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Tropical Cyclones

What is a Tropical Cyclone?

A tropical cyclone is a large and powerful low pressure area rotating counterclockwise in the northern hemisphere, clockwise in the southern hemisphere, and containing rising warm air, that forms over warm water.  A tropical cyclone can be up to six miles high, and hundreds of miles wide.  They generally travel at ten to fifteen miles per hour but can travel as fast as 40 miles per hour.  Less powerful tropical cyclones are called Tropical Storms and even weaker storms are termed Tropical Depressions.  Almost all tropical cyclones form in a band from 10 to 30 degrees on either side of the equator in an area termed the Inter tropical Discontinuity or the Inter tropical Convergence Zone, in an area of high humidity where the seawater temperature is at least 80 degrees Fahrenheit down to a depth of at least 150 feet, or 50 meters.  In these areas an existing disturbed weather system, frequently a tropical wave, can be influenced by the Coriolis Effect, caused by the earth’s rotation, which leads to cyclonic rotation.  As the storm system continues to rotate it sucks up vast amounts of humidity which rises into the atmosphere then condenses and releases latent heat.  A small percentage of this released heat is converted into mechanical energy which leads to increased wind speeds.  These increased wind speeds in turn allow the storm to pull up even more warm moist air and a self perpetuating positive feedback loop is formed with ever-increasing wind speeds.  The storm will continue to progress until its system is destabilized by moving over an area where it cannot continue to draw up warm moist air, such as an area of cooler water temperature or a land mass, or disrupted by strong winds from another weather system.

These storms are called hurricanes when they form in the Atlantic Ocean, and tropical cyclones in the Indian Ocean and Bay of Bengal.  Other regional names for these storms are typhoons, severe tropical cyclones, and severe cyclonic storms.  In this article the terms tropical cyclone and typhoon and hurricane are used interchangeably.  Tropical cyclones can inflict terrible damage due to thunderstorms and tornadoes, violent winds, torrential rain with accompanying flooding and landslides, incredible waves, and extremely high tides associated with tidal surges.  Extremely strong winds can reach out as little as 25 miles from the center of a small tropical cyclone and as far as 150 miles from the center of a large tropical cyclone.  Less-strong but still damaging winds can extend as far as 300 miles from the center of a large tropical cyclone.  These are very dangerous storms. 

Tropical Cyclones in Bangladesh

Tropical cyclones generally strike Bangladesh in two seasons, March through July and September through December, with the greatest majority of storms arriving in May and October.

Typhoon Strength and Speed

In the United States, hurricanes are tracked by the National Hurricane Center, part of the National Weather Service, which is itself part of the National Oceanic and Atmospheric Administration (NOAA).  In the Pacific Ocean area, the Joint Typhoon Warning Center, based in Hawaii, provides typhoon tracking services.  There are very slight differences in the terminology used by these two services.  In Bangladesh tracking is based on multiple sources including the World Meteorological Organization.

There are certain differences in the ways tropical cyclones and hurricanes are measured in the United States and other countries, in particular how the storms’ strength and speed are measured.

In the United States, these storms’ sustained wind speeds are measured in statute miles per hour, while in Bangladesh the typhoon’s speed is measured in kilometers per hour and the sustained winds within the storm are measured in meters per second.  The conversion is:  1 knot (kt) = 1.15 mile per hour (mph) = 1.85 kilometers per hour (km/h) = 0.514 meters per second (m/s).

Second, the time period over which maximum sustained wind speed is measured is different:  in the United States the wind speed is measured over a one-minute period at a height of 10 meters above the earth’s surface, in most other countries it is measured over a three-minute period and in a few countries over a ten-minute period.  In practice, this means that an identical storm would show lower wind speeds in other countries and higher wind speeds in the United States.  Put differently, to an American this means that a tropical cyclone in other countries with winds of a given speed will be more destructive than would be expected for a storm of that size.  Note that gusts of wind are not factored into the sustained wind speed, and these gusts’ speeds can be 20% or even higher than the speed of the sustained winds.

Third, in other countries, the air pressure of the eye of the storm is measured in hectoPascals (one hectoPascal [hPa] = 1 millibar [mb]) while in the United States it is measured in inches of mercury (1 inch of mercury = 25.4 mm of mercury = 33.86 millibars = 33.86 hectoPascals).  In general, the lower the air pressure in the eye of the storm, the more powerful the storm.

