Hurricanes

UPDATED: 14 November 2023: clarified extent of Atlantic hurricane season

UPDATED: 15 February 2022: added definition of the development stage invest; update on Saffir-Simpson scale

POSTED: 10 June 2020

• Hurricanes

• hurricanes are strictly tropical cyclones (i.e. need tropical waters to feed)
• hurricanes are 100 - 1500km across
• hurricanes travel at 0-60km/h and are classified into 5 categories, according to wind speeds and lowest pressure (Saffir-Simpson scale, see below)
• pushed by trade winds, hurricanes initially move from east to west. They then get diverted by the Coriolis effect, large-scale high-pressure systems and other features in the atmosphere.
• hurricanes cannot cross the equator.
  They would need to go from a cyclone to an anti-cyclone, if the wind direction were to remain, or go from counterclockwise to clockwise winds, if the low-pressure system were to remain. Both are physically not possible.
• hurricanes cannot cross continents.
  They quickly dissipate once they made landfall. Remnants of hurricanes can continue to bring intense rainfall further inland (see 2008 Hurricane Ike). Sometimes, Atlantic hurricanes can cross the narrow lands of Central America and become Pacific hurricanes and vice versa, the latter occurring less often. If this happens, then the storms get renamed, taking on a name of the predetermined list for that ocean, for example Atlantic Hurricane Cesar became Pacific Hurricane Douglas in 1996.


• hurricane season: during early summer through early fall, typically June - October in the northern hemisphere; the most severe hurricanes typically occur later in the season (after summer heat warmed ocean surface water)
• Atlantic hurricane season: recently (2022??), NOAA declared the Atlantic hurricane season to be June - November. A Google search yields September 10 as the peak of the Atlantic hurricane season, according to a statement found on the NOAA website. Giving a specific date is somewhat silly and should be understood only as a statistical estimate as ocean conditions vary vastly from year to year

Hurricanes are destructive due to
• intensive rain fall
• high wind speeds
• large waves in the open ocean
• storm surge approaching the coast

NB: Over 90% of hurricane deaths are caused by the storm surge ¹.



• Hurricane Surges

Storm surges are particularly high surf conditions that are associated with great storms.
Factors that contribute to a high storm surge:
• high winds push waves to pile up; particularly on the right side of a storm where winds speeds are highest
• low pressure in the storm pulls sea surface up (by up to 1m)
• high tides; particularly strong during full moon and new moon (spring tides)

NB: In the northern hemisphere, the surge is particularly high on the right side of the eye (see winds speeds for an explanation).




• The Features of a Hurricane

A hurricane feeds itself by initial evaporation of warm ocean water and absorbing large amounts of latent heat, in a low pressure system. Due to its great latent heat, the air can rise to great heights. The heat gets released during condensation which can make the air rise still higher, creating an even lower pressure. This process continues until the hurricane does no longer cross over tropical waters.
a hurricane has:
• eye:area of calm weather in the center, dry air descends; 35 - 60 km across; well-developed in stronger storms
• eye wall: area of the strongest (rotary) winds
• rain bands: several spiral arms outside of the eye wall; windspeeds increase toward center; (strong) thunderstorms can form in the inner rain bands

DID YOU KNOW? 25% of all hurricanes spawn tornadoes 1.

• Conditions for a Hurricane to Form

• form only in tropical oceans (latitudes less than 20°); storm feeds itself through release of latent heat from tropical ocean water
• need weak winds aloft (low high-altitude winds) so that strong low-pressure system can build up

Hurricanes weaken and start to dissipate when they encounter land or colder oceans (e.g. off Southern California) or land.

In rare circumstances, hurricanes can feed existing extratropical storms to make those more powerful (e.g. to fuel Nor'Easters) or to reach farther inland (e.g. 2008 Hurricane Ike whose impact could be felt as far as Canada).

Sometimes, hurricanes have enough energy to cross over land-bridges/isthmuses into another ocean (e.g. 1997 East Pacific Hurricane Linda, the most powerful hurricane in the East Pacific, started out as a tropical wave off the coast of Africa). But in general, hurricanes cannot reach far into a continent.


