about cyclones

8/10/2019 About Cyclones

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All you wanted to know about Cyclones :::: GEOGRAPHY

The term cyclone refers to a wide variety of broad, low

pressure system with cyclonic rotation, thatis counterclockwise if t he system is in the nort hern

hemisphere and clockwise in the southern

hemisphere.

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The f irst t ype to explore is called a tropical cyclone .

These are the mos t intense cyclones in terms o f both pressure (in all cyclones, a lower pressure

means a stronger storm) and wind speed.

Tropical cyclones o riginate f rom disorganized areas o f low pressure that move over warm ocean

water, where they feed on moisture and become stro nger and better organized.

These sto rms can only develop over warm ocean water and weaken rapidly if they hit land.
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These tro pical systems tend to be called cyclonesin the so uthern hemisphere, and they rotate in a

clockwise direction.

In the northern hemisphere, where cyclones occur in the Atlantic Ocean, Caribbean and northeastern

Pacif ic Ocean they are called hurricanesand those in the South China Sea and regions o f Asia are

called typhoons.

Northern hemisphere cyclones rotate ant i-clockwise.

Weaker tropical cyclonesin these regions are referred to astropical storms if sustained winds

are be tween 63 and 118 km/h(39 and 73 mph) and as tropical depressions if winds are less

than 63 km/h.

In addition to producing damaging winds these systems of ten produce heavy flooding. Tropical

cyclones usually f orm in the summer of early autumn when the oceans are warmest .


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Cyclone intensity is measured by wind speed, and cyclones are accorded a category based on this:

Hurricanes, which are cyclones in the northern hemisphere are class if ied according to the Saffir-Simpson

Scale,which is also based on wind speed - not wind gusts , but rather sustained wind speeds.

The highest class if ication on this scale is category 5, in which sustained winds exceed 251 km/h (156 mph).


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Next up are mid-latitudeor extratropicalcyclones.

Unlike tropical cyclones these systems arepowered by temperature gradients, or changes in

te mperature across a distance.

Because theydo not need as much moisture as tropical cyclones these systems can develop

over land or over cool wate r.

These systems f orm along f ronts , or boundaries betweenair masses of different temperatures

and dew points (a measure of absolute humidity).

As a mid- latitude cyclone develops, a warm f ront will usually develop on the western s ide of the low

while a cold f ront develops on the equator ward side (this is the so uthern side in the northern

hemisphere and northern side in the southern hemisphere).

These systems are usually not as intense as tropical cyclonesbut are typically larger in geographic

extent, especially if you include the f ronts t hey connect with. They do so metimes produce gale fo rce and

even hurricane force winds, especially at sea. Unlike tro pical cyclones these cyclones usually occur in fall,

winter, or spring, when temperature contrasts are greatest.

Depending on where and when they occur these systems can cause blizzards, f looding, or outbreaks

of severe weather and tornadoes.

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There are also subtropicalcyclones, which have characterist ics of both tro pical and extratro pical

cyclones. These typically f ollow the same naming conventions as t ropical cyclones. A cyclone can transit ion

between these three types.


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Mesocyclone

A mesocyclone is a dense, swirling pack o f cloud and winds between half a mile and six miles wide.

To the eye, it looks like a thin, vertical band of black clouds that spins f rom beneath thunderclouds.

A mesocyclone turns into a tornado if it hits the ground and continues to churn up wet , warm air.The United States experiences approximately 1,700 mesocyclones a year, with 50 percent o f these

turning into to rnadoes.

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Polar or Arctic Cyclones

Arctic or polar cyclones occur in Antarct ic regions and can reach up to 1,200 miles wide.

Polar cyclones dif f er with others because they are not seaso nal.

They can occur at any time of the year, unlike in the Gulf of Mexico when during late summer, the riskof a hurricane increases.

Polar cyclones can also f orm quickly (sometimes less than 24 hours), and their direction or

movement cannot be predicted.