Because of these differences, we have placed each system on a separate chart to reduce confusion.  We recommend that viewers refer to all charts.  The chart labeled United States System of Hurricane Classification is an amalgamation of the National Hurricane Center, Joint Typhoon Warning Center, and other classification systems. The Joint Typhoon Warning center updates its information every six hours, while the information in Bangladesh is updated more frequently as a storm approaches.

Bangladesh System of Tropical Cyclone Classification

Tropical Cyclone Classification Maximum winds
Depression Up to 62 kph [33 knots or 38 mph]
Cyclonic Storm 63 to 87 kph [34 to 47 knots or 38 to 54 mph]
Severe Cyclonic Storm 88 to 117 kph [48 to 63 knots or 55 to 73 mph]
Severe Cyclonic Storm of Hurricane Intensity Over 118 kph [64 knots or 74 mph]

Bangladesh also uses a 1 to 10 scale to classify tropical cyclones with 10 being the most severe.

Alert Stage:  Signal No. I, II, and III

Warning Stage:  Signal No. IV

Disaster Stage:  Signal No. V, VI, VII and VIII, IX, and X

India System of Tropical Cyclone Classification

Category Sustained winds –knots, 3-minute average Sustained winds – kilometers per hour, 3-minute average
Super Cyclonic Storm Over 120 knots Over 222 kph
Very Severe Cyclonic Storm 64 to 119 knots 118 to 221 kph
Severe Cyclonic Storm 48 to 63 knots 88 to 117 kph
Cyclonic Storm 34 to 47 knots 62 to 87 kph
Deep Depression 28 to 33 knots 52 to 61 kph
Depression Under 27 knots Under 51 kph


United States System of Hurricane Classification

Description Category Central Pressure Mb  Inches Wind Speed Miles per Hr Knots Wind Speed KPH Meters/Sec Storm Surge Feet Meters
Tropical Depression 38




Tropical Storm 39 to 72 mph

34 to 63 kt

63 to 117 kph

17.5 to 33 m/s

1 Under 980 mb
Under 28.84″
74 to 95 mph  64 to 82 kt 118 to 153 kph

33 to 42 m/s

4 to5 ‘

1.5 m

2 965 to 979 mb
28.5 to 28.91″
96 to 110 mph   83 to 95 kt 154 to 177 kph

43 to 49 m/s

6 to 8 ‘

2 to 2.5 m

3 945 to 964 mb

27.91 to 28.47″

111 to 130 mph

96 to 113 kt

178 to 209 kph

49 to 58 m/s

9 to 12 ‘

2.5 to 4 m

4 920 to 944 mb

27.17 to 27.88″

131 to 155 mph

114 to 135 kt

210 to 249 kph

59 to 69 m/s

13 to 18 ‘

4 to 5.5 m

Super Typhoon Over 150 mph

Over 130 kt

Over 241 kph

Over 65 m/s

5 944 to 920 mb

27.88 to 27.17″

Over 155 mph

Over 135 kt

Over 250 kph

Over 70 m/s

Over 18′

> 5.5 m

Also, if you access the Hong Kong Observatory’s web site, you will find another classification system for tropical storms.  The system is based on the World Meteorological Organization with the nomenclature slightly different from nomenclature used in Bangladesh.  That system is listed below, with additional information on wind speeds in knots and miles hour in brackets.

Hong Kong Observatory System

Tropical Cyclone Classification Maximum winds (10-minute mean)
Tropical Depression Up to 62 kph [33 knots or 38 mph]
Tropical Storm 63 to 87 kph [34 to 47 knots or 38 to 54 mph]
Severe Tropical Storm 88 to 117 kph [48 to 63 knots or 55 to 73 mph]
Typhoon Over 118 kph [64 knots or 74 mph]

The US military uses a scale to indicate the urgency of making preparations for a typhoon.  If you receive information on typhoons from the US military you may encounter the terms noted below.  Note that these conditions do not indicate the relative wind speeds and destructive capability of the storm (such as the Category 1, 2, etc. Saffir-Simpson Hurricane Scale system used in the United States), but rather how soon the storm is expected to arrive and the general category, damaging or destructive, of the winds.  Individual commands have established local procedures to be implemented with each condition of readiness, which may affect schools, availability of services, and working hours.

