Frequently Asked Questions on Tropical Cyclones

What is a tropical cyclone?

  • A tropical cyclone is a rotational low pressure system in tropics when the central pressure falls by 5 to 6 hPa from the surrounding and maximum sustained wind speed reaches 34 knots (about 62 kmph). It is a vast violent whirl of 150 to 800 km, spiraling around a centre and progressing along the surface of the sea at a rate of 300 to 500 km a day.
  • The word cyclone has been derived from Greek word ‘cyclos’ which means ‘coiling of a snake’. The word cyclone was coined by Heary Piddington who worked as a Rapporteur in Kolkata during British rule. The terms “hurricane” and “typhoon” are region specific names for a strong “tropical cyclone”. Tropical cyclones are called “Hurricanes” over the Atlantic Ocean and “Typhoons” over the Pacific Ocean.


Why do 'tropical cyclones' winds rotate counter-clockwise (clockwise) in the Northern (Southern) Hemisphere?

  • As the earth’s rotation sets up an apparent force (called the Coriolis force) that pulls the winds to the right in the Northern Hemisphere (and to the left in the Southern Hemisphere). So, when a low pressure starts to form over north of the equator, the surface winds will flow inward trying to fill in the low and will be deflected to the right and a counter-clockwise rotation will be initiated. The opposite (a deflection to the left and a clockwise rotation) will occur south of the equator.
  • This Coriolis force is too tiny to effect rotation in, for example, water that is going down the drains of sinks and toilets. The rotation in those will be determined by the geometry of the container and the original motion of the water. Thus, one can find both clockwise and counter-clockwise flowing drains no matter what hemisphere you are located. If you don’t believe this, test it out for yourself.


What does maximum sustained wind mean ? How does it relate to gusts in tropical cyclones?

  • India Meteorological Department (IMD) uses a 3 minutes averaging for the sustained wind. The maximum sustained wind mentioned in the bulletins used by IMD is the highest 3 minutes surface wind occurring within the circulation of the system. These surface winds are observed (or, more often, estimated) at the standard meteorological height of 10 m (33 ft) in an unobstructed exposure (i.e., not blocked by buildings or trees).
  • The National Hurricane Centre uses a 1 minute averaging time for reporting the sustained wind. Some countries also use 10 minutes averaging time for this purpose. While one can utilize a simple ratio to convert from peak 10 minute wind to peak 1 minute wind or 3 minute wind, such systematic differences to make interbasin comparison of tropical cyclones around the world is problematic. However there is no significant difference between the maximum sustained wind reported in different basis with different averaging method.


What is the energy potential of a tropical cyclone?

  • Tropical Cyclone can be compared to a heat engine. The energy input is from warm water and humid air over tropical oceans. Release of heat is through condensation of water vapour to water droplets/rain. Only a small percentage (3%) of this released energy is converted into Kinetic energy to maintain cyclone circulation (windfield). A mature cyclone releases energy equivalent to that of 100 hydrogen bombs.

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What is a tropical cyclone?

  • The low pressure system over Indian region are classified based on the maximum sustained winds speed associated with the system and the pressure deficit/ number of closed isobars associated with the system. The pressure criteria is used, when the system is over land and wind criteria is used, when the system is over the sea. The system is called as low if there is one closed isobar in the interval of 2 hPa. It is called depression, if there are two closed isobars, a deep depression, if there are three closed isobars and cyclonic storm if there are four or more closed isobars. The detailed classification based on wind criteria are given in the Table below. Considering wind criteria, the system with wind speed of 17-27 knots is called as depression and the low pressure system with maximum sustained 3 minutes surface winds between 28-33 knots is called a deep depression. The system with maximum sustained 3 minutes surface winds of 34 knots or more is called as cyclonic storm
  • System

    Pressure
    deficient

    hPa

    Associated
    wind speed

    Knots
    (Kmph)

    Low pressure area

    1.0

    <17(<32)

    Depression

    1.0- 3.0

    17-27 (32–50)

    Deep Depression

    3.0 – 4.5

    28-33 (51–59)

    Cyclonic Storm

    4.5- 8.5

    34-47 (60-90)

    Severe Cyclonic Storm (SCS)

    8.5-15.5

    48-63 (90-119)

    Very Severe Cyclonic Storm

    15.5-65.6

    64-119 (119-220)

    Super Cyclonic Storm

    >65.6

    >119(>220)

Are all cyclonic storms equally dangerous?

  • No, all cyclonic storms are not equally dangerous. More the pressure drop at the central region more will be the severity of the storm. The cyclonic storms are generally categorised according to the maximum wind associated with the storm. If the maximum wind is between 34 – 47 knots (about 60-90 kmph) it is called a Cyclonic storm. Severe Cyclonic storm will have maximum wind speed between 48 – 63 knots (about 90-120 kmph). If the maximum wind is 64-119 knots it will be called a very severe Cyclonic storm and when the wind is 120 knots and above it will be called super cyclonic storm. There is very little association between intensity (either measured by maximum sustained winds or by the lowest central pressure) and size (measured by radius of gale force winds)


What are the super cyclone, super-typhoon, a major hurricane and an intense hurricane?

  • When the maximum sustained 3 minutes surface winds are more than 119 knots, the low pressure system is called as “Super Cyclone” over north Indian Ocean. Similarly, “Super-typhoon” is a term utilized by the U.S. Joint Typhoon Warning Centre for typhoons that reach maximum sustained 1 minute surface winds of at least 130 knots (65 m/s). This is the equivalent of a strong Saffir-Simpson category 4 or category 5 hurricane in the Atlantic basin or a category 5 severe tropical cyclone in the Australian basin.


Where do tropical cyclones form?

  • The tropical cyclones form over ocean basins in lower latitudes of all oceans except south Atlantic and southeast Pacific. The tropical cyclones develop over the warm water of the Bay of Bengal and the Arabian Sea. The favourable ocean basins for development of cyclonic storms are shown in the figure below.

  • TC breeding grounds are located over certain ocean basins. Arrows indicate average trajectories over different basins


What is the size of a tropical cyclone over the north Indian Ocean

  • The size of a tropical cyclone over Indian seas varies from 50-100 km radius to 2000 km with an average of 300 –600 km.


What is the structure of a tropical cyclone?

  • A fully developed tropical cyclone has a central cloud free region of calm winds, known as the “eye” of the cyclone with diameter varying from 10 to 50 km. Surrounding the eye is the “wall cloud region” characterised by very strong winds and torrential rains, which has the width of about 10 to 150 km. The winds over this region rotate around the centre and resemble the “coils of a snake”. Wind speed fall off gradually away from this core region, which terminate over areas of weaker winds with overcast skies and occasional squall .There may be one or more spiral branch in a cyclone where higher rainfall occurs. The vertical extent of the cyclone is about 15 km. The INSAT imagery of Orissa Super cyclone on 29th October, 1999 is shown in the figure below.

