Sunday, October 13, 2013

The term cyclone refers to a wide variety of broad, low pressure system with cyclonic rotation, that is counterclockwise if the system is in the northern hemisphere and clockwise in the southern hemisphere. 

  • The first type to explore is called a tropical cyclone
  • These are the most intense cyclones in terms of both pressure (in all cyclones, a lower pressure means a stronger storm) and wind speed. 
  • Tropical cyclones originate from disorganized areas of low pressure that move over warm ocean water, where they feed on moisture and become stronger and better organized. 
  • These storms can only develop over warm ocean water and weaken rapidly if they hit land. 

  • These tropical systems tend to be called cyclones in the southern hemisphere, and they rotate in a clockwise direction. 
  • In the northern hemisphere, where cyclones occur in the Atlantic Ocean, Caribbean and northeastern Pacific Ocean they are called hurricanes and those in the South China Sea and regions of Asia are called typhoons
  • Northern hemisphere cyclones rotate anti-clockwise. 
  • Weaker tropical cyclones in these regions are referred to as tropical storms if sustained winds are between 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 often produce heavy flooding. Tropical cyclones usually form in the summer of early autumn when the oceans are warmest. 

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 classified according to the Saffir-Simpson Scale, which is also based on wind speed - not wind gusts, but rather sustained wind speeds. 

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


Next up are mid-latitude or extratropical cyclones. 

  • Unlike tropical cyclones these systems are powered by temperature gradients, or changes in temperature across a distance. 
  • Because they