes 10 to 15 m (33 to 49 ft) high. These waves move outward, away from the storm, and carry the storm's energy to distant beaches.

Although the hurricane's high winds inflict a great deal of damage, it is the huge waves, high seas, and flooding that normally cause most of the destruction. The flooding is due, in part, to winds pushing water onto the shore and to the heavy rain. Flooding is also aided by the low pressure of the storm, which allows the ocean level to rise (perhaps half a meter). The combined effect of high water, high winds, and the net transport of water toward the coast, produces the storm surge -- an abnormal rise of several meters in the ocean level.

Considerable damage may also occur from hurricane-spawned tornadoes. About one-fourth of hurricanes that strike the United States produce tornadoes.

In summary, the possible dangers associated with a hurricane are:

• strong winds
• storm surge
• heavy rains, and
• associate tornadoes

In an effort to estimate the possible damage of a hurricane's sustained winds and the storm surge could do to a coastal area, the Saffir-Simpson scale was developed (see table below).

Saffir-Simpson Hurricane Damage-Potential Scale

Scale Number (Category)	Central Pressure mb (in.)	Winds mi/hr (knots)	Storm Surge ft (m)	Damage
1	>980 (>28.94)	74-95 (64-82)	4-5 (~1.5)	Damage mainly to trees, shrubbery and unanchored mobile homes
2	965-979 (28.50-28.91)	96-110 (83-95)	6-8 (~2.0-2.5)	Some trees blown down; major damage to exposed mobile homes; some damage to roofs of buildings
3	945-964 (27.91-28.47)	111-130 (96-113)	9-12 (~2.5-4.0)	Foliage removed from trees; large trees blown down; mobile homes destroyed; some structural damage to small buildings
4	920-944 (27.17-27.88)	131-155 (114-135)	13-18 (~4.0-5.5)	All signs blown down; extensive damage to roofs, windows, and doors; complete destruction of mobile homes; flooding inland as far as 10 km (6 mi); major damage to lower floors of structures near shore
5	<920 (<27.17)	>155 (>135)	>18 (>5.5)	Severe damage to windows and doors; extensive damage to roofs of homes and industrial buildings; small buildings overturned and blown away; major damage to lower floors of all structures less than 4.5 m (15 ft) above sea level within 500 m of shore

Figure 4: The number of hurricanes (by each category) that have made landfall along the coastline of the United States from 1900 to 1999.

Links

Information about notable hurricanes of the last decade can be found in NOAA Hurricane Research Division.

Summary of 2004 Hurricane positions

NOAA 2004 Hurricane overview and summary

Typhoon web page

More on this factors can be found at The University of Illinois web pages.

Animations of recent hurricanes are available from the Atmos. Sciences Weather Imagery