What is the eye? How is it formed and maintained? What is the eyewall? What are spiral bands?

  • The “eye” is a roughly circular area of comparatively light winds and fair weather found at the centre of a severe tropical cyclone. Although the winds are calm at the axis of rotation, strong winds may extend well into the eye. There is little or no precipitation and sometimes blue sky or stars can be seen. The eye is the region of lowest surface pressure and warmest temperatures aloft – the eye temperature may be 10°C warmer or more at an altitude of 12 km than the surrounding environment, but only 0-2°C warmer at the surface in the tropical cyclone. Eyes range in size from 8 km to over 200 km across, but most are approximately 30-60 km in diameter.
  • The eye is surrounded by the “eyewall”, the roughly circular ring of deep convection, which is the area of highest surface winds in the tropical cyclone. The eye is composed of air that is slowly sinking and the eyewall has a net upward flow as a result of many moderate – occasionally strong – updrafts and downdrafts. The eye’s warm temperatures are due to compressional warming of the subsiding air. Most soundings taken within the eye show a low-level layer, which is relatively moist, with an inversion above – suggesting that the sinking in the eye typically does not reach the ocean surface, but instead only gets to around 1-3 km of the surface.
  • The exact mechanism by which the eye forms remains somewhat controversial. One idea suggests that the eye forms as a result of the downward directed pressure gradient associated with the weakening and radial spreading of the tangential wind field with height (Smith, 1980). Another hypothesis suggests that the eye is formed when latent heat release in the eyewall occurs, forcing subsidence in the storm’s centre (Shapiro and Willoughby, 1982). It is possible that these hypotheses are not inconsistent with one another. In either case, as the air subsides, it is compressed and warms relative to air at the same level outside the eye and thereby becomes locally buoyant. This upward buoyancy approximately balances the downward directed pressure gradient so that the actual subsidence is produced by a small residual force.
  • Another feature of tropical cyclones that probably plays a role in forming and maintaining the eye is the eyewall convection. Convection in tropical cyclones is organized into long, narrow rainbands which are oriented in the same direction as the horizontal wind. Because these bands seem to spiral into the centre of a tropical cyclone, they are called “spiral bands”. Along these bands, low-level convergence is a maximum, and therefore, upper-level divergence is most pronounced above. A direct circulation develops in which warm, moist air converges at the surface, ascends through these bands, diverges aloft, and descends on both sides of the bands. Subsidence is distributed over a wide area on the outside of the rainband but is concentrated in the small inside area. As the air subsides, adiabatic warming takes place, and the air dries. Because subsidence is concentrated on the inside of the band, the adiabatic warming is stronger inward from the band causing a sharp contrast in pressure falls across the band since warm air is lighter than cold air. Because of the pressure falls on the inside, the tangential winds around the tropical cyclone increase due to increased pressure gradient. Eventually, the band moves toward the centre and encircles it and the eye and eyewall form.
  • Thus, the cloud-free eye may be due to a combination of dynamically forced centrifuging of mass out of the eye into the eyewall and to a forced descent caused by the moist convection of the eyewall. This topic is certainly one that can use more research to ascertain which mechanism is primary.
  • Some of the most intense tropical cyclones exhibit concentric eyewalls, two or more eyewall structures centreed at the circulation centre of the storm. Just as the inner eyewall forms, convection surrounding the eyewall can become organized into distinct rings. Eventually, the inner eye begins to feel the effects of the subsidence resulting from the outer eyewall, and the inner eyewall weakens, to be replaced by the outer eyewall. The pressure rises due to the destruction of the inner eyewall are usually more rapid than the pressure falls due to the intensification of the outer eyewall, and the cyclone itself weakens for a short period of time.


How does the cyclone look like in a Radar ?

  • According to Radar imagery, a matured cyclone consists of eye, eye wall, spiral bands, pre-cyclone squall lines and streamers as shown in the above figure.


What is the wind structure in a cyclone?

  • The ideal wind and cloud distribution in a cyclone is shown in the following figure.
  • The band of maximum winds may vary between 10 and 150 Km. In this belt, speed decreases rapidly towards the eye of the cyclone. But it decreases slowly and in an irregular fashion outward from the eye wall.

How do the cyclones form and intensify?

  • In the tropics, weak pressure waves move from east to west. These are called easterly waves. Under favourable situation, a low pressure area forms over the area of an easterly trough. This gives rise to low level convergence. If the sea is warm (sea surface temperature > = 26.50 C) and there is sufficient upper level divergence i.e air is blown off at higher levels from the area of low pressure, the pressure gradually falls. Low level convergence coupled with upper level divergence gives rise to vertical motion taking moist air upwards. These moistures condense at higher levels (middle troposphere) and give out latent heat of condensation. Due to release of heat of condensation the area warms up resulting into further fall in pressure. This process continues and a low pressure
  • Hence, for tropical cyclogenesis, there are several favourable environmental conditions that must be in place. They are:-
    1. Warm ocean waters (of at least 26.5°C) throughout a sufficient depth (unknown how deep, but at least on the order of 50 m). Warm waters are necessary to fuel the heat engine of the tropical cyclone.
    2. An atmosphere which cools fast enough with height such that it is potentially unstable to moist convection. It is the thunderstorm activity which allows the heat stored in the ocean waters to be liberated for the tropical cyclone development.
    3. Relatively moist layers near the mid-troposphere (5 km). Dry mid levels are not conducive for allowing the continuing development of widespread thunderstorm activity.
    4. A minimum distance of at least 500 km from the equator. For tropical cyclogenesis to occur, there is a requirement for non-negligible amounts of the Coriolis Force (attributed to earth’s rotation) to provide the near gradient wind balance to occur. Without the Coriolis Force, the low pressure of the disturbance cannot be maintained. This is the reason why the narrow corridor of width of about 300 km on either side of the equator is free from cyclones. Because of this there is no inter-hemispheric migration of tropical cyclones across the equator.
    5. A pre-existing near-surface disturbance with sufficient vorticity (rotation) and convergence. Tropical cyclones cannot be generated spontaneously. To develop, they require a weakly organized system with sizable spin and low level inflow.
    6. Low values (less than about 10 m/s or 20 kts) of vertical wind shear between the lower (1.5 km) and the upper troposphere (12 km). Vertical wind shear is the magnitude of wind change with height. Large values of vertical wind shear disrupt the incipient tropical cyclone and can prevent genesis, or, if a tropical cyclone has already formed, large vertical shear can weaken or destroy the tropical cyclone by interfering with the organization of deep convection around the cyclone centre.
  • The above conditions are necessary, but not sufficient as many disturbances that appear to have favourable conditions do not develop. However, these criteria fit well over the north Indian Ocean

What is the role of easterly waves on cyclogenesis in north Indian Ocean ?

