Tropical Weather

The tropical zone is the area surrounding the equator that is exposed to direct solar rays during the year. Tropical areas receive higher annual precipitation and experience low temperature variation between seasons. Localized low-pressure systems in the ocean can develop into weather disturbances when the ocean temperature increases. As temperature and pressure continue to increase, weather disturbances develop into tropical depressions and tropical storms.

Origin of Tropical Weather

The tropical zone is the portion of the Earth that surrounds the Earth’s equator. The zone ranges from approximately 23.5 degrees north (the Tropic of Cancer) to 23.5 degrees south (the Tropic of Capricorn).

Because of the relationship between the Earth's rotation and its orbit around the Sun, the portion of the Earth that lies directly under the Sun varies between the Tropics of Cancer and Capricorn during the year. The tropical zone can be described as the portion of the Earth that lies directly under the Sun for some portion of the year.

The tropics generally receive high annual precipitation and relatively stable temperatures that vary little between seasons. Rather than warm and cold seasons, tropical areas have wet and dry seasons because precipitation in the tropics varies more markedly throughout the year than temperature.

The differential distribution of solar energy causes weather patterns within the tropics. Solar radiation strikes the Earth along the tropical zone, exciting the atoms of atmospheric gases, which causes them to vibrate and collide with one another. In the lowest level of the atmosphere, called the troposphere, excited pockets of atmospheric gases rise because they lose density and pressure as they spread. The gases continue to rise until they reach the limit of the troposphere, the tropopause, where warm air is forced toward the poles because the atmosphere is highly stratified.

As warm air moves toward the Tropics of Capricorn and Cancer, it gradually cools, condenses, and falls to the surface. By approximately 30 degrees north and south, tropical air currents moving close to the surface converge with cooler air moving toward the equator from the poles. Some tropical air is pushed back toward the equator, thus constituting a circulation cell, a pattern of rising and falling air currents that cycles air between the surface and the tropopause.

Two tropical or Hadley cells move air from the equator to about 30 degrees north, south, and back. A portion of the tropical air from the Hadley cells continues moving toward the poles, cycling through two additional circulation cells and distributing the solar energy contained within tropical air over the Earth's surface.

Low-pressure Systems and Storm Generation

Directly over the equator, where the northern and southern tropical cells meet, is the area called the Intertropical Convergence Zone (ITCZ), which is known colloquially as the doldrums.

The ITCZ does not remain directly over the equator because the Sun's position relative to the Earth changes during Earth’s orbit. The ITCZ, therefore, migrates with the seasons, moving through an area that takes the zone between 5 and 10 degrees north or south of the equator. The ITCZ migrates north in the summer and south in the winter months, corresponding to temperature variations, precipitation increases, and storm activity.

The ITCZ receives the highest level of solar heating during the year and, therefore, develops into a low-pressure area as the air within the ITCZ loses density in response to solar excitation. Low-pressure areas are characterized by cloud cover and strong winds. Because the Earth rotates counterclockwise, the winds spreading from the ITCZ are deflected relative to the poles; this phenomenon, known as the Coriolis effect, causes the ITCZ winds to move to the northeast and southeast from the equator. These winds, called the trade winds, carry air currents toward the ends of the tropical zone, where they gradually cool, increasing density and pressure. This creates two high-pressure areas, called the subtropical high-pressure zones, which shift (along with the migration of the ITCZ) between 10 and 30 degrees north and south of the equator.

Weather fronts develop at the point where low- and high-pressure systems meet. A cold front is an area where a dominant stream of cold air pushes under a bed of warmer air at its leading edge. This causes warm air to rise and condense rapidly; precipitation develops along the cold front's leading edge as the water vapor condenses.

A warm front is an area in which dominant streams of warm air encounter pockets of cold air. The warm air rises over the cold air, thereby leading to the condensation of water vapor and precipitation following in the wake of the front. In general, rising air produces rainfall, while falling air creates dry conditions.

Tropical

Monsoons are seasonal variations in precipitation that accompany the movement of the ITCZ. Monsoons result from the differential heating of the ocean and nearby landmasses and changes in the direction of wind patterns. Monsoon seasons are the periods with the highest annual precipitation during the year. Monsoons affect portions of Africa, Asia, North America, and South America.

Wind currents typically move from the continents to the ocean through most of the year. In the summer months, the solid materials of the continental crust heat faster and reach higher average temperatures than the liquid water of the ocean, which disseminates and thereby regulates heat more efficiently. As this occurs, a low-pressure area develops over the land as hot air disperses into the atmosphere. This causes a reversal in the direction of prevailing winds as cooler air pockets over the ocean are drawn over the land. The mixture of warm and cool air rises and leads to condensation and precipitation moving inward over the continent. This continues until heat levels reach equilibrium between the two areas and the monsoons decrease.

Monsoons also occur in the winter because the land loses heat more rapidly than the water, producing a high-pressure system over the land. When this reaches a certain level, warmer air pockets over the ocean are drawn over the land and begin rising, blending with cooler air to produce precipitation. These two systems are responsible for the seasonal cycle of monsoons alternating with dry seasons in the tropics.

Tropical Storms

Tropical storms develop within the trade winds out of localized disturbances called easterly waves, troughs of low pressure moving from east to west across the ocean. Easterly waves develop when a portion of the ocean is heated such that water vapor begins to rise from the surface through evaporation, thereby reducing pressure along a narrow band. As this zone of low-pressure moves, the area behind it forms a zone of divergence, where the winds flow out of the system.

