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. Frequent showers and non-violent thunderstorms often accompanied with heavy rainfall are typical for tropical regions. Intense and well organized storm systems are relatively rare.

Surface winds in the tropics generally blow from the east -- northeast (in the northern hemisphere), or southeast (in the southern hemisphere) -- these reliable and steady winds are called the trade winds. Occasionally, a large undulation or ripple in the normal trade wind pattern will develop and move slowly from east toward west. These disturbances in the flow are called tropical waves, or easterly waves. Because variaions of surface air pressure in the tropics are so slight compared with the middle latitudes, tropical waves are best shown by plotting steamlines of the wind patterns as shown in this example. The example also indicates where areas of convergence (which forces rising motion resulting in the development of clouds and rain) and divergence occur relative to the position of the easterly wave. The typical westward movement of a tropical easterly wave is shown in this animation. Although the majority of tropical waves die out before becoming major systems, if conditons are favorable, tropical waves can intensify, develop a central low pressure region around which the winds rotate, and grow into hurricanes. In fact, most severe hurricanes that affect the United States begin as easterly waves, which move of the continent of Africa. More information and a diagram can be found in this African Easterly Wave Link

The general name applied to any tropical low pressure system is a tropical cyclone. As a tropical low pressure system intensifies, it is classified according to wind speed.

Tropical Depression - surface winds < 39 mi/hr
Tropical Storm - surface winds 39-74 mi/hr (This is when storm is first given a name)
Hurricane - surface winds > 74 mi/hr (In some parts of the world they are called Typhoons or simply severe tropical cyclones.

Tropical storms and hurricanes are the only natural disasters which have their own names (e.g., Andrew, Camille, Hugo or the 2004 Florida hurricanes, Charles, Frances, Ivan, and Jeanne). Names seem appropriate because we commonly come to know hurricanes long before they strike land, often watching these storms move across the oceans for days or even weeks. By contrast isolated severe thunderstorms and tornadoes develop suddenly and last only hours. Hurricanes are much larger than an individual severe thunderstorm cell and are more correctly described as being composed of an organized clustering of thunderstorms. They are quite powerful and release great amounts of energy (mostly in the form of latent heat during condensation). In fact the energy released by a single, strong hurricane can be greater than the total annual energy consumption of the United States and Canada combined.

Anatomy of a Hurricane

Hurricane Elena as seen from
the space shuttle

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). By local convention these storms are called hurricanes in the northern Atlantic Ocean, the northeast Pacific Ocean (east of the dateline), and the south Pacific Ocean (east of 160° East Longitude). In the northwest Pacific Ocean (west of the dateline), they are called typhoons. In the southwest Pacific Ocean (west of 160° East Longitude) and throughout the Indian Ocean, they are called severe tropical cyclones.

To the right 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. Notice that the cloud tops in the eye wall region extend above the other clouds. 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 extremely 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.

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°C.

Also, hurricanes do not develop in the region 4 degrees either side of the equator. Within this region Coriolis force is negligible. Coriolis force is required for the initiation of rotation.

Hurricanes grow stronger as long as the air aloft moves outward away from the storm center more quickly than the surface air moves in toward the center. They dissipate rapidly when they move over colder water or over a large landmass.

Below is a summary list of five requirements for tropical storm development and intensification. Diagrams and further information will be provided during the lecture.

Warm, humid surface air and an unstable atmosphere
- Latent heat release during condensation is the driving energy for tropical storm intensification.
(a)Warm ocean surface waters (Temperature > 26.5°C [80°F]). This is the "fuel" for hurricanes.
(b)Depth of warm water > 60 m (200 ft)
- As cyclone develops, winds churn up water, bringing up water from below the ocean surface. If this water is too cold, hurricanes cannot develop.
Pre-existing large-scale surface convergence and/or upper-level divergence
- An area of low level convergence is present with easterly waves.
Must be further than 4° Latitude away from the Equator
- In order to get winds to rotate counterclockwise around an area of low pressure, there must be a Coriolis effect. The Coriolis force at the equator is zero and gets larger as one moves toward the poles.
Absence of strong vertical wind shear
- Wind shear inhibits deep convection, essentially ripping tropical storms apart. Generally, tropical storms are best able to strengthen when the rising warm air near the center of the storm goes straight up. Wind shear can cause the updrafts to bend over, which is not favorable for the storm to strengthen.

Only when all of these conditions are met do tropical cyclones develop into hurricanes. Tropical cyclones are fairly common, but only a small % of these become hurricanes.

During lecture, I will explain to you how tropical cyclones intensify to become hurricanes using a simple diagram. When conditions are just right, tropical storms intensify via a positive feedback loop.

The figure below shows a more realistic look at anatomy of a hurricane. As opposed to the simple explanation for intensification which only showed a central updraft, real hurricanes have banded structures of rising and sinking motion.