  • It has been recognized since at least the 1930s that lower tropospheric westward traveling disturbances often serve as the “seedling” circulations for a large proportion of tropical cyclones. These disturbances are known as easterly waves. The waves move generally toward the west in the lower tropospheric trade wind flow. They are first seen usually in October to April. The waves have a period of about 3 or 4 days and a wavelength of 2000 to 2500 km. One should keep in mind that the “waves” can be more correctly thought of as the convectively active troughs along an extended wave train. Though, these waves are generated frequently, but it appears that the number that is formed has no relationship to how much tropical cyclone activity there is over the north Indian Ocean each year. It is currently completely unknown, how, easterly waves change from year to year in both intensity and location and how these might relate to the activity ?

Is there any extra-tropical cyclone?

  • Extra-tropical cyclones are low pressure systems with associated cold fronts, warm fronts, and occluded fronts. The extra-tropical cyclone is a storm system that primarily gets its energy from the horizontal temperature contrasts that exist in the atmosphere. Extra-tropical cyclones are low pressure systems with associated cold fronts, warm fronts, and occluded fronts. Tropical cyclones, in contrast, typically have little to no temperature differences across the storm at the surface and their winds are derived from the release of energy due to cloud/rain formation from the warm moist air of the tropics. Structurally, tropical cyclones have their strongest winds near the earth’s surface, while extra-tropical cyclones have their strongest winds near the tropopause – about 12 km up. These differences are due to the tropical cyclone being “warm-core” in the troposphere (below the tropopause) and the extra-tropical cyclone being “warm-core” in the stratosphere (above the tropopause) and “cold-core” in the troposphere. “Warm-core” refers to being relatively warmer than the environment at any level.
  • Often, a tropical cyclone will transform into an extra-tropical cyclone as it recurves poleward and to the east. Occasionally, an extra-tropical cyclone will lose its frontal features, develop convection near the centre of the storm and transform into a full-fledged tropical cyclone. Such a process is most common in the north Atlantic and northwest Pacific basins. The transformation of tropical cyclone into an extra-tropical cyclone (and vice versa) is currently one of the most challenging forecast problems.

What is the annual frequency of Cyclones over the Indian Seas? What is its intra-annual variation?

  • The average annual frequency of tropical cyclones in the north Indian Ocean (Bay of Bengal and Arabian Sea) is about 5 (about 5-6 % of the Global annual average) and about 80 cyclones form around the globe in a year. The frequency is more in the Bay of Bengal than in the Arabian Sea, the ratio being 4:1. The monthly frequency of tropical cyclones in the north Indian Ocean display a bi-modal characteristic with a primary peak in November and secondary peak in May. The months of May-June and October-November are known to produce cyclones of severe intensity. Tropical cyclones developing during the monsoon months (July to September) are generally not so intense.
  • The frequencies of Cyclonic systems over north Indian Ocean during 1891-2006 are given in the figure below.

What are the average, most, and least tropical cyclones occurring in this basin?

  • The most, least and average numbers of cyclonic storms and severe cyclonic storms over the north Indian ocean is given in the Table below:
    1. Minimum No. of cyclones in a year – One (1949)
    2. Maximum No. of cyclones in a year – Ten (1893,1926,1930,1976)
    3. Out of total disturbances – 35% intensify to Cyclones
  • 16 % intensify to severe cyclones
  • 07% intensify to very severe cyclones

  • Basin

    Cyclonic
    storm

    Severe cyclonic
    storm

    Most

    Least

    Average

    Most

    Least

    Average

    N Indian Ocean

    10

    1

    5.4

    6

    0

    2.5

How many severe tropical storms occur around the world and over north Indian Ocean every year?

  • About 20-30 severe tropical storms occur around the world every year. Over the north Indian Ocean, 2-3 severe cyclonic storms form out of total 5-6 cyclonic storms


How many cyclones cross different coastal states of India?

  • The frequencies of cyclonic storms crossing different coastal states of India during 1891-2006 are shown in the figure below. The frequency of severe cyclonic storms is maximum for Andhra Pradesh while that of cyclone is maximum for Orissa. Considering west coast only, Gujarat is most vulnerable.

Which is the most intense tropical cyclone on record?

  • Typhoon Tip in the Northwest Pacific Ocean on 12 October, 1979 was measured to have a central pressure of 870 hPa and estimated surface sustained winds of 165 knots (85 m/s). Typhoon Nancy on 12 September, 1961 is listed in the best track data for the Northwest Pacific region as having an estimated maximum sustained winds of 185 knots (95 m/s) with a central pressure of 888 hPa. However, it is now recognized that the maximum sustained winds estimated for typhoons during the 1940s to 1960s were too strong and that the 95 m/s (and numerous 83 to 93 m/s reports) is somewhat too high.
  • Note that Hurricane Gilbert’s 888 hPa lowest pressure (estimated from flight level data) in mid September, 1988 is the most intense [as measured by lowest sea level pressure] for the Atlantic basin, it is almost 20 hPa weaker (higher) than the above Typhoon Tip of the Northwest Pacific Ocean.
  • While the central pressures for the Northwest Pacific typhoons are the lowest globally, the North Atlantic hurricanes have provided sustained wind speeds possibly comparable to the Northwest Pacific. From the best track database, both Hurricane Camille (1969) and Hurricane Allen (1980) have winds that are estimated to be 165 knots (85 m/s). Measurements of such winds are inherently going to be suspect as instruments often are completely destroyed or damaged at these speeds.
  • Orissa super cyclone, 1999 which crossed Orissa coast near Paradip on 29th October, 1999 was the most intense cyclonic storm over north Indian Ocean in the recorded history of the region. The estimated sustained maximum surface wind speed was about 140 knots at the time of landfall and lowest estimated central pressure was 912 hPa.
  • A few cyclones that have originated over the Bay of Bengal have reached the intensity of Super Cyclones and have caused great devastations to life and property. The estimates of maximum sustained winds of these systems are estimated from satellite imageries. The list of very intense Cyclones in the Bay of Bengal since 1990 is given below.
  • .Place of
    landfall

    Date
    of landfall

    Maximum sustained winds (kmph) – estimated on the
    basis of satellite imageries

    Chittagong

    13 November,
    1970

    224

    Chirala, Andhra
    Pradesh

    19 November,
    1977

    260

    Rameshwaram

    24 November
    1978

    204

    Sriharikota

    14 November,
    1984

    213

    Bangla Desh

    30 November,
    1988

    213

    Kavali, Andhra
    Pradesh

    9 November,
    1989

    235

    Machlipatnam,
    AP

    9 May ,1990

    235

    Chittagong

    29 April, 1991

    235

    Teknaf
    (Myanmar)

    2 May, 1994

    204

    Teknaf

    19 May, 1997

    235

    Paradip, Orissa

    29 October,
    1999

    260

    89.80E, Bangladesh

    15 November,
    2007

    220

    16.00N, Myanmar

    02 May, 2008

    200


Which are the largest and smallest tropical cyclones on record?