A zone of convergence forms to the front of the wave, where high- and low-pressure winds meet and spiral upward. This spiraling, rising column of current, which carries significant moisture from evaporated water vapor, is the origin of tropical storms.

The defining characteristic of a tropical storm is the column of warmer air at the center of the storm, known as the core, which creates the differential pressure and density that drive the continued movement of the system. For a tropical storm to develop, the water underlying the storm system must be 26 degrees Celsius (79 degrees Fahrenheit) or warmer, with high local humidity levels. A tropical storm can develop when these factors merge with local weather disturbances.

Phases of Tropical Storms

Tropical storms begin as smaller, localized tropical disturbances. In most cases, tropical disturbances result in isolated thunderstorms that dissipate as the warm currents at the core are drawn into the upper troposphere. If the localized pressure difference grows, the storm becomes a tropical depression, with a more defined core and wind speeds between 37 and 63 kilometers (23 and 39 miles) per hour. At this point, meteorological organizations generally give the tropical storm an identification number, which is tracked for further developments.

When wind speeds increase to 64 and 118 km (40 to 73 mi) per hour, the storm is reclassified as a tropical storm or cyclone. At this point, the storm system is designated with a unique name and may seriously threaten coastal settlements. Cyclones with wind speeds exceeding 119 km (74 mi) per hour are called mature tropical cyclones and are characterized by a well-defined core, sometimes called the eye of the storm. Mature tropical cyclones that develop in the eastern part of the Pacific basin or the Atlantic basin are called hurricanes. In contrast, those that develop in the western portion of the Pacific basin are called typhoons.

Once a cyclone forms, it will move in response to changes in local wind speed and pressure. The path of a cyclone is highly unpredictable, which increases the danger it poses to terrestrial ecosystems and human settlements. Cyclones can change speed and direction rapidly and without significant warning, making it difficult to predict the cyclone's path. In general, most of the storms that originate above the equator tend to move northwest, while those below the equator move southwest.

Tropical storms follow seasonal patterns related to the north and south variations of the ITCZ. The peak for cyclones in the Southern Hemisphere falls between January and March, while the peak for cyclones in the Northern Hemisphere falls between June and November. The northwest Pacific basin is the most active area for tropical cyclones, with storms forming during any month of the year. Most cyclones are active for less than a week, though particularly strong storms have lasted nearly a month before dissipating. Tropical storms dissipate when the latent heat energy in the ocean and atmosphere is reduced so the storm can no longer sustain its motion.

The National Weather Service in the United States names each tropical storm using a system of alternating male and female names chosen to reflect the alphabetical position of the storm for that year. The first named storm in any year will therefore be given a name beginning with the letter A. If the last storm of the preceding year was given a male name, then the new storm will be given a female name and vice versa. The list of potential names is recycled through a six-year period. When a hurricane causes significant damage to human settlements, the name of the hurricane may be retired from the list of potential storm names. Hurricane Allison (2001) and Hurricane Agnes (1972) are two hurricanes whose names have been retired from usage because of the damage wrought by those storms. Harvey, Irma, Katrina, Maria, and Sandy are hurricane names that have been retired in the twenty-first century.

In some hurricanes, wind speeds can exceed 180 km (112 mi) per hour, sufficient to destroy buildings and fell trees. When a cyclone is active, water levels in the area directly beneath and around the eye of the storm rise in response to the low pressure of the core, creating what is called a storm surge. Many hurricane- or typhoon-related deaths result from flood waters accompanying the storm surge. Hurricane Katrina, which hit the Atlantic and Gulf coasts of the United States (US) in 2005, caused more than $191 billion in damage and led to over 1,400 deaths. It is considered the costliest hurricane in US history. Global climate change's warming of the Earth's oceans is only expected to cause more frequent and destructive hurricanes and other forms of tropical weather, a trend that became apparent in the mid-2020s. 

In addition, approximately 25 percent of hurricanes that impact the continental surface have associated tornadoes, which are vortexes of wind currents that can move over dry land. Associated tornadoes can spread wind damage for miles surrounding the impact zone.

Principal Terms

circulation cell: zones of concentrated air circulation caused by the rotation of the Earth and the cyclic distribution of heat through the atmosphere

cold front: area in which a dominant stream of colder air pushes under a pocket of warmer air, causing the warm air to rise and leading to precipitation at the leading edge of the front

easterly waves: localized zones of low pressure oriented parallel to the Earth’s rotational axis and moving from east to west across the ocean; they form an important generative component of tropical weather patterns

Intertropical Convergence Zone: low-pressure area created by the interaction of the tropical circulation cells on either side of the equator and by differential heating of the equatorial atmosphere

monsoon: period of increased precipitation caused by the differential distribution of heat and pressure between the lithosphere and hydrosphere

storm surge: rising water levels beneath and surrounding the warm core of a tropical storm system related to the effects of low- and high-pressure interactions within the storm

tropical cyclone: tropical storm marked by clear rotation around a central, warm column of air and wind speeds above 119 kilometers (74 miles) per hour

tropical zone: area between the Tropic of Cancer and the Tropic of Capricorn that falls directly under the Sun during a part of the year

troposphere: lowest level of Earth’s atmosphere lying between 0 and 10 km (0 and 6 mi) above sea level within which weather patterns develop

warm front: area in which a dominant warm current of air rises over a pocket of cold air to cause condensation and precipitation at the trailing edge of the front

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