17.6: Cyclones
- Page ID
- 32280
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Cyclones can be the most intense storms on Earth. A cyclone is a system of winds rotating counterclockwise in the Northern Hemisphere around a low pressure center. The swirling air rises and cools, creating clouds and precipitation.
There are two types of cyclones: middle latitude (mid-latitude) cyclones and tropical cyclones. Mid-latitude cyclones are the main cause of winter storms in the middle latitudes. Tropical cyclones are also known as hurricanes.
An anticyclone is the opposite of a cyclone. An anticyclone’s winds rotate clockwise in the Northern Hemisphere around a center of high pressure. Air comes in from above and sinks to the ground. High pressure centers generally have fair weather.
Mid-Latitude Cyclones
Mid-latitude cyclones, sometimes called extratropical cyclones, form at the polar front when the temperature difference between two air masses is large. These air masses blow past each other in opposite directions. The Coriolis effect deflects winds to the right in the Northern Hemisphere, causing the winds to strike the polar front at an angle. Warm and cold fronts form next to each other. Most winter storms in the middle latitudes, including most of the United States and Europe, are caused by mid-latitude cyclones.The warm air at the cold front rises and creates a low pressure cell. Winds rush into the low pressure and create a rising column of air. The air twists, rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Since the rising air is moist, rain or snow falls.
Mid-latitude cyclones form in winter in the mid-latitudes and move eastward with the westerly winds. These two- to five-day storms can reach 1000 to 2500 km (625 to 1600 miles) in diameter and produce winds up to 125 km per hour (75 mph). Like tropical cyclones, they can cause extensive beach erosion and flooding. Mid-latitude cyclones are especially fierce in the mid-Atlantic and New England states where they are called nor’easters, because they come from the northeast. About 30 nor’easters strike the region each year.
Hurricanes
Tropical cyclones have many names. They are called hurricanes in the North Atlantic and Eastern Pacific Oceans, typhoons in the Western Pacific Ocean, tropical cyclones in the Indian Ocean, and willy-willies in the waters near Australia. By any name, they are the most damaging storms on Earth. Hurricanes arise in the tropical latitudes (between 10°N and 25°N) in summer and autumn when sea surface temperature are 26.5°C (80°F) or higher. The warm seas create a large humid air mass. The warm air rises and forms a low pressure cell, known as a tropical depression. Thunderstorms materialize around the tropical depression.
If the temperature reaches or exceeds 26.5°C (80°F) the air begins to rotate around the low pressure (counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere). As the air rises, water vapor condenses, releasing energy from latent heat. If wind shear, or the change in wind speed and/or direction over a short distance, is low, the storm builds into a hurricane within two to three days.
Hurricanes are large systems with high winds. The exception is the relatively calm eye of the storm where the air is rising upward. Rainfall can be as high as 2.5 cm (1 inch) per hour, resulting in about 20 billion metric tons of water released daily in a hurricane. The release of latent heat generates enormous amounts of energy, nearly the total annual electrical power consumption of the United States from one storm. Hurricanes can also generate tornadoes. Hurricanes are strange phenomena because they are deadly monsters, yet have a gentle, but cold heart. The anatomy of a hurricane is fairly simple, though the processes involved are quite complex. As a low pressure disturbance forms, the warm, moist air rushes towards the low pressure and rises upward to form towering thunderstorms. Around the low pressure disturbance is a wall of clouds called an eyewall. Within the eyewall, the wind speeds are greatest, the clouds are the tallest, atmospheric pressure is at its lowest, and precipitation is most intense.
At the center or heart of the hurricane is called the eye. Within the eye of a hurricane, winds are light, precipitation is minimal, and occasionally the skies above are clear. It is the calm region of the tropical storm, but that is what makes it so dangerous. Many people tend to go outside as the eye moves overhead because they believe the storm is over. But what some don’t realize is that “round two” is coming from behind. Moving away from the eyewall are organized, intense thunderstorms, called spiral rain bands, that rotate around and toward the storm’s eyewall.