Plus, they can last f rom a day up to several weeks.

Mos t f requently, polar cyclones develop above northern Russia and Siberia.

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Miscellaneous

Some terms related to Tropical Cylcones ...to understand it furthur !!!

What is a "CDO"?

"CDO"is an acronym that s tands f or "central dense overcast".

This is the cirrus cloud shield that results f rom the thundersto rms in the eyewall of a tropical cyclone

and its rainbands.

Bef ore the t ropical cyclone reaches very severe cyclonic sto rm (64 knots ,), typically the CDO is

unifo rmly showing the cold cloud tops o f the cirrus with no eye apparent.

Once the storm reaches the hurricane strength threshold, usually an eye can be seen in either the

infrared or visible channels o f the satellites.

Tropical cyclones that have nearly circular CDO's are indicative of f avourable, low vertical shear

environments.

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

bands"?


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EYE

The "eye"is a roughly circular area of comparatively light winds and f air weather f ound at the centre

of a severe tro pical cyclone.

Although the winds are calm at the axis of rotat ion, strong winds may extend well into the eye.

There is little or no precipitat ion and sometimes blue sky or s tars can be seen.

The eye is the region of lowest surface pressure and warmest t emperatures alof t - the eye

temperature may be 10C warmer or more at an altitude of 12 km than the surro unding environment,

but only 0-2C warmer at the surface in the tropical cyclone.

Eyes range in size f rom 8 km to over 200 km across, but mos t are approximately 30-60 km in

diameter.

EYEWALL

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 f low as a result

of many moderate - o ccasionally strong - updraf ts and downdraf ts .

The eye's warm te mperature s 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 - suggest ing that the sinking in the eye typically does not reach the ocean surf ace,

but instead only gets to around 1-3 km of the surf ace.

1. The exact mechanism by which the eye f orms remains somewhat controvers ial. One idea suggests

that the eye forms as a result of the downward directed pressure gradient asso ciated with the weakening

and radial spreading of the tangential wind f ield with height (Smith, 1980).

2. Another hypothesis suggests that the eye is formed when latent heat release in the eyewall occurs,

f orcing subs idence in the storm's centre (Shapiro and Willoughby, 1982).


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It is poss ible that t hese hypotheses are not incons istent 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.

SPIRAL BANDS

Another f eature o f tropical cyclones that probably plays a ro le in f orming and maintaining t he eye isthe 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 t hese bands, low-level convergence is a maximum, and theref ore, upper-level divergence is

mos t pronounced above.

A direct circulat ion develops in which warm, moist air converges at the surf ace, ascends through

these bands, diverges alof t, 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, adiabat ic warming takes place, and the air dries.

Because subsidence is concentrated on the inside of the band, the adiabatic warming is st ronger

inward f rom the band causing a sharp contrast in pressure falls across t he band since warm air is

lighter than cold air.

Because of the pressure falls o n the inside, the tangential winds around the t ropical 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- f ree eye may be due to a combination of dynamically f orced centrif uging of mass o ut o f

the eye into t he eyewall and to a f orced descent caused by the moist convection of the eyewall.

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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 t roposphere (above 9

km).

This results in large vertical wind shear. Strong vertical wind shear inhibits cyclone development.

Also the potential zo ne f or the development o f cyclones shif ts to North Bay of Bengal during

southwest monsoon season.

During this seaso n, the low pressure system upto the intensity of depress ions f orm along the

monsoo n trough, which extends f rom northwest India to the nort h Bay of Bengal.

The Depression f orming 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 f or t heir non-

intensif ication into intense cyclones.

What is the life pe riod of cyclones? Which t ropical cyclone lasted the longest?


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Life period of a Tropical Cyclone over the nort h Indian Ocean is 5-6 days.

It will have hurricane intensity f or 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-15thNov, 1886 &

16th- 29thNov, 1964).