• Developing Stages of a Hurricane

Invest: a developing weather system for which a tropical cyclone forecast center (e.g. the National Hurricane Center) is interested in collecting data and watch its development. There is no particular likelihood that such a storm would actually develop into a tropical storm or even into a hurricane.


• tropical disturbance: development of a low-pressure zone; weak surface winds; 150-550 km across; maintained for 24 h or more
• tropical depression: gets identifying number (e.g. TD nine); development of a cyclone; 1-minute average wind speeds 37 - 63 km/h
• tropical storm: isobars are packed together; 1-minute average surface wind speeds 64 - 118 km/h
• hurricane: 5 categories; winds speeds greater than 119 km/h

• The Saffir-Simpson Scale

The Saffir-Simpson scale casts the hurricanes into 5 categories (1-5). It has been introduced and used by the National Hurricane Center (NHC) since 1974. For a long time, the main criteria were the highest sustained wind speeds (in the eyewall) and the lowest air pressure in the eye. Larger hurricanes typically also have higher storm surges and are more destructive where they encounter land. Windspeed, central low pressure and storm surge were used in the Saffir-Simpson scale until 2009.

Starting in 2010, the NHC started using only the wind speed as criterion to categorize hurricanes. Over the years, small changes have been made to the wind speed limits in each category. The table below includes the windspeeds in use as of 2021, and the central surface air pressure and storm surges as used until 2009.

Category	Descriptor	Sustained Wind Speed (mph)	Sustained Wind Speed (km/h)	Air Pressure in eye (mbar)	Storm Surge height (m)	Typical Damage
1	minimal	74-95	119-153	980 or more	1.2-1.5	branches broken; unanchored mobiles homes damaged; some flooding of coastal areas; no damage to buildings
2	moderate	96-110	154-177	965-979	1.6-2.4	some roofs, doors, and windows damaged; mobile homes seriously damaged; some trees blown down; small boat moorings broken
3	extensive	111-129	178-208	945-964	2.5-3.6	some structural damage to small buildings; large trees blown down; mobile homes destroyed; structures along coastal areas destroyed by flooding and battering
4	extreme	130-156	209-251	920-944	3.7-5.4	some roofs completely destroyed; extensive window and door damage; major damage and flooding along coast; widespread evacuation of regions within up to 10km of the coast may be necessary
5	catastrophic	over 156	over 251	less than 920	greater than 5.4	many roofs and buildings completely destroyed; extensive flooding; widespread evacuation of regions within up to 16km of the coast may be necessary



NB: some hurricanes are difficult to classify. An example was Hurricane Ike in 2008. It has the wind speeds and low pressure of a category 2 but was much larger for its category. The large associated wind field (its large diameter) made more act like a category 4, i.e. its storm surge was that of a category 4. The IKE (integrated kinetic energy) scale, that goes from 1 to 6, presents a measure of storm surge destructive potential and may be a more accurate way to predict imminent coastal flooding. On this scale, Hurricane Ike was a 5.6 and had the highest destructive potential of all Atlantic hurricanes recorded so far (see also Wikipedia).
Some scientists suggested in 2005 to add another category (6) for storms with wind speeds greater than 280 or 290 km/h (174 or 180 mph). New suggestions were made after Hurricane Irma in 2017. In a newspaper article in 2018, a NOAA scientist even suggested to start category 6 for windspeeds ≥ 314 km/h (≥ 195 mph) have a category 7 (wind speeds ≥ 370 km/h; ≥ 230 mph).

• Wind Speeds in a Hurricane

locally, the wind speed is a combination of the hurricane moving as a whole and the wind speeds in the rainbands and eyewall. The strongest winds are found where both point in the same direction and the weakest are found where they blow in opposite directions. Since the winds blow counterclockwise in the northern hemisphere, the winds are:
• strongest on the right of the eye
• weakest on the left of the eye
• intermediate near the front and back ends of the storm
in the southern hemisphere, winds blow clockwise and the strongest winds are found to the left of the eye.