Hurricanes are assigned to categories based on their wind speed. The categories are listed on the Saffir-Simpson Scale.
| Category | Wind Speed (mph) | Estimated Damage |
|---|---|---|
| 1 (Weak) | 74–95 | Some damage: Above normal; no real damage to structures |
| 2 (Moderate) | 96–110 | Extensive damage: Some property damage; considerable damage to vegetation |
| 3 (Strong) | 111–130 | Devastating damage: Some buildings damaged; mobile homes destroyed |
| 4 (Very strong) | 131–156 | Catastrophic damage: Complete roof failure on small residences; major beach erosion of beach areas |
| 5 (Devastating) | Over 156 | Catastrophic damage: Complete roof and some building failure on most residential and industrial buildings |
Hurricanes move with the prevailing winds. In the Northern Hemisphere, they originate in the trade winds and move to the west. When they reach the latitude of the westerlies, they switch direction and travel toward the north or northeast. Hurricanes may cover 800 km (500 miles) in one day. Damage from hurricanes comes from the high winds, rainfall, and storm surge. Storm surge occurs as the storm’s low pressure center comes onto land, causing the sea level to rise unusually high. A storm surge is often made worse by the hurricane’s high winds blowing seawater across the ocean onto the shoreline. Flooding can be devastating, especially along low-lying coastlines such as the Atlantic and Gulf Coasts. Hurricane Michael in 2018 had peak winds of 260 km per hour (160 mph) and storm surges up to 4.3 m (14 ft).
Hurricanes typically last for 5 to 10 days. Over cooler water or land, the hurricane’s latent heat source shut downs and the storm weakens. When a hurricane disintegrates, it is replaced with intense rains and tornadoes.
There are about 100 hurricanes around the world each year, plus many smaller tropical storms and tropical depressions. As people develop coastal regions, property damage from storms continues to rise. However, scientists are becoming better at predicting the paths of these storms and fatalities are decreasing. There is, however, one major exception to the previous statement: Hurricane Sandy.
For over a decade, the conversation in the United States over hurricanes was directed toward Hurricane Katrina. But in late October 2012, a Category 3 hurricane called Hurricane Sandy, sometimes called Superstorm Sandy, struck the eastern coast of the United States. It is estimated that the storm system caused $68 billion in damage and killed nearly 300 people.
What made Hurricane Sandy different from most tropical storms was how powerful it was so late in the hurricane season. The concern is that as the planet warms from anthropogenic (human-enhanced) climate change, the oceans will likely warm up, allowing hurricanes to become stronger, travel farther toward the poles, and the hurricane season last longer.
References
- Dynamic Earth: Introduction to Physical Geography. Authored by: R. Adam Dastrup. Located at: http://www.opengeography.org/physical-geography.html. Project: Open Geography Education. License: CC BY-SA: Attribution-ShareAlike
- Anatomy of a hurricane. Authored by: Kelvinsong. Located at: https://commons.wikimedia.org/wiki/F...rricane-en.svg. License: CC BY 3.0
- Cyclone map. Provided by: NASA. Located at: https://gpm.nasa.gov/education/articles/how-do-hurricanes-form. License: Public Domain: No Known Copyright
- Nor'easter formation. Provided by: NOAA. Located at: https://scijinks.gov/noreaster/. License: Public Domain: No Known Copyright
- Low pressure system over Iceland. Provided by: NASA. Located at: https://en.Wikipedia.org/wiki/File:Low_pressure_system_over_Iceland.jpg. License: Public Domain: No Known Copyright
- Mid-Latitude Cyclone over the United States. Provided by: NASA. Located at: https://earthobservatory.nasa.gov/NaturalHazards/view.php?id=52297. License: Public Domain: No Known Copyright
- Hurricane Floyd. Provided by: NOAA. Located at: https://commons.wikimedia.org/wiki/File:Hurricane_Floyd_1999-09-14.jpg. License: Public Domain: No Known Copyright
- Heat Index. Provided by: NOAA. Located at: http://www.srh.noaa.gov/jetstream/global/hi.html. License: Public Domain: No Known Copyright