  • Typhoon Tip had gale force winds 34 knots (17 m/s), which extended out for 1100 km in radius in the Northwest Pacific on 12 October, 1979. Tropical Cyclone Tracy had gale force winds that only extended 50 km radius when it struck Darwin, Australia, on 24 December,1974.
  • Considering north Indian Ocean, Orissa super cyclone of October, 1999 and the cyclone, ‘Ogni’ were the largest and smallest cyclones during 1891-2007.


Which tropical cyclone over north Indian Ocean have caused the most deaths and most damage?

  • The death toll in the infamous Bangladesh Cyclone of 1970 has had several estimates, some wildly speculative, but it seems certain that at least 300,000 people died from the associated storm tide [surge] in the low-lying deltas.


Why there are fewer cyclones over the Arabian Sea as compared to the Bay of Bengal?

  • Cyclones that form over the Bay of Bengal are either those develop insitu over southeast Bay of Bengal and adjoining Andaman Sea or remnants of typhoons over Northwest Pacific and move across south China sea to Indian Seas. As the frequency of typhoons over Northwest Pacific is quite high (about 35 % of the global annual average), the Bay of Bengal also gets its increased quota.
  • The cyclones over the Arabian Sea either originate insitu over southeast Arabian Sea (which includes Lakshadweep area also) or remnants of cyclones from the Bay of Bengal that move across south peninsula. As the majority of Cyclones over the Bay of Bengal weaken over land after landfall, the frequency of migration into Arabian Sea is low.
  • In addition to all the above the Arabian Sea is relatively colder than Bay of Bengal and hence inhibits the formation and intensification of the system.

Why there are very few Tropical Cyclones during southwest monsoon season?

  • The southwest monsoon is characterized by the presence of strong westerly winds in the lower troposphere (below 5 km) and very strong easterly winds in the upper troposphere (above 9 km) .This results in large vertical wind shear. Strong vertical wind shear inhibits cyclone development.
  • Also the potential zone for the development of cyclones shifts to North Bay of Bengal during southwest monsoon season. During this season, the low pressure system upto the intensity of depressions form along the monsoon trough, which extends from northwest India to the north Bay of Bengal. The Depression forming over this area crosses Orissa – West Bengal coast in a day or two. These systems have shorter oceanic stay which is also one of the reasons for their non-intensification into intense cyclones.


What is the life period of cyclones? Which tropical cyclone lasted the longest?

  • Life period of a Tropical Cyclone over the north Indian Ocean is 5-6 days. It will have hurricane intensity for 2-4 days as against 6 days of global average. Life period of the longest lived Tropical cyclone in Indian seas is 14 days (2nd -15th Nov, 1886 & 16th – 29th Nov, 1964). Hurricane/Typhoon John lasted 31 days as it traveled both the Northeast and Northwest Pacific basins during August and September, 1994. (It formed in the Northeast Pacific, reached hurricane force there, moved across the dateline and was renamed Typhoon John, and then finally recurved back across the dateline and renamed Hurricane John again.) Hurricane Ginger was a tropical cyclone for 28 days in the North Atlantic Ocean back in 1971. It should be noted that prior to the weather satellite era (1961) many tropical cyclones’ life cycles could be underestimated.


How are Tropical Cyclones monitored by IMD?

  • IMD has a well-established and time-tested organization for monitoring and forecasting tropical cyclones. A good network of meteorological observatories (both surface and upper air) is operated by IMD, covering the entire coastline and islands. The conventional observations are supplemented by observational data from automatic weather stations (AWS), radar and satellite systems. INSAT imagery obtained at hourly intervals during cyclone situations has proved to be immensely useful in monitoring the development and movement of cyclones.

How is cyclone monitored by satellite technique ?

  • The satellite technique can be used to find out the centre and intensity of the system. It can also be used to find out various derived parameters which are useful for monitoring and prediction of the cyclones and associated disastrous weather.
  • Dvorak’s technique based on pattern recognition in the cloud imagery based on satellite observation is used to determine the intensity of cyclonic storm. For this purpose a T. No. where T stands for tropical cyclone is assigned to the system. This scale of T Nos. varies from T 1.0 to T 8.0 at the interval of 0.5. The T 2.5 corresponds to the intensity of a cyclonic storm. The detailed classification of cyclonic disturbances based on above technique is given below:
  • T” CLASSIFICATION OF CYCLONIC STORM
    AND

  • CORRESPONDING WIND SPEED AND PRESSURE DEFECT
    (
    ?P)

  • T. Number/

    C.I.
    Number

    Classification of Cyclonic
    Disturbance

    Wind
    speed in Knots

    Wind
    speed In Kmph

    ? P

    Wind
    criteria in Knots

    Wind
    criteria in Kmph

    T1.0

    L

    ?17

    ?31

    T1.5

    D

    25

    46.3

    17-27

    31-49

    T2.0

    DD

    30

    55.6

    4.5

    28-33

    50-61

    T2.5

    CS

    35

    64.9

    6.1

    34-47

    62-88

    T3.0

    45

    83.4

    10.0

    T3.5

    SCS

    55

    101.9

    15.0

    48-63

    89-117

    T4.0

    VSCS

    65

    120.5

    20.9

    64-119

    119-221

    T4.5

    77

    142.7

    29.4

    T5.0

    90

    166.8

    40.2

    T5.5

    102

    189.0

    51.6

    T6.0

    115

    213.1

    65.6

    T6.5

    SuCS

    127

    235.4

    80.0

    120 AND ABOVE

    222 AND
    ABOVE

    T7.0

    140

    259.5

    97.2

    T7.5

    155

    287.3

    119.1

    T8.0

    170

    315.1

    143.3


What is the utility of Radar in cyclone monitoring ?

  • The radar can be utilized to find out the location of the cyclonic storm more accurately when the system comes within radar range. In addition it can find out convective cloud cluster, wind distribution, rainfall rate etc.

What is the present network of Cyclone Detection Radars?

  • A network of conventional Cyclone Detection Radars (CDRs) has been established at Kolkata, Paradip, Visakhapatnam, Machilipatnam, Chennai and Karaikal along the east coast and Goa, Cochin, Mumbai and Bhuj along the west coast. These conventional radars are being phased out and replaced by Doppler Weather Radars (DWRs). DWR have already been installed and made operational at Chennai, Kolkata, Visakhapatnam and Machlipatnam. An indigenously developed DWR Radar by Indian Space Research Organisation (ISRO) has been installed at Sriharikota.
  • It is proposed to replace all the conventional radars by DWRs during the next 3-4 years.