Tropical Cyclone Conditions of Readiness (TCCOR)

TCCOR 4                 Destructive winds of 50 knots (58 mph) or greater expected within 72 hours;

TCCOR 3                 Destructive winds of 50 knots (58 mph) or greater expected within 48 hours;

TCCOR 2                 Destructive winds of 50 knots (58 mph) or greater expected within 24 hours;

TCCOR 1                 Destructive winds of 50 knots (58 mph) or greater expected within 12 hours;

TCCOR 1C              CAUTION:  Destructive winds of 50 knots (58 mph) or greater expected within 12 hours, damaging winds of 34 to 49 knots (39 to 56 mph) occurring on station at present;

TCCOR 1E              EMERGENCY:  Destructive winds of 50 knots (58 mph) or greater occurring on station;

TCCOR 1 R             RECOVERY:  Destructive winds have moved off station, but are still between 35 and 49 knots (39 to 56 mph);

ALL CLEAR:          Winds of 35 knots (39 mph) or greater have moved off station.

Other Terms and Systems

Depending on what source of information you use to keep informed about an approaching typhoon, you might encounter a few additional terms used to describe these storms:

Destructive Winds.  A term used by the US Military to describe winds of 50 knots (58 mph) or greater, sustained or gusts.  Extensive damage to structures and facilities is likely.

Intense Hurricane.   Same as Major Hurricane, below.

Major Hurricane.  A term used by the National Hurricane Center to describe hurricanes that reach wind speeds of 50 m/s (96 knots or 111 mph).  This is equivalent of a Category 3, 4 or 5 hurricane.

Saffir-Simpson Hurricane Scale.  A 1 to 5 rating based on a hurricane or typhoon’s intensity.  It is used to give an estimate of the potential property damage and flooding expected along a coast from a hurricane’s landfall.

Super Typhoon.  A term used by the Joint Typhoon Warning Center to describe typhoons that reach wind speeds of at least 65 m/s (130 knots or 150 mph).  This is equivalent to a Category 4 or 5 hurricane.

Typhoon Damage

Typhoons can do great damage.  In the United States, hurricanes are classified on the Saffir-Simpson Hurricane Scale from 1, the least powerful, to 5, the most powerful.  The categories and descriptions below are from NOAA.

Category 1.  Winds of 74 to 95 mph.  Storm surge (see information below) generally 4-5 feet above normal.  No real damage to building structures.  Damage primarily to unanchored mobil homes, shrubbery, and trees.  Some damage to poorly constructed signs.  Some coastal road flooding and minor pier damage.

Category 2.  Winds of 96 to 110 mph.  Storm surge generally 6-8 feet above normal.  Some roofing material, door, and window damage to buildings.  Considerable damage to shrubbery and trees with some trees knocked down.  Considerable damage to mobile homes, poorly constructed signs, and piers.  Coastal and low-lying escape routes flood 2-4 hours before arrival of the hurricane center.  Small craft in unprotected anchorages break moorings.

Category 3.  Winds of 111 to 130 mph.  Storm surge generally 9-12 feet above normal.  Some structural damage to small residences and utility buildings with a minor amount of curtainwall failures.  Damage to shrubbery and trees with foliage blown off trees and large trees blown down.  Mobile homes and poorly constructed signs are destroyed.  Low-lying escape routes are cut by rising water 3-5 hours before arrival of the center of the hurricane.  Flooding near the coast destroys smaller structures with larger structures damaged by battering from floating debris.  Terrain continuously lower than 5 feet above mean sea level may be flooded inland 8 miles (13 km) or more.  Evacuation of low-lying residences within several blocks of the shoreline may be required.

Category 4.  Winds of 131 to 155 mph.  Storm surge generally 13-18 feet above normal.  More extensive curtainwall failures with some complete roof structure failures on small residences.  Shrubs, trees, and all signs are blown down.  Complete destruction of mobile homes.  Extensive damage to doors and windows.  Low-lying escape routes may be cut by rising water 3-5 hours before arrival of the center of the hurricane.  Major damage to lower floors of structures near the shore.  Terrain lower than 10 feet above sea level may be flooded requiring massive evacuation of residential areas as far inland as 6 miles (10 km).

Category 5.  Winds of over 155 mph.  Storm surge generally greater than 18 feet above normal.  Complete roof failure on many residences and industrial buildings.  Some complete building failures with small utility buildings blown over or away.  All shrubs, trees, and signs blown down.  Complete destruction of mobile homes.  Severe and extensive window and door damage.  Low-lying escape routes are cut by rising water 3-5 hours before the arrival of the center of the hurricane.  Major damage to lower floors of all structures less than 15 feet above sea level and within 500 yards of the shoreline.  Massive evacuation of residential areas on low ground within 5-10 miles (8-16 km) of the shoreline may be required.