• Where do Hurricanes occur? - Naming Convention

Hurricanes are named by location. A hurricane changes its name, when a hurricane crosses from one region into another.



Name	Area of Occurrence	Rate of Occurrence (*)
hurricane	Atlantic	12%, northern hemisphere only
hurricane	Eastern Pacific	15%, northern hemisphere only
typhoon	Western Pacific	30% northern hemisphere, 12% southern
cyclone	Indian Ocean	12% west, northern hemisphere, 12% west southern; 7% east southern

(*)in terms of fraction of all hurricanes occurring worldwide.

The international dateline, which is approximately aligned with the 180° longitude, is the boundary between the Eastern and Western Pacific.

An example for a storm that got renamed was the category 5 Hurricane Ioke, which formed in the Central Pacific ocean in late August 2006 (the first ever cat 5 hurricane to form there). When it crossed the dateline, it became Typhoon Ioke. Had Ioke been in the Western Pacific at the time when it was a category 5, it would have been called a Super-Typhoon.

The western Pacific Ocean has the most and strongest hurricanes.

There are no hurricanes in the southeastern Pacific and the southeastern Atlantic. Hurricanes in the Eastern Pacific traveling north along the West Coast of Central America also usually dissipate when they pass Baja California. The reason for this is that eastern boundary currents in these oceans are usually cold (i.e. the Humboldt Current off South America, the Benguela Current off Southwestern Africa and the California Current). Hurricanes can't form and feed in such waters.

There are usually also no hurricanes in the southwestern Atlantic because wind conditions are unfavorable (too much wind shear). The only hurricane to make landfall in the South Atlantic was Hurricane Catarina in March 2004 off Brazil. The hurricane made landfall as a category 1 system (check out at earth observatory). The paths of hurricanes are controlled by the Coriolis effect and on where high pressure systems are located and how strong they are. In the northern hemisphere, hurricanes tend to take on a clockwise path (deflected to the right). In the southern hemisphere, hurricane (mostly cyclones actually) tend to take on a counterclockwise path (deflected to the left).

• Hurricanes in the North Atlantic Ocean

• In the Atlantic Ocean, hurricanes often form as storms in the Sahel zone south of the Sahara Desert. Pushed by the trade winds, these travel west as tropical waves. When encountering tropical waters, these storms can grow quickly to tropical storm strength by the time they reach the Cape Verde Islands (Cape Verde-type tropical storm). They then move west across the Atlantic, gaining strength by sucking up warm, tropical Atlantic Ocean water.
• Tropical storms can also form in the Caribbean Sea and Gulf of Mexico (Caribbean Sea and Gulf of Mexico-type tropical storm). During summer, the ITCZ is sufficiently shifted toward the north so that tropical air from the northeast and southeast can converge over the Caribbean, forming a low-pressure system.
• On average, there are 5 Atlantic hurricanes every year.



• The Path of Atlantic Hurricanes

The path of Atlantic hurricanes is controlled by:
• trade winds that blow west
• Coriolis effect that gives the path a curvature to the right
• size and position of the Bermuda High, a warm and dry high pressure system that sits over the North Atlantic over much of the year; it is called Bermuda High in the U.S. and Azores High in Europe
• if the high is small and to the north, hurricanes turn north early and may not affect the U.S. coast
• a large high directs hurricanes along the U.S. east coast
• if the high is located further south, hurricanes approach the Caribbean and Gulf of Mexico



• Atlantic Hurricanes and El Niño

• less hurricanes in El Niño years:
• the western Sahel region tends to be drier
• the Central Atlantic tends to be cooler
• east-blowing high-altitude wind tend to disrupt forming hurricanes
• weak trade winds can't push hurricanes across ocean
• more hurricanes when El Niño is absent:
• the western Sahel region tends to be wetter
• the Central Atlantic tends to be warmer
• strong trade winds blow growing hurricanes across ocean
• El Niños and the East Pacific
• Some argue that hurricanes are less likely also in the Eastern Pacific during El Niño years.
• However, category 5 Hurricane Linda, the strongest hurricane in the East Pacific, formed in 1997, a strong El Niño year.
  Category 4 Hurricane Pauline a few weeks later made landfall along the Mexican west coast and killed 230 people.
  The season of 2006, which was accompanied by a weak El Niño, was quite productive in the number of storms, with many more storms than in the Atlantic.