What are the basic differences between conventional analog type of Cyclone Detection Radar and the Doppler Weather Radar?

  • While conventional weather radar can look deeper into a weather system to provide information on intensity rain-rate, vertical extent, the capability to probe internal motion of the hydrometers and hence to derive information on velocity and turbulence structure has become available only with the advent of Doppler Weather Radar (DWRs) which provide vital information on radial velocity from which wind field of a tropical disturbance in the reconnaissance area of DWR can be derived. In addition to above, a number of derived parameters useful for cyclone monitoring and prediction are also available from DWR.

What are the causes of disaster during cyclone?

  • The dangers associated with cyclonic storms are generally three fold.
    1. Very heavy rains causing floods.
    2. Strong wind.
    3. Storm surge.

When does a coastal station start experiencing bad weather associated with a Cyclone?

  • Coasts come under the influence of bad weather in the form of heavy rain, gale winds (exceeding 65 kmph) when the cyclone moves closer to the coast within 200km. Heavy rainfall generally commences about 9-12 hours before cyclone landfall. Gale force winds commence about 6-9 hours in advance of cyclone landfall. Maximum storm surge may appear at or near the landfall time.

What is the amount of rainfall expected during a cyclone? Which sector gets more rainfall? What is the impact of heavy rainfall ?

  • Intensive Rainfall occurs to the left of the Cyclone. Maximum rainfall occurs close to the centre of the storm. Secondary maximum of rainfall occurs 2º away from Primary maximum to the right of the storm centre. Slow moving/big size cyclones give more rainfall, whereas, fast moving/small size ones give less rainfall. More than 90% of rainfall is limited within 200 Km radius of the storm. Extensive rainfall occurs in the left forward sector for westward moving system and forward sector for northward moving system and right forward sector for those re-curving to east and northeast.
  • The governing factors for rainfall distribution and intensity are intensity, speed and size of the storm and local effects such as topography and orientation of the coast.

What are the largest rainfalls associated with tropical cyclones over north Indian Ocean?

  • The rainfall can vary from trace/ nil rainfall when the system moves skirting the coast to maximum rainfall upto 50-60 cm per day. In the recent super cyclone which crossed Orissa coast near Paradip on 29th October 1999, Paradip recorded 24 hr cumulative rainfall of about 52 cm at 0830 IST of 30th October 1999.


What may be the wind speed in most severe storm?

  • The wind speed may be as high as 300 kmph.

What is the wind speed at the centre of a storm? What is weather there?

  • Nearly calm wind with fair weather prevails at the centre of the storm.

How is the damage that cyclones cause related with wind ?

  • The amount of damage does not increase linearly with the wind speed. Instead, the damage produced increases exponentially with the winds.

Which sector of the cyclone experiences strongest winds?

  • In general, the strongest winds in a cyclone are found on the right side of the storm. The “right side of the storm” is defined with respect to the storm’s motion: if the cyclone is moving to the west, the right side would be to the north of the storm; if the cyclone is moving to the north, the right side would be to the east of the storm, etc. The strongest wind on the right side of the storm is mainly due to the fact that the motion of the cyclone also contributes to its swirling winds. A cyclone with a 145 kmph winds while stationary would have winds up to 160 kmph on the right side and only 130 kmph on the left side if it began moving (any direction) at 16 kmph. While writing the cyclone warning bulletins, this asymmetry is taken into consideration.
  • For tropical cyclones in the Southern Hemisphere, these differences are reversed: the strongest winds are on the left side of the storm. This is because the winds swirl clockwise south of the equator in tropical cyclones.

What causes each cyclone to have a different maximum wind speed for a given minimum sea-level pressure?

  • The basic horizontal balance in a tropical cyclone above the boundary layer is between the sum of the Coriolis ‘acceleration’ and the centripetal ‘acceleration’, balanced by the horizontal pressure gradient force. This balance is referred to as gradient balance, where the Coriolis ‘acceleration’ is defined as the horizontal velocity of an air parcel, v, times the Coriolis parameter, f. Centripetal ‘force’ is defined as the acceleration on a parcel of air moving in a curved path, directed toward the centre of curvature of the path, with magnitude v2/r, where v is the horizontal velocity of the parcel and r the radius of curvature of the path. The centripetal force alters the original two-force geostrophic balance and creates a non-geostrophic gradient wind. The reason that different peak winds can result in different central pressures is caused by the fact that the radius, r, of the peak wind varies. A storm with 40 m/s peak winds with a 100 km RMW will have a much lower pressure drop than one with a 25 km RMW.

Why do very severe cyclone or hurricane force winds start at 64 knots ?

  • In 1805-06, Commander Francis Beaufort RN (later Admiral Sir Francis Beaufort) devised a descriptive wind scale in an effort to standardize wind reports in ship’s logs. His scale divided wind speeds into 14 Forces (soon after pared down to thirteen) with each Force assigned a number, a common name, and a description of the effects such a wind would have on a sailing ship. And since the worst storm an Atlantic sailor was likely to run into was a hurricane that name was applied to the top Force on the scale.
  • During the 19th Century, with the manufacture of accurate anemometers, actual numerical values were assigned to each Force level, but it wasn’t until 1926 (with revisions in 1939 and 1946) that the International Meteorological Committee (predecessor of the WMO) adopted a universal scale of wind speed values. It was a progressive scale with the range of speed for Forces increasing as you go higher. Thus Force 1 is only 3 knots in range, while the Force 11 is eight knots in range. So Force 12 starts out at 64 knots (74 mph, 33 m/s).
  • There is nothing magical in this number, and since hurricane force winds are a rare experience chances are the committee which decided on this number didn’t do so because of any real observations during a hurricane. Indeed the Smeaton-Rouse wind scale in 1759 pegged hurricane force at 70 knots (80 mph, 36 m/s). Just the same, when a tropical cyclone has maximum winds of approximately these speeds we do see the mature structure (eye, eyewall, spiral rainbands) begin to form, so there is some utility with setting hurricane force in this neighborhood.

What is a Storm Surge?

  • Storm Surge is an abnormal rise of sea level as the cyclone crosses the coast. Sea water inundates the coastal strip causing loss of life, large scale destruction to property & crop. Increased salinity in the soil over affected area makes the land unfit for agricultural use for two or three seasons.
  • Storm surge depends on intensity of the cyclone (Maximum winds and lowest pressure associated with it and Coastal bathymetry (shallower coastline generates surges of greater heights).

In which direction of a storm the surge will appear?

  • The on shore wind gives rise to storm surge. Thus the forward right sector of a storm gives rise to storm surge.

What is storm tide?

  • The storm tide is the combination of storm surge and the astronomical tide

What is the interaction of astronomical tide with storm surge?

