Tropical Weather

The tropics can be defined as the area of the Earth found between the Tropic of Cancer and the Tropic of Capricorn. In this region, the sun will be directly overhead during some part of the year. The temperature of the tropics does not vary much from season to season because considerable solar insolation is received by locations regardless of the season. Weather in the tropics is dominated by convective storms that develop mainly along the intertropical convergence zone, the subtropical high pressure zone, and oceanic disturbances in the trade winds that sometimes develop into hurricanes.

Figure 1: Solar influence on the movement of the Earth's global circulation patterns. (Source: PhysicalGeography.net)

One of the most important weather features found in the tropics is the intertropical convergence zone. The intertropical convergence zone is distinguished by a wide band of cumulus and cumulonimbus clouds that are created by dynamic atmospheric lifting due to convergence and convection.

Normally, the intertropical convergence zone delineates the location where the noonday sun is directly overhead on the globe. Because of the high sun, the intertropical convergence zone receives the greatest quantity of daily solar insolation in the tropics. At the intertropical convergence zone, this energy is used to evaporate large amounts of water and is coverted into sensible heat at the ground surface and within the atmosphere. Often, these processes lead to an almost daily development of convective thunderstorms by providing moisture and heat for the development of cumulonimbus clouds.

The intertropical convergence zone also represents the location of convergence of the northeast and southeast trade winds. The convergence of these wind systems enhances the development of convective rain clouds at the tropics.

The intertropical convergence zone moves seasonally with the tilt of the Earth's axis (Figure 1). The convective rains that accompany the passage of the intertropical convergence zone are the primary source of precipitation for locations roughly 10 to 23.5° North and South latitide. The animation in Figure 1 demonstrates the seasonal movement of this weather phenomenon.

Weather disturbances in the trade winds, known as easterly waves, are another source of cloud development and precipitation in the tropics. Easterly waves develop first as a weak disturbance in the atmosphere usually because of the presence of localized warmer sea temperatures. This is seen on a weather map as a wave in the isobars. On the eastern side of the wave, convergence occurs forming thunderstorms (divergence on the western side). If the convergence is strong enough the storm system may evolve into a hurricane.

The other important weather feature in the tropics is the subtropical high pressure zone. Air flow in the subtropical high pressure zone is primarily descending. This creates clear skies, low humidities and hot daytime temperatures. Like the intertropical convergence zone, the subtropical high pressure zone migrates seasonally (see Figure 1). It generally influences locations 10 to 23.5° North and South during some part of the year.

Hurricanes

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. (Source: PhysicalGeography.net)

Hurricanes are intense cyclonic storms that develop over the warm oceans of the tropics (Figure 3). These tropical storms go by other names in the various parts of the world: India/Australia - cyclones; western North Pacific - typhoons; and the Philippines - baguio. By international agreement, the term tropical cyclone is used by most nations to describe hurricane-like storms that originated over tropical oceans. 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. To be classified as a hurricane, sustained wind speeds must be greater than 118 kilometers per hour at the storm's center. Wind speed in a hurricane is directly related to the surface pressure of the storm. The following graph (Figure 2) shows the relationship between surface pressure and sustained wind speed for a number of tropical low pressure systems.

Figure 3: Hurricane as seen from the space shuttle (Source: NASA).

Hurricanes have no fronts associated with them like the mid-latitude cyclones of the polar front. They are also smaller than the mid-latitude cyclone, measuring on average 550 kilometers in diameter. Mature hurricanes usually develop a cloud-free eye at their center (Figure 4). In the eye, air is descending creating clear skies. The eye of the hurricane may be 20 to 50 kilometers in diameter. Surrounding the eye are bands of organized thunderstorm clouds formed as warm air moves in and up into the storm. The strongest winds and heaviest precipitation are found in the area next to the eye where a vertical wall of thunderstorm clouds develops from the Earth's surface to the top of the troposphere.

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° Celsius. Ocean temperatures this high only occur in selected regions and during particular seasons. Hurricane development does not occur if a temperature inversion exists in the atmosphere. Inversions develop in the tropics when subtropical high pressure systems produce sinking air. Also, hurricanes do not develop in the region 5° either side of the equator. Within this region Coriolis force is negligible. Coriolis force is required for the initiation of cyclonic flow.

Figure 4: Satellite view of hurricane Floyd just before it made landfall in North Carolina, September 15, 1999. Notice the eye is clearly visible in this image. (Source: PhysicalGeography.net)

Hurricanes dissipate when their energy supply is substantially reduced. This occurs either with landfall or storm movement into cooler seas. Most hurricanes live for about a week. However, if a hurricane remains over warm water its life can be extended. In 1992, hurricane Tina was an active tropical storm for 24 days over the North Pacific. The map in Figure 5 shows the typical areas of hurricane development and the usual paths they take during their life.

Figure 5: Typical areas where hurricanes begin their development and the common paths of storm movement. (Source: PhysicalGeography.net)

Hurricanes go through a number of different stages of development. Initially, these powerful storms begin their lives as an unorganized group of thunderstorms that develop over the specific areas of the tropical oceans. However, not all of these types of tropical disturbances become hurricanes. To develop into a hurricane, significant cyclonic circulation must occur around the disturbances. This type of circulation enhances the development of the group of thunderstorms by providing additional moisture and latent heat energy. With more moisture and latent heat energy, the strength and number of the thunderstorms in the tropical disturbance increases causing the disturbance to intensify. The thunderstorms also begin to organize themselves into spiral bands that swirl cyclonically toward the center of the storm. If the sustained wind speed around the disturbance increases to between 37 and 63 kilometers per hour, the storm becomes classified as a tropical depression. Tropical depressions appear on the weather map as a cyclonic low with several closed isobars circling the storm's center. A tropical depression can continue to intensify and become a tropical storm which is the next stage in hurricane development. Tropical storms have a lower central pressure, several more closed isobars on a weather map, and winds that are between 64 and 118 kilometers per hour. Finally, tropical storms officially become hurricanes when their sustained wind speed exceeds 118 kilometers per hour.

Tropical storms and hurricanes are the most deadly and destructive type of severe weather on our planet. One of the most destructive hurricanes of the 20th century was Andrew in August of 1992. This storm, which had a minimal pressure reading of 922 millibars, caused an estimated 26 billion dollars worth of damage mainly in Dade County, Florida. The deadliest storm of the 20th century is probably hurricane Mitch, which hit Central America in late October and early November 1998. Estimates suggest that over 11,000 people died from this storm. Most of these deaths were cause by flooding and mudslides due to heavy rains.

The damage that hurricanes inflict is caused by high wind speeds, heavy rainfall, storm surge and tornadoes. Wind speed in a hurricane is usually directly related to atmospheric pressure (see Figure 2). The lower the pressure the faster the winds blow. Wind speed also varies within the storm. As discussed earlier, winds are usually strongest at the edge of the hurricane's eye. High winds inflict damage by blowing down objects, creating choppy waves and high seas, and by inundating coastal areas with seawater. Rainfall within a hurricane can often exceed 60 centimeters in a 24 hour period. If this rainfall occurs on land, flooding normally occurs. Storm surge is increase in the height of the ocean's surface in the region beneath and around the eye of the storm. It occurs when low atmospheric pressure causes the ocean surface to expand and because the hurricane's cyclonic winds blow seawater towards the eye. Hurricane Camille (1969) had a storm surge of more than seven meters with a central pressure of 909 millibars. A considerable amount of damage can also occur because of hurricane-generated tornadoes. About 25% of the hurricanes that make landfall have tornadoes. Some scientists also suspect that the thunderstorms that occur near the eye of a hurricane can produce very strong downbursts (vertical downward movements of air).

The year 1998 was extremely bad for the development of tropical storms and hurricanes in the Atlantic ocean. During the late summer and fall of that year fourteen tropical storms developed of which ten became hurricanes. Table 1 describes some of the characteristics of each of the storms that developed into hurricanes. Seven of the tropical storms and hurricanes made landfall in the United States causing about 6.5 billion dollars worth of damage. The strongest hurricane of the 1998 season was hurricane Mitch. This storm made landfall in Honduras and killed over 11,000 people in Central America.

Further Reading

• PhysicalGeography.net