Friday, January 6, 2012


Tropical Weather
The tropics can be defined as the area of the Earth found between the Tropic of Cancer (23.5° North) and the Tropic Capricorn (23.5° South). 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 high quantities of solar insolation are received here regardless of the time of the year. Weather in the tropics is dominated by convective storms that develop mainly along the intertropical convergence zone (ITCZ), the subtropical high pressure zone, and oceanic disturbances in the trade winds that sometimes develop into hurricanes.

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. In general, the intertropical convergence zone delineates the location where the noonday Sun is directly overhead. 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 converted 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. 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 latitude. Figure 7u-1describes the seasonal movement of the intertropical convergence zone and other systems associated with our planet’s global circulation.

Figure 7u-1: Seasonal movement of the Earth's global circulation patterns. The major circulation cells are identified on the “Equinox” figure. In the “Equinox” figure, the intertropical convergence zone (ITCZ) occurs at the equator where the Trade Winds converge creating a low-pressure center and updrafts (red arrows). The upward movement of the moist tropical air above the intertropical convergence zone produces a band of thunderstorm activity that brings precipitation to the tropics. At the zone where the Hadley Cell and Ferrel Cell meet, tropospheric airflow is in a downward direction moving from the top of the troposphere to the Earth’s surface (green arrows). These downdrafts give rise to the subtropical high-pressure zone on the Earth’s surface. Surface air also moves upward into the troposphere at the boundary between the Ferrel Cell and the Polar Cell. At this location, frontal lifting associated with mid-latitude cyclones moves warm subtropical air over cold polar air producing clouds and precipitation. Note how the Earth’s circulation patterns change their position in the December and June solstice figures.

The other important weather feature in the tropics is the subtropical high-pressure zone. Airflow in the subtropical high-pressure zone is primarily descending. This creates clear skies, low humidity, and hot daytime temperatures. Like the intertropical convergence zone, the subtropical high-pressure zone migrates seasonally (see Figure 7u-1). It generally influences locations 10 to 23.5° North and South during some part of the year.

Easterly Waves
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 ocean temperatures. On a weather map these weather systems appear as a wave in the isobars (Figure 7u-2). On the eastern side of this wave, convergence occurs forming numerous thunderstorms (divergence occurs on the western side). If the convergence is strong enough, the storm system may intensify and organize into a hurricane. On average, about 10% of the weather disturbances associated with easterly waves develop into hurricanes.

Figure 7u-2: Most tropical storms develop from a weather feature known as an easterly wave. On weather maps, these features appear as a “wave” in the isobars (lines of equal pressure) that travel from east to west. Numerous thunderstorms form on the East side of the easterly wave because of convergence. If this mass of thunderstorms develops cyclonic airflow a hurricane often forms.

Hurricanes are intense cyclonic storms that develop over the warm oceans of the tropics (Figure 7u-4). 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 7u-3) shows the relationship between surface pressure and sustained wind speed for a number of tropical low pressure systems.

Figure 7u-3: 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.

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. One of the largest hurricanes ever measured was Typhoon Tip (October 12, 1979) which had a diameter of about 2100 kilometers. Mature hurricanes usually develop a cloud-free eye at their center (Figure 7u-4 and 7u-5). 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 move in and up into the storm (Figure 7u-6). 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.

Figure 7u-4: Hurricane as seen from the space shuttle (Source: NASA).

Figure 7u-5: 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: NASA).

Figure 7u-6: Graphical model showing a vertical cross-section of the air circulation, clouds, and precipitation associated with a hurricane.

Hurricane Development, Movement, and Dissipation
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 (80° F). Further, this warm surface water must exist in a layer that is at least 200 meters (600 feet) deep. Ocean temperatures this high only occur in selected regions of our planet and during particular seasons. Hurricane development can also be prevented by the presence of a temperature inversion in the atmosphere. Inversions develop in the tropics when subtropical high pressure systems produce sinking air.

Hurricanes go through a number of different stages of development. Initially, these powerful storms begin their lives as a group of unorganized 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 (23 to 39 miles 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, 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.