  • In general one may expect that if there is a storm surge of x metres and tidal wave of y metres then during high tide total surges would be x+y and during low tide x-y. But, it is found that there is an interaction of storm surge with astronomical tide, and during high tide time the total surge is little less than x+y and during low tide time it is little more than x-y.


What are the disaster potential of Storm Surge?

  • Disaster potential due to cyclones is due to high storm surges occurring at the time of landfall. The storm surges are by far the greatest killers in a cyclone. as sea water inundates low lying areas of the coastal regions causing heavy floods, erosion of beaches and embankments, damage to vegetation and reducing soil fertility. Flooding due to storm surges pollute drinking water sources resulting in shortage of drinking water and causing out-break of epidemics, mostly water borne diseases Very strong winds (Gales) may cause uprooting of trees, damage to dwellings, overhead installations, communication lines etc., resulting in loss of life and property. Past records show that very heavy loss of life due to tropical cyclones have occurred in the coastal areas surrounding the Bay of Bengal. Cyclones are also often accompanied by very intense & heavy precipitation (exceeding 40-50 cm in a day or about 10cm or more per hour in some places)

What is the vulnerability our coastline from the point of view of storm surge potential?

  • Entire Indian coast can be categorized into 4 zones
    1. Very high risk zones (Surge height > 5m)
    2. High risk Zone (Surge height between 3-5m)
    3. Moderate risk zone (Surge height between 1.5 to 3m)
    4. Minimal risk zone ( Surge height < 1.5m)
  • Accordingly
    1. The coastal areas and off-shore islands of Bengal and adjoining Bangladesh are the most storm-surge prone (~ 10-13m) – VHRZ
    2. East coast of India between Paradip and Balasore in Orissa (~ 5-7m) – VHRZ
    3. Andhra coast between Bapatla and Kakinada holding estuaries of two major rivers Krishna and Godavari (~ 5-7m) – VHRZ
    4. Tamilnadu coast between Pamban and Nagapattinam (~ 3-5m) – HRZ
    5. Gujarat along the west coast of India (~ 2-3m) -MRZ

Can we predict storm surge?

  • The storm surge is predicted by IMD using nomograms and dynemic model developed by IIT, Delhi. Both these models taken into consideration different characteristics, the cyclones and the coastal bathymetry to predict the storm surge.

Which tropical cyclone has produced the highest storm surge?

  • The Bathurst Bay Hurricane, also known as Tropical Cyclone Mahina, struck Bathurst Bay, Australia in 1899. It produced a 13 m (about 42 ft) surge, but other contemporary accounts place the surge at 14.6 m (almost 48 ft). Considering cyclones over north Indian Ocean, cyclone of 1970 has produced maximum storm surge of 13 metres in recent years. Some of the significant storm surges (metres) over the region are mentioned below.
  • Hooghly river (WB), October, 1737 : 13
  • Contai (WB), October, 1864 : 10-13
  • Bangladesh cyclone, November, 1970 : 13
  • Paradip, Orissa,October, 1971 : 4-5
  • Balasore Orissa, May, 1989 : 3-6
  • Orissa Super Cyclone, October, 1999 : 5-6

What is the damage potential of a deep depression (28 – 33 knots) and what are the suggested actions?

  • Road/Rail: Some breaches in Kutcha road due to flooding
    Agriculture: Minor damage to Banana trees and near coastal agriculture due to salt spray. Damage to ripe paddy crops
    Marine Interests: Very rough seas. Sea waves about 4-6 m high.
    Coastal Zone: Minor damage to Kutcha embankments
    Overall Damage Category: Minor
    Suggested Actions: Fishermen advised not to venture into sea


What is the damage potential of a cyclonic storm (34-47 knots or 62 to 87 kmph) and what are the suggested actions?

  • Structures: Damage to thatched huts
    Communication and power: Minor damage to power and communication lines
    due to breaking of tree branches.
    Road/Rail: Major damage to Kutcha and minor damage to Pucca roads.
    Agriculture: Some damage to paddy crops, Banana, Papaya trees and orchards.
    Marine Interests: High to very high sea waves about 6-9 m high.
    Coastal Zone: Sea water inundation in low lying areas after erosion of Kutcha embankments
    Overall Damage Category: Minor to Moderate
    Suggested Actions: Fishermen advised not to venture into sea


What is the damage potential of a severe cyclonic storm 48-63 Knots (88-117 Kmph) and what are the suggested actions?

  • Structures: Major damage to thatched houses / huts. Roof tops may blow off. Unattached metal sheets may fly.
    Communication and power: Minor damage to power and communication lines.
    Road/Rail: Major damage to Kutcha and some damage to Pucca roads. Flooding of escape routes.
    Agriculture: Breaking of tree branches, uprooting of large avenue trees.
  • Moderate damage to Banana and Papaya trees: Large dead limbs blown from trees.
    Marine Interests: Phenomenal seas with wave height 9-14 m. Movement in motor boats unsafe.
    Coastal Zone: Major damage to coastal crops. Storm surge upto 1.5m (area specific) causing damage to embankments/ salt pans. Inundation upto 5 Km in specific areas.
    Overall Damage Category: Moderate
    Suggested Actions: Fishermen advised not to venture into sea. Coastal hutment dwellers advised to move to safer places. Other people in the affected areas to remain indoors.

What is the damage potential of a very severe cyclonic storm (64-90 Knots or 118-167 Kmph) and what are the suggested actions?

  • Structures: Total destruction of thatched houses/ extensive damage to Kutcha houses. Some damage to Pucca houses. Potential threat from flying objects.
    Communication and power: Bending/ uprooting of power and communication poles.
    Road/Rail: Major damage to Kutcha and Pucca roads. Flooding of escape routes. Minor disruption of railways, overhead power lines and signaling systems.
    Agriculture: Widespread damage to standing crops plantations, orchards, falling of green coconuts and tearing of palm fronds Blowing down bushy trees like mango.
    Marine Interests: Phenomenal seas with wave heights more than 14m. Visibility severely affected. Movement in motor boats and small ships unsafe.
    Coastal Zone: Storm surge up to 2 m, Inundation up to 10 Km in specific areas. Small boats, country crafts may get detached from moorings.
    Overall Damage Category: Large
    Suggested Actions: Fishermen not to venture into sea. Evacuation from coastal areas needs to be mobilized. People advised to remain indoors. Judicious regulation of rail and road traffic needed.


What is the damage potential of a very severe cyclonic storm (91-119 Knots or 168-221 Kmph) and what are the suggested actions?