• The Naming of Hurricanes

Since 1953, Atlantic tropical storms have been named from lists originated by the National Hurricane Center. Similar lists exist for other oceans as well. They are maintained and updated by an international committee of the World Meteorological Organization. The original name lists featured only women's names. Since 1979, men's and women's names alternate. Six lists are currently used in rotation (e.g. the 2008 list will be used again in 2014).
In the past, if there were more storms than names in a given season, they were given names from the Greek alphabet, starting with Alpha. This happened in the 2005 and the 2020 hurricane season. In 2021 (which also ran out of names), the use of the Greek alphabet was terminated, and naming started over using the regular alphabet.
See NOAA website for details.

The name of a particularly deadly or destructive hurricane gets retired and replaces with a new one e.g. 2005 Hurricane Katrina's name was replaced by Katia for the 2011 season. Some of the names retired in recent years for Atlantic Ocean, Caribbean Sea, Gulf of Mexico (from NOAA):


• 1998: Georges, Mitch
• 1999: Floyd, Lenny
• 2000: Keith
• 2001: Allison, Iris, Michelle
• 2002: Isidore, Lili
• 2003: Fabian, Isabel, Juan
• 2004: Charley, Frances, Ivan, Jeanne
• 2005: Dennis, Katrina, Rita, Stan, Wilma
• 2006: -
• 2007: Dean, Felix, Noel
• 2008: Gustav, Ike, Paloma
• 2009: -
• 2010: Igor, Thomas
• 2011: Irene
• 2012: Sandy
• 2013: Ingrid
• 2014: -
• 2015: Erika, Joaquin
• 2016: Matthew, Otto
• 2017: Harvey, Irma, Maria, Nate
• 2018: Florence, Michael
• 2019: Dorian
• 2020: Laura, Eta, Iota

• 1997 Hurricane Linda: Hurricanes close to home

In 1997, Hurricane Linda formed to become the strongest ever recorded in the east Pacific. Linda actually started as a tropical wave off the coast of West Africa and traveled undeveloped across the Atlantic. It crossed Central America where convection increased and fueled the storm.
• Sep 9: tropical disturbance (early UTC)
• Sep 9: became tropical depression (1200 UTC)
• Sep 10: became Tropical Storm Linda (0000 UTC)
• Sep 11: became category 1 Hurricane Linda (0000 UTC); developed from depression into hurricane in only 36 h
• Sep 12: reached peak intensity with 160 knt winds and 902 mb minimum central pressure at 0600 UTC; went from category 1 to 5 in only 30 h
• Sep 17: dissipated far west of Baja California
As Linda was getting stronger, predictions at some point hinted a landfall on Baja California and it was considered whether this hurricane could move up all the way the Southern California even though this would be very unusual (see below).

some more numbers:
• maximum sustained winds: 185 mph
• gusts up to 220 mph
• lowest central pressure: 902 mb
• no landfall but hit Socorro Island
• no fatalities but $ 3.2 Mio damages
some short-distances and long-distance effects:
• cut power to meteorological station on Socorro Island
• closed resorts in Baja California
• flooding in mainland Mexico
• 15 ft storm surge in Southern California
• 5 people swept off a jetty in Newport Beach
• severe flooding and mudslides in Southern California; destroyed 2 homes and damaged 77

• Hurricane History in Southern California

• no hurricane ever made landfall in Southern California
• landfall of a tropical storm in 1939; $ 2 Mio damages
• some hurricanes approached but made landfall somewhere else
however, points to consider for SoCal
• meteorological records do not go back very far
• a recent graduate study at SIO revealed that there was indeed a San Diego Hurricane
  • newspaper accounts from Northern California and information from ship logs
  • only little information from sparsely populated San Diego (1860 population: 4325)
  • 02 October 1858 hurricane made landfall near San Diego
  • reports of ships blown ashore
  • a house completely destroyed
  • from estimated wind speeds and pressury -> category 1/F2 winds
  • estimated damages if such a hurricane struck today: $ 100 Mio
• reason why hurricanes currently make no landfalls in SoCal: the cold California Current from the north provide water too cold to sustain a hurricane; so a north-moving hurricane dissipates before making landfall
• the worry: global warming could weaken the California Current, allowing east Pacific Hurricanes to migrate further north

• Barrier Islands and Coastal Erosion

Hurricanes and other great storms cause extensive damage to coastline. Stretches that are particularly vulnerable:
• barrier islands along Gulf of Mexico and East Coast
• exposed areas along northeaster part of East Coast
• northern part of West Coast (winter storms)
• other extremely vulnerable coasts include those below sea level (e.g. New Orleans and Bangladesh), that need to be protected by levees that can fail

Barrier Islands:
• natural sand bars that are deposited by longshore current
• lagoons form on landward side of bars
• coast is protected by barrier islands
• barrier islands often only a few meters high
• barrier islands are particularly vulnerable to storms and sea level rise
• sea level rises at about 1ft/century
• barrier islands are often developed for tourism
• the deadliest natural disaster to strike the U.S. was in 1900, when a hurricane made landfall near Galveston, TX, which is built on a barrier island; 6000 (some sources cite 8000) people got killed, when hurricane forecast did not yet exist;
• Galveston now protected by seawall, but threats still exist

• Coastal Protection

Several measures exist to protect costal areas and structures at the beach:
• sea walls: built to ward off waves; e.g. at Scripps Beach to protect Hubbs Hall
• rip-rap and boulders: structures made of concrete or natural rock placed either in undercuts to protect cliffs or around structures, e.g. piers. An example is found on Scripps Beach to protect the IGPP building that sits on the cliffs above. Rip-rap and boulders take up some of the wave energy that would otherwise be used by the waves to erode that targets.
• jetties: structures in the form of dams to protect passages into a harbor, e.g. at Mission Bay.
• breakwaters: built often parallel to the coastline to divert currents or approaching waves

• Is Protection a Good Thing?

Some measures, such as seawalls, are only temporary fixes:
• Often, the waves erode the beach at the base of the seawalls much faster than they would without the seawall; eventually the seawalls lose support and collapse
• sand in front of the seawalls is often transported further out into the ocean and deposited offshore instead of on the beach; the beach then loses its supply of sand
• at places where the beach sand comes from local cliff erosion and not from long-shore transport of sand from somewhere else, the beach is eroded quickly to never come back
• much of the U.S. East Coast is losing its beach sand which has to be replenished by expensive measures

NB: as many of the San Diego beaches, Scripps Beach, loses its beach sand in winter. During summer, sand is deposited as the longshore drift (driven by the California Current) brings sand from further north (e.g. San Diegito River). In winter, storms that come from the North along the coast bring winds and strong surf from the west that pull the beach sand offshore. In years with particularly strong storms, the beach can be stripped completely of its beach sand, exposing the rocks beneath. The last time this happened at Scripps Beach was during the El Niño winter 97/98.

• Recommended Reading:

• ¹Natural Disasters in General: Earthshock by Andrew Robinson, Thames and Hudson Publ., 1993, ISBN: 0-500-27738-9

1. in the eye
2. in the rainbands
3. in the eyewall
4. in the footx

1. Atlantic ocean
2. Indian Ocean
3. South Pacific
4. East Pacific
5. West Pacific

1. low high-altitude winds
2. lots of tropical ocean water
3. strong high-altitude winds
4. the jet stream stops blowing

1. true
2. false

1. Mexican hurricanes form in the southern hemisphere
2. no hurricanes ever form in the eastern Pacific
3. all eastern Pacific hurricanes travel southward
4. the cold California Current prevents hurricanes to sustain themselves