Figure 7u-7 shows the tropical and subtropical ocean areas where hurricanes typically form on our planet. These areas generally extend pole ward to a maximum latitude of about 25 to 30° North or South. The exclusion of hurricanes from around the equator is related to the fact that Coriolis force is almost negligible here. A specific threshold quantity of Coriolis force is required to initiate cyclonic flow. Note that hurricanes do not form over the southeastern Pacific Ocean, South Atlantic Ocean, and off the coast of northern Africa. In these regions, cool ocean temperatures or the presence of temperature inversions restrict formation.

Figure 7u-7: Locations where hurricanes began their development. Data is for the period 1950-2000.

Hurricanes are seldom motionless after their initial formation. Typical paths of movement are displayed on Figures 7u-8 and 7u-9 for the various ocean basins that generate hurricanes. On average, hurricanes that form in the North Atlantic and North Pacific move in a west or northwestward path. In reality, the track taken by any individual storm is often very chaotic. Hurricanes can suddenly change both their speed and direction of travel.

Figure 7u-8: Typical areas where hurricanes begin their development and the common paths of storm movement.

Figure 7u-9: This map superposes the tropical storm tracks available from the National Hurricane Centerand the Joint Typhoon Warning Center from the beginning of their records to September 2006. Colors associated with storm tracks indicate the intensity of the storm according to the Saffir-Simpsom intensity scale. Note the absense of storms in southeast Pacific and South Atlantic Oceans. Cool ocean currents keep sea temperatures from reaching temperatures needed for tropical cyclone development. (Source: NASA - Earth Observatory).

Figure 7u-10 describes the annual number of hurricanes that developed in the North Atlantic, northeast Pacific, and northwest Pacific Ocean basins from 1951 to 2002. Of these three regions, the northwest Pacific Ocean basin produced the most hurricanes for this period, an average of 17.7 storms per year (Figure 8.36). The mainland of North America is often influenced by storms that form in the North Atlantic and northeast Pacific which produce a yearly average of 5.9 and 7.4 hurricanes, respectively. Most of the storms that form in the northeast Pacific travel away from North America out to open ocean (see Figure 7u-8).

Figure 7u-10: Yearly hurricane numbers for the North Atlantic, northeast Pacific, and northwest Pacific Ocean basins for the period 1951-2002. Mean number of hurricanes for each ocean basin is displayed in red.

Tropical storms and hurricanes also have very specific seasonal patterns. The peak season for these weather events in the Southern Hemisphere is January to March. In the Northern Hemisphere, most tropical storms and hurricanes develop in the months of June to November. Figure 7u-11describes the month frequency of storm formation in the three ocean basins found in the Northern Hemisphere for the period 1951 to 2002. Each basin has slightly different patterns. Tropical storms and hurricanes rarely form during the months December to April in the North Atlantic and North Pacific basins. The Northwest Pacific basin has a significant number of storms forming in all months.

Figure 7u-11: Combined tropical storm and hurricane monthly frequency for the North Atlantic, northeast Pacific, and northwest Pacific Ocean basins. Data is for the period 1950-2000.

Hurricanes dissipate when the availability of latent heat energy 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.

Hurricane Classification
The most commonly used system to classify the strength of hurricanes is the Saffir-Simpson scale (Table 7u-1). This classification scheme was developed so the public could quickly estimate the potential damage a hurricane’s winds and storm surge could have on a coastal area. The first two categories describe the two stages that occur before a storm becomes a hurricane: TD – tropical depression and TS – tropical storm. Hurricanes are categorized into 5 scales of intensity that ranges from 1 to 5. A major hurricane is considered to be category 3 and above.

Table 7u-1: Saffir-Simpson scale of hurricane intensity.
Pressure (mb)
Winds (mph)
Winds (kmph)
Surge (meters)
Tropical Depression
< 39
< 62
Tropical Storm
> 980
< 920

Figure 7u-12 describes the number of category 1 to 5 hurricanes that developed in the North Atlantic, northeast Pacific, and northwest Pacific Ocean basins for the period 1951 to 2002. Note that the northwest Pacific basin produces proportionally more major storms than the other two basins.

Figure 7u-12: Frequency of Saffir-Simpson category 1, 2, 3, 4, and 5 hurricanes for the North Atlantic, northeast Pacific, and northwest Pacific Ocean basins. Data is for the period 1951-2002.

