How Hurricanes Form, What Causes, Hurricane Models

Hurricane Sandy near FloridaFew events on Earth rival the sheer power of a hurricane. Hurricanes produce enough rain to fill over 22 million Olympic-sized swimming pools, and results in the release of 600 trillion watts of heat energy (or 200 times the worldwide electrical generating capacity as of January 1, 1996, according to US Department of Energy). Also known as tropical cyclones and typhoons, these fierce storms can generate 50-foot (15-meter) waves, redefine coastlines and destroy entire cities. In the Northern Hemisphere, the hurricane season runs from June 1 to Nov. 30, while the Southern Hemisphere’s hurricane season is from activity from January to March. A hurricane builds energy as it moves across warm parts of the ocean near the equator, sucking up warm, moist tropical air from the surface.

Hurricane Definition: What is a Hurricane

To understand how a hurricane works, you have to understand the principle of atmospheric pressure. The gases that make up Earth’s atmosphere are subject to the planet’s gravity. In fact, the atmosphere weighs about 5.5 quadrillion tons (4.99 quadrillion metric tons). Gas molecules near the surface are compressed by the weight of the air above. When air heats up, its molecules move farther apart, making it less dense. This air then rises to higher altitudes where air molecules are less dense. When warm, low-pressure air rises, cool, high-pressure air takes its place. This movement is called a pressure gradient force.

Pressure Gradient Force

When warm, moist air from the ocean’s surface begins to rise rapidly, its ¬≠water vapor condenses to form storm clouds and droplets of rain. The condensation releases heat called latent heat of condensation. This latent heat warms the cool air, causing it to rise. This rising air is replaced by more warm, humid air from the ocean below. And the cycle continues, drawing more warm, moist air into the developing storm and moving heat from the surface to the atmosphere. This exchange of heat creates a pattern of wind that circulates around a center, like water going down a drain.

Converging winds at the surface are colliding and pushing warm, moist air upward. This rising air reinforces the air that’s already ascending from the surface, so the circulation and wind speeds of the storm increase. In the meantime, strong winds at higher altitudes (up to 30,000 feet or 9,000 meters) help to remove the rising hot air from the storm’s center, maintaining a continual movement of warm air from the surface and keeping the storm organized. If the high-altitude winds don’t blow at the same speed at all levels (if wind shears are present) the storm becomes disorganized and weakens. As high-pressure air is sucked into the low-pressure center of the storm, wind speeds increase.

How Hurricanes Form

How Hurricane Forms DiagramHurricanes develop in warm, tropical regions where the water is at least 80 degrees Fahrenheit (27 degrees Celsius). The storms also require moist air and converging equatorial winds. Most Atlantic hurricanes begin off the west coast of Africa, starting as thunderstorms that move out over the warm, equatorial ocean waters.¬≠ A hurricane’s low-pressure center of relative calm is called the eye. The area surrounding the eye is called the eye wall, where the storm’s most violent winds occur. The bands of thunderstorms that circulate outward from the eye are called rain bands. These storms play a key role in the evaporation/condensation cycle that feeds the hurricane.

The rotation of a hurricane is a product of the Coriolis effect, a natural phenomenon caused by the Earth’s rotation, that causes fluids and free-moving objects to veer to the right of their destination in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection gets storms spinning. As a result, hurricanes in the Northern Hemisphere rotate counterclockwise and clockwise in the Southern Hemisphere. The effect bends hurricanes to the right (clockwise) in the Northern Hemisphere and to the left (counterclockwise) in the Southern Hemisphere.

Hurricanes often begin their lives as clusters of clouds and thunderstorms called tropical disturbances. These low-pressure areas feature weak pressure gradients and little to no rotation. Most of these disturbances die out, but a few persevere to gain hurricane status. In these cases, the thunderstorms in the disturbance release latent heat, which warms areas in the disturbance. This causes the air density inside the disturbance to lower, dropping the surface pressure. Wind speeds increase as cooler air rushes underneath the rising warm air. As this wind is subject to the Coriolis effect, the disturbance begins to rotate. The incoming winds bring in more moisture, which condenses to form more cloud activity and releases latent heat in the process.