  • Structures: Extensive damage to all types Kutcha houses, some damage to old badly managed Pucca structures. Potential threat from flying objects.
    Communication and power: Extensive uprooting of power and communication poles.
    Road/Rail: Disruption of rail / road link at several places.
    Agriculture: Extensive damage to standing crops plantations, orchards. Blowing down of Palm and Coconut trees. Uprooting of large bushy trees.
    Marine Interests: Phenomenal seas with wave heights more than 14m. Movement in motor boats and small ships not advisable.
    Coastal Zone: Storm surge up to 2 – 5 m, Inundation may extend up to 10-15 Km over specific areas. Large boats and ships may get torn from their moorings, country crafts may get detached from moorings
    Overall Damage Category: Extensive
    Suggested Actions: Fishermen not to venture into sea. Evacuation from coastal areas essential. Diversion / suspension of rail traffic may be required.

What is the normal movement of a Tropical Cyclone?

  • Tropical Cyclones move as a whole. They casually move west-northwestwards or northwestwards in the northern hemisphere. The average speed is 15-20 kmph (360-480 km per day). They may change their direction of movement towards north. During this change their speed of movement decreases to 10 kmph or even less. A larger fraction of such storms later turn towards northeast and move northeastwards very fast at a speed of 25 kmph or more.

What are fast and slow moving cyclones?

  • When the speed of movement is 10-14 kmph, it is called as slow moving cyclone. It is called as moderately moving cyclone, if the speed of movement is 15-25 kmph. If the speed of movement is more than 25 kmph, is called as fast moving cyclone.


How track prediction is done in IMD ?

  • Various Techniques are available for Track Prediction of the storm as mentioned below:
    Methods based on climatology, persistence and both Climatology & Persistence (CLIPER)
    Synoptic Techniques – Empirical Techniques
    Satellite Techniques
    Statistical Techniques using climatology, persistence and synoptic
    Analogue Techniques
    Numerical weather prediction models

What are the numerical weather prediction models used operationally for Tropical Cyclone track Prediction and storm surges in IMD?

  • T-254 model of NCMRWF, MM5 mesoscale model
    Quasi-Lagrangian Limited Area Model (QLM) for track prediction
    Weather Research and Forecast (WRF) mesoscale model for intensification and track prediction
    Prediction Models of IIT – Delhi and NIOT Chennai for Storm Surge rediction
    In addition to above, IMD forecasters make use of various forecasts available from international NWP models like BCBCMRF, UKMET and COLA etc.

What is our accuracy of landfall prediction?

  • Probability of correct forecast decreases with increasing forecast validity period. Mean forecast errors for 12, 24, 48 and 72 hours are about 50, 140, 300 and 500 km respectively, which are comparable to corresponding figures of other centres like National Hurricane Centre, Miami, which monitor Atlantic Hurricanes; Typhoon Warning Centre, Tokyo, which monitors Typhoons of Northwest Pacific etc.

How does IMD predict intensity of the cyclone ?

  • Subjective techniques like Climatology, Synoptic and Satellite (Dvorak) techniques and radar techniques are used. Though the performance of NWP models in intensity prediction is not satisfactory, they provide valuable guidance in intensity prediction also.


What is the role of upper tropospheric westerly trough ?

  • An Upper tropospheric westerly trough is important for tropical cyclone forecasting as they can force large amounts of vertical wind shear over tropical disturbances and tropical cyclones which may inhibit their strengthening. There are also suggestions that these troughs can assist tropical cyclone genesis and intensification by providing additional forced ascent near the storm centre and/or by allowing for an efficient outflow channel in the upper troposphere. The location of this trough and its intensity can also influence the movement of the storm and hence can be used for cyclone track forecasting.

Why Tropical Cyclones weaken over land after landfall ?

  • After just a few hours, a tropical cyclone over land begins to weaken rapidly because the storm lacks the moisture and heat sources that the ocean provided. This depletion of moisture and heat hurts the tropical cyclone’s ability to produce thunderstorms near the storm centre. Without this convection the cyclone cannot survive.
  • However, there are instances like Orissa super cyclone of October 1999, which maintained its intensity of cyclonic storm even 24 hours after landfall. During this period, it remained practically stationary over coastal Orissa.


Doesn't the friction over land kill tropical cyclones?

  • No, during landfall, the increased friction over land acts – somewhat contradictory – to both decrease the sustained winds and also to increase the gusts felt at the surface. The sustained winds are reduced because of the dampening effect of larger roughness over land (i.e. bushes, trees and houses over land versus a relatively smooth ocean). The gusts are stronger because turbulence increases and acts to bring faster winds down to the surface in short (a few seconds) bursts.

What are the abnormal characteristics associated with Tropical Cyclones?

  • Majority of Tropical Cyclones are associated with some sort of abnormal behavior such as
    Rapidly changing trends in motion and intensity
    Remaining quasi-stationary close to landfall
    Development or intensification close to a populated coastline
    Approaching a vulnerable coastline at an acute angle so that even minor forecast errors introduce large landfall uncertainties
    Threatening the coastal community during high pitch of seasonal activity such as harvesting, festivals, holidays etc.

What is the organizational set up in IMD for Cyclone forecasting and Warning?

  • The Cyclone Warning Organization in India has a 3-tier system to cater to the needs of the maritime States. These are : Cyclone Warning Division set up at IMD Head Quarters to co-ordinate and supervise cyclone warning operations in the country and to advise the Govt. at the apex level; Area Cyclone Warning Centres at Chennai, Mumbai and Kolkata and Cyclone Warning Centres at Visakhapatnam, Ahmedabad and Bhubaneswar. The cyclone warning work is also supervised and coordinated by the Forecasting Division at Pune.

What are the different bulletins issued by IMD in connection with the cyclone?

    1. Weather and Sea area bulletins.
    2. Bulletins for Indian Navy.
    3. Bulletins for Departmental Exchange.
    4. Port Warnings
    5. Fisheries warnings
    6. Four Stage Warnings
    7. Bulletins for AIR
    8. Bulletins for Press
    9. Coastal bulletins

How frequently IMD issues these bulletins?

  • When cyclone is beyond the range of coastal cyclone detection radar, (more than 400 km away from coast), cyclone warnings are issued 6 times a day to air stations and each warning is broadcast at frequent intervals interrupting routine programme. When the cyclone comes within radar range and tracked by radar, cyclone warnings are issued every hour to air stations. During cyclone period, concerned air stations keep round the clock watch for broadcasting cyclone warnings.

Is there any order in mentioning the disastrous weather in the bulletin?

  • A certain order depending upon the intensity and proximity of the system to the coast will be observed during cyclone period while indicating the adverse weather.

    In case of a cyclone expected to strike the coast in
    About 12 hrs: tidal wave / gales / heavy rain fall
    Next 12-24 hrs: gales / tidal wave / heavy rain fall
    About 24 hrs: rain / gales / tidal wave

What areare fishermen warning?