Hurricane/Typhoon John lasted 31 days as it t raveled both the Northeast and Northwest Pacif ic

basins during August and September, 1994. (It f ormed in the Northeast Pacif ic, reached hurricane

f orce there, moved across the dateline and was renamed Typhoo n John, and then f inally recurvedback across t he dateline and renamed Hurricane John again.)

Hurricane Ginger was a t ropical cyclone f or 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 monitore d by IMD?

IMD has a well-established and time-tested organizat ion f or monito ring and forecast ing tropicalcyclones.

A good network o f meteoro logical observatories (both surf ace and upper air) is o perated by IMD,

covering the entire coastline and islands.

The conventional observations are supplemented by observational data f rom automatic weathe r

stations (AWS), radar and satellite systems.

INSAT imageryobtained at hourly intervals during cyclone situations has proved to be immensely

usef ul in monito ring the development and movement o f cyclones.

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

Hemisphere?

As the earth's ro tation sets up an apparent force (called the Coriolis force) that pulls the winds

to the right in the Northern Hemisphere (and to the lef t in the Southern Hemisphere).

So, when a low pressure starts to f orm over nort h of the equato r, the surface winds will f low inward

trying to f ill in the low and will be def lected to the right and a counter-clockwise rotation will be

initiated. The opposite (a def lection to the lef t and a clockwise rotation) will occur south of the

equator.

This Coriolis f orce is too tiny to ef f ect ro tation in, for example, water that is going down the drains

of sinks and to ilets.

The rotation in those will be determined by the geometry of the container and the original mot ion of

the water.

Thus, one can f ind both clockwise and counter-clockwise f lowing drains no matter what hemisphere

you are located. If you don't believe this, test it out f or yourself .

What causes each cyclone to have a different maximum wind speed f or a given minimum sea- levelpressure?

The basic horizo ntal balance in a tro pical cyclone above the boundary layer is between the sum of

the Coriolis 'accelerat ion' and the centripet al 'acceleration', balanced by the horizontal


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pressure gradient force.

This balance is referred to as gradient balance, where the Corio lis 'acceleration' is def ined as t he

horizo ntal velocity o f an air parcel, v, times the Coriolis parameter, f. Centripetal 'f orce' is def ined as

the acceleration on a parcel of air moving in a curved path, directed toward the centre of curvature of

the path, with magnitudev2/r, where vis the horizontal velocity of the parcel and rthe radius o f

curvature of the path.

The centripetal force alters the original two-f orce geost rophic balance and creates a non-

geostrophic gradient wind.

The reason that dif f erent peak winds can result in dif f erent central pressures is caused by the fact

that the radius, r, of the peak wind varies. A st orm 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.

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What is a Storm Surge?

Storm Surge is anabnormal rise of sea level as the cyclone crosses t he coast.Sea water inundates the coastal st rip causing loss o f life, large scale destruction to property &

crop.

Increased salinity in the soil over af f ected area makes the land unf it f or agricultural use for two or

three seasons.

Storm surge depends o n intensity o f the cyclone (Maximum winds and lowest pressure associated

with it and Coastal bathymetry (shallower coast line generates surges of greater heights).

The st orm surge is predicted by IMD using nomograms and dynemic model developed by IIT,

Delhi.Both these models taken into consideration dif f erent characteristics, the cyclones and the

coastal bathymetry to predict the sto rm surge.

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System Pressure deficienthPa

Associated wind speedKnots (Kmph)

Low pressure area 1.0 119(>220)

What are t he super cyclone, super- typhoon, a major hurricane and an inte nse hurricane?

When themaximum sustained 3 minutes surface winds are more than 119 knots, the low


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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 t yphoons

that reach maximum sustained 1 minute surf ace winds of at least 130 knot s (65 m/s).

This is the equivalent o f a st rong Saff ir-Simpson category 4 or category 5 hurricane in the Atlantic

basin or a category 5 severe tro pical cyclone in the Australian basin.

about cyclones

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