Hurricane Life Cycle

Hurricane Life Cycle

If a tropical disturbance continues to find enough warm moist air and encounters optimal wind and pressure conditions, it will just keep growing. It can take anywhere from hours to days for a tropical disturbance to develop into a hurricane. If the cycle of cyclonic activity continues and wind speeds increase, the tropical disturbance advances through three stages:
Tropical Depression: wind speeds of less than 38 mph
Tropical Storm: wind speeds of 39 to 73 mph
Hurricane: wind speeds greater than 74 mph

Between 80 and 100 tropical storms develop each year around the world. Many of them die out before they can grow too strong, but around half of them eventually achieve hurricane status. Hurricanes vary widely in physical size. Some storms are compact, with only a few bands of wind and rain trailing behind them. Other storms are looser in which the bands of wind and rain spread out over hundreds or thousands of miles. Hurricane Floyd, which hit the eastern United States in September 1999, was felt from the Caribbean islands to New England. Once a hurricane has formed and intensified, the only remaining path for the atmospheric juggernaut is dissipation. Eventually, the storm will encounter conditions that deny it the warm, moist air it requires. When a hurricane moves onto cooler waters at higher latitudes, gradient pressure decreases, winds slow, and the entire storm loses power. That supply of warm, moist air also vanishes when the hurricane makes landfall. Condensation and the release of latent heat diminishes, and the friction of an uneven landscape decreases wind speeds. This causes winds to move more directly into the eye of the storm, eliminating the large pressure difference that fuels the storm’s awesome power.

Hurricane Categories

Hurricane Categories Chart

Hurricanes can unleash incredible damage when they hit. With enough advance warning though, cities and coastal areas can give residents the time they need to fortify the area with flood barriers, hurricane shutters, etc, and even evacuate. To better classify each hurricane and prepare those affected for the intensity of the storm, meteorologists rely on rating systems.

American meteorologists use a scale to classify hurricanes called the Saffir-Simpson Hurricane Wind Scale, which ranks storms by wind speed on a scale of 1 to 5.

Category 1 Hurricane

Wind Speed:
74-95 mph
64-82 kt
119-153 km/h

Very dangerous winds will produce some damage: Well-constructed frame homes could have damage to roof, shingles, vinyl siding and gutters. Large branches of trees will snap and shallowly rooted trees may be toppled. Extensive damage to power lines and poles likely will result in power outages that could last a few to several days.

Category 2 Hurricane

Wind Speed:
96-110 mph
83-95 kt
154-177 km/h

Extremely dangerous winds will cause extensive damage: Well-constructed frame homes could sustain major roof and siding damage. Many shallowly rooted trees will be snapped or uprooted and block numerous roads. Near-total power loss is expected with outages that could last from several days to weeks.

Category 3 Hurricane

Wind Speed:
111-129 mph
96-112 kt
178-208 km/h

Devastating damage will occur: Well-built framed homes may incur major damage or removal of roof decking and gable ends. Many trees will be snapped or uprooted, blocking numerous roads. Electricity and water will be unavailable for several days to weeks after the storm passes.

Category 4 Hurricane

Wind Speed:
130-156 mph
113-136 kt
209-251 km/h

Catastrophic damage will occur: Well-built framed homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Most trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last weeks to possibly months. Most of the area will be uninhabitable for weeks or months.

Category 5 Hurricane

Wind Speed:
157 mph or higher
137 kt or higher
252 km/h or higher

Catastrophic damage will occur: A high percentage of framed homes will be destroyed, with total roof failure and wall collapse. Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months. Most of the area will be uninhabitable for weeks or months.