  • A fisherman warning is warning message for fishermen who ply on coastal areas or may go out at sea. Dangers to fisherman due to storm are strong winds and associated high seas, due to which fishing boats may capsize. Hence, the fishermen are issued warning when one of the following conditions of weather is expected along and off any coast
  • Strong off-shore and on-shore winds (or with appropriate direction), speed exceeding 45 kmph

  • Squally weather – frequent squalls with rain; or persistent type of strong gusty winds (>20kts; 36kmph) accompanied by rain.

  • Gales and

  • State of sea very rough or above (wave heights are four metres or more).


What is sea area bulletin?

  • Issued by ACWC for deep sea
    Normally twice a day (based on 03 and 12 UTC
    Thrice a day in case of depression/ deep depression (additional bulletin based on 18 UTC)
    Six times a day in case of a cyclone. There is also provision of special bulletin.
    The bulletin contains significant system, expected weather, wind, state of sea, port warning etc.


What is coastal weather bulletin?

  • Issued by area cyclone warning centre/ cyclone warning centre for coastal shipping
    Normally twice a day (based on 03 and 12 UTC
    Issued based on sea area bulletin
    Thrice a day in case of depression/ deep depression (additional bulletin based on 18 UTC)
    Six times a day in case of a cyclone. There is also provision of special bulletin
    The bulletin contains significant system, expected weather, wind, state of sea, port warning etc


What is the meaning of widespread /fairly wide spread/scattered/isolated rainfall?

  • The rainfall distribution as mentioned in the bulletin are based on following classification

  • DISTRIBUTION

    NO. OF PLACES

    DESCRIPTION

    Isolated

    One or two places

    <25% of area gets rainfall

    Scattered

    A few places

    (26 –50)% of area gets rainfall

    Fairly Widespread

    A many places

    (51 – 75)% of area gets rainfall

    Wide Spread

    Most place

    (76 – 100)% of area gets
    rainfall


What is meaning of the reference time mentioned in the bulletin ?

    • EARLY HOURS 0000 – 0400 HRS. IST

    • MORNING 0400 – 0800 HRS. IST

    • FORENOON 0800 – 1200 HRS. IST

    • AFTERNOON 1200 – 1600 HRS. IST

    • EVENING 1600 – 2000 HRS. IST

    • NIGHT 2000 – 2400 HRS. IST

    • EARLY MORNING 0400 – 0600 HRS. IST

    • AROUNDNOON 1100 – 1300 HRS. IST)


How are Cyclone Warnings disseminated ?

  • The different telecommunication channels used are as follows

    • Landline

    • T/P (Internal)

    • Telex

    • Telephone

    • Telefax

    • VHF/HFRT (Internal)

    • Cyclone Warning Dissemination System (CWDS)

    • Police Wireless

    • AFTN (Aviation)

    • Internet (e-mail)

    • Websites

    • Radio/TV network

    • Interactive voice response system (IVRS)

    • Mobile Phones

What are the bulletins available in the website? What is the website address ?

  • There are two cyclone related bulletins issued by Cyclone Warning Division, IMD, New Delhi. These are as follows.
    Bulletin for Indian coast

  • Regional Specialised Meteorological Centre (RSMC) bulletin
  • Bulletin for Indian coast


What is IVRS ? How does it work ?

  • IVRS stands for interactive voice response system. The requests for weather information and forecasts from the general public are automatically answered by this system. For this purpose, the person has to dial a toll-free Number “18001801717” from anywhere in the country. This system has been installed at 26 Meteorological Centres/ Regional Meteorological Centres. The data on maximum & minimum temperatures and Rainfall for a large number of towns/cities are provided. The local weather forecasts of cities and multi-hazard warnings including cyclone warnings are also provided.


What is Cyclone Warning Dissemination System (CWDS)?

  • This is a unique scheme not tried anywhere in the world. The scheme has been extremely successful during the cyclones for last 24 years and gained considerable confidence of the public of this country.
    • Designed by ISRO and implemented by IMD in the
      mid-eighties, the CWDS is used all these years to disseminate cyclone warnings
      effectively.

    • Selective addressing (Separate messages for each district)
      is done by transmitting a digital code followed by the actual warning
      message

    • Cyclone
      warnings are generated in English and other local languages (Tamil, Telugu,
      Oriya, Bengali, Marathi, Gujarathi etc)

    • Though
      Radio/TV broadcast are for one and all, the messages through CWDS can be
      accessed only at centres equipped with a receiver and addressed specifically
      for receiving the message

    • CWDS is
      one-way communication system and will be complimentary to other systems of
      cyclone warning dissemination. Facility of acknowledgement is available in the
      upgraded (Digital) version of CWDS

    • The
      present CWDS network covers 252 stations spread over coastal areas of maritime
      districts along the east and e west coast

    • Through
      World Bank assistance Govt. of Andhra Pradesh had installed 100 Digital CWDS
      receivers along Andhra Coast. For this purpose a digital up-linking station
      also functions at Chennai.


Who are the recipients of Cyclone Warnings?

  • Warnings are issued for general public, fishermen, farmers and different categories of users such as central and state government officials responsible for disaster mitigation and relief, industrial and other establishments located in the coastal areas, ports, coastal shipping, railways, aviation, transport, communication and power authorities.

On the event of any doubt about approach of a cyclonic storm to whom a common man can approach to get authentic information (in absence of relevant AIR bulletins)?

  • Normally all collectors of coastal districts (subjected to adverse weather due to cyclonic storm) are intimated by sending warning messages through fax. They in turn inform junior officers under their control to take necessary action. These informations will be therefore available with the state Govt. officials. More over if any one is having phone facilities he may contact nearest cyclone warning centre/ Area cyclone warning centre or Cyclone Warning Division at IMD Head Quarters, New Delhi to get most authentic information about storms over Bay of Bengal. Also one can take advantage of IVRS system to get latest information.


How does IMD keep liaison with State officials?

  • Area Cyclone Warning Centres (ACWCs) and Cyclone Warning Centres (CWCs) maintain liaison with the concerned state Governments in state and district levels on cyclone related activities. The cyclone warning bulletins are communicated to the Chief Secretary, Revenue Secretary, Special Relief Commissioner, State control room, State Disaster Management Authority and concerned district collectors every three hourly. In addition, the Chief Secretary is personally briefed by Director, ACWC/CWC regularly. Before the cyclone season, ACWC/CWC organizes the precyclone preparedness meeting under the chairmanship of Chief Secretary where all the high state Govt. officials from various departments participate.

What are the devastations which can not be protected by a common man and has to be mentally prepared to accept the loss?

  • Inundations caused by storm surge, uprooting of trees and damage caused by that, flooding of low lying areas due to heavy rain and damage to houses and communication due to very strong winds.

Counter reading since 01 June Visitors Last Updated On August 22 2017 22:00:06.