NOAA estimates that damage from a hurricane increases exponentially as the category increases.  The damage of a Category 2 hurricane is estimated at 10 times that of a Category 1 hurricane, a Category 3 hurricane 40 times, a Category 4 hurricane at 250 times, and a Category 5 hurricane is estimated to produce 500 times the damage of a Category 1 hurricane.

Storm Surges

Part of the danger of a tropical cyclone or hurricane comes from the storm surge, and it has been estimated that up to 90% of all deaths associated with a typhoon or hurricane are due to the storm surge.  A storm surge is a large dome of water, 50 to 100 miles (80 to 160 kilometers) or more wide, that is caused by a combination of the hurricane’s winds at the sea surface which physically pushes the water into a mound, the low pressure associated with the storm (water rises approximately 1 cm per 1 Millibar of a drop in pressure), and the physical formation of a coastline.  If the coastline consists of shallow water the dome of water cannot flow away from the center of the mound and the mass of water continues to grow larger.  If the coastline consists of deep water the mound of water can disperse and in these areas storm surges are much less destructive.

In Bangladesh a combination of high tides (tidal ranges in Bangladesh are approximately 3 meters in western Bangladesh to 5 meters in eastern Bangladesh), a low and flat terrain in the coastal region, and the funnel shape of the Bay of Bengal magnifies the effect of the storm surge.  Storm surges are generally located at the right hand side of the storm as viewed from above.  A storm surge can be over 18 feet deep, and can reach up to 25 miles inland.  If the tide is high at the time a storm surge arrives, the height of the tide has to be added to the height of the storm surge to determine the height of the storm tide.  Storm tides can be exceptionally high: a 24 foot high storm tide was observed in Florida in 1995 during Hurricane Opal.  Predicting the height of storm surges is very difficult because of numerous factors involved, and even the most complex estimates of the height of a surge can be off by a factor of plus or minus 20% of the actual surge height.  Since the weight of seawater is close to one ton, approximately 1,730 pounds, per cubic yard (the weight of seawater can also be expressed as 1,027 kilograms or 2,260 pounds per cubic meter), the force of pounding waves and flooding can destroy or cause extensive damage to anything in the surge’s path.  The danger of flooding caused by storm surges should not be underestimated.  Storm surges cannot be prevented.  Your only option is to seek higher ground or locate one of many cyclone shelters found in Bangladesh.

Heavy rains, too, contribute to the total tropical cyclone damage.  In addition to flooding caused by the rain, landslides of saturated earth can also occur.  These landslides are of particular concern after rain has fallen at a rate of 20 mm or more per hour or when 100 mm of rain falls nonstop. 

Major Typhoons in Bangladesh

Typhoons are very destructive to property and in terms of lives lost.  Bangladesh has experienced both.  The following recent tropical cyclones are notable for their loss of human life in Bangladesh.  The number of deaths in each storm vary by reporter, and the numbers listed are approximate and may not be accurate.  What we should remember is that these storms are dangerous.

Location Date Number of Deaths
Feni-Chittagong coast October 26, 1962 50,000
Barisal-Chittagong coast May 10, 1965 20,000
Khulna-Chittagong coast November 12, 1970 300,000
Patuakhali-Cox’s Bazar coast April 29, 1991 150,000


Staying Informed Tropical Cyclone Nargis, May 8, 2008, led to approximately 3,500 deaths in Bangladesh, but the Government of Bangladesh, partner nations, and non-governmental organizations have worked hard on disaster preparedness.  This does not mean that we do not need to be cautious in dealing with these dangerous storms, whether we are in our homes, enjoying the scenery of Bangladesh, or at sea for work or pleasure.

Local media generally has good coverage of approaching tropical storms.  Television stations broadcast tracks of typhoons and show information on the typhoon’s intensity and speed.  This information should be understandable even by those who do not speak or read Bangla.  There is also an abundance of excellent resources on the Internet; try a search under tropical cyclone and choose from a multitude of sites.  Below are five that we find useful for tracking storms:

Hurricane Season – Know Before You Go

General Information – Natural Disasters

Pacific Disaster Center – Global Hazards Atlas

India Meteorological Department – Cyclone Warnings

U.S. Navy Joint Typhoon Warning Center

Weather Underground – Bangladesh

World Meteorological Organization – Severe Weather Information Centre

Bangladesh Meteorological Department
  (This site may not work using Microsoft Internet Explorer)