Hurricane Damage

Hurricane DamageThe word “hurricane” derives from “Hurakan,” a destructive Mayan god. No matter how you choose to sum up or personify these powerful acts of nature, the damage hurricanes inflict stems from several different aspects of the storm. Hurricanes deliver massive downpours of rain. A particularly large storm can dump dozens of inches of rain in just a day or two, much of it inland. That amount of rain can create flooding, potentially devastating large areas in its path. In addition, high sustained winds within the storm can cause widespread structural damage to both man-made and natural structures. These winds can roll over vehicles, collapse walls and blow over trees. The prevailing winds of a hurricane push a wall of water, called a storm surge, in front of it. If the storm surge happens to coincide with high tide, it causes beach erosion and significant inland flooding.

The hurricane itself is often just the beginning. The storm’s winds often spawn tornadoes and waterspouts, which are smaller, more intense cyclonic storms that cause additional damage. The extent of hurricane damage doesn’t just depend on the strength of the storm, but also the way it makes contact with the land and local construction codes. In many cases, the storm merely grazes the coastline, sparing the shores its full power. Hurricane damage also greatly depends on whether the left or right side of a hurricane strikes a given area. The right side of a hurricane packs more punch because the wind speed and the hurricane’s speed of motion complement one another there. On the left side, the hurricane’s speed of motion subtracts from the wind speed. Also, regions in which buildings are reinforced with flood barriers, impact windows and hurricane shutters have a much better chance of surviving a hurricane than a building with substandard construction.

Hurricane Tracking

Category Five Atlantic Hurricanes Path MapTo monitor and track the development and movement of a hurricane, meteorologists rely on remote sensing by satellites, as well as data gathered by specially equipped aircraft. On the ground, Regional Specialized Meteorological Centers, a network of global centers designated by the World Meteorological Organization, are charged with tracking and notifying the public about extreme weather. Weather satellites use different sensors to gather different types of information about hurricanes. They track visible clouds and air circulation patterns, while radar measures rain, wind speeds and precipitation. Infrared sensors also detect vital temperature differences within the storm, as well as cloud heights. Meteorologists take all the storm data they receive and use it to create computer forecast models. Based on a great deal of current and past statistical data, these virtual storms allow scientists to forecast a hurricane’s path and changes in intensity well in advance of landfall. With this data, governments and news agencies can warn residents of coastal areas and greatly reduce the loss of life during a hurricane. Long-term forecasting now allows meteorologists to predict how many hurricanes will take place in an upcoming season and to study trends and patterns in global climate.

Hurricane Names

The practice of naming hurricanes originated with meteorologists, not media outlets. Often more than one tropical storm is active at the same time, so what better way to tell them apart than by naming them. For several hundred years, residents of the West Indies often named hurricanes after the Catholic saint’s day on which the storm made landfall. If a storm arrived on the anniversary of a previous storm, a number was assigned. For example, Hurricane San Felipe struck Puerto Rico on Sept. 13, 1876. Another storm struck Puerto Rico on the same day in 1928, so this storm was named Hurricane San Felipe the Second. During World War II, weather officials only gave hurricanes masculine names. These names closely followed radio code names for letters of the alphabet. This system, like the West Indian saints system, drew from a limited naming pool. In the early 1950s, weather services began naming storms alphabetically and with only feminine names. By the late 1970s, this practice was replaced with the equal opportunity system of alternating masculine and feminine names. The World Meteorological Organization (WMO) continues this practice to this day.

The first hurricane of the season is given a name starting with the letter A, the second with the letter B and so on. As the storms affect varying portions of the globe, the naming lists draw from different cultures and nationalities. Hurricanes in the Pacific Ocean are assigned a different set of names than Atlantic storms. For example, the first hurricane of the 2001 hurricane season was a Pacific Ocean storm near Acapulco, Mexico, named Adolf. The first Atlantic storm of the 2001 season was named Allison. If a hurricane inflicts significant damage, a country affected by the storm can request that the name of the hurricane be “retired” by the WMO. A retired name can’t be reissued to a tropical storm for at least 10 years. This helps to avoid public confusion and to simplify both historical and legal record keeping.