Hurricane Damage and Destruction
Tropical cyclones are the most deadly and destructive type of severe weather event on our planet. One of the most destructive hurricanes in the last 100 years was the Bhola cyclone (category 3) that made landfall on the Bangladesh coastline on November 12 of 1970. This storm had a minimal pressure reading of 966 millibars and maximum sustained winds of about 185 km/hr. Estimates suggest that over 300,000 people died from this storm. Most of these deaths were caused by flooding due to the storm surge and heavy rains. The high death toll caused by this hurricane was due to lack of preparedness. It is believed that less than 1% of the population took shelter in fortified buildings. One of the deadliest Atlantic storms is hurricane Mitch (category 5). Hurricane Mitch encountered populated areas of Central America twice: in late October and early November 1998. Estimates suggest that over 11,000 people died from this storm. Most of these deaths were caused 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 7u-3 above). 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 (24 inches) in a 24-hour period. If this rainfall occurs on land, flooding often occurs. Storm surge is an 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 – category 5) had a storm surge of more than seven meters (23 feet) with a central pressure of 909 millibars. A considerable amount of damage can also occur because of tornadoes. About 25% of the hurricanes that make landfall have associated tornadoes. Some scientists also suspect that the thunderstorms that occur near the eye of a hurricane can produce very strong down bursts (vertical downward movements of air).

The year 2005 was extremely bad for the development of tropical storms and hurricanes in the Atlantic Ocean. During the late summer and fall of that year, 28 tropical storms developed of which 15 became hurricanes. Table 7u-2 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 more than 100 billion US dollars worth of damage. Katrina was the most destructive storm of the 2005 season causing over 81 billion dollars of damage. This hurricane made landfall at the mouth of the Mississippi River on August 29th and flooded 80% of New Orleans (Figure 7u-13). The strongest hurricane of the 2005 season was hurricane Wilma with a minimum storm pressure of 882 mb (Figure 7u-14). This is the lowest pressure reading ever recorded for the Altantic Ocean in modern times.

Table 7u-2: Characteristics of the Atlantic hurricanes of the 2005 season.

Name of HurricaneBornDiedLifespanMaximum
Sustained Winds
Storm Pressure
Damage in
US Dollars
Direct and
Indirect Deaths
CindyJuly 3July 74 days120 km/hr991 mb320 Million3
DennisJuly 4July 139 days240 km/hr930 mb5 Billion89
EmilyJuly 10July 2111 days260 km/hr929 mb550 Million15
IreneAug 4Aug 1814 days165 km/hr970 mbNoneNone
KatrinaAug 23Aug 318 days275 km/hr902 mb> 81 Billion> 1836
MariaSept 1Sept 109 days185 km/hr962 mb3.1 Million3
NateSept 5Sept 105 days145 km/hr979 mbNoneNone
OpheliaSept 6Sept 2317 days140 km/hr976 mb70 Million3
PhilippeSept 17Sept 247 days130 km/hr985 mbNoneNone
RitaSept 18Sept 268 days285 km/hr895 mb10 Billion120
StanOct 1Oct 54 days130 km/hr977 mb2 Billion> 1600
VinceOct 8Oct 113 days120 km/hr988 mbNoneNone
WilmaOct 15Oct 2510 days295 km/hr882 mb29 Billlion63
BetaOct 26Oct 315 days185 km/hr962 mbNoneNone
EpsilonNov 29Dec 810 days140 km/hr979 mbNoneNone


Figure 7u-13: Photo taken on August 29, 2005 in New Orleans, Louisiana showing Interstate 10 at West End Boulevard and looking towards Lake Pontchartrain. (Source: United States Coast Guard, photo taken by Kyle Niemi).

Figure 7u-14: On the morning of October 19, 2005, the center of category 5 hurricane Wilma had a pressure of 882 millibars, the lowest pressure ever measured in an Atlantic hurricane. Wilma started its life on October 17th in the western Caribbean, southwest of the island of Jamaica.

Pidwirny, M. (2006). "Tropical Weather and Hurricanes". Fundamentals of Physical Geography, 2nd Edition. 6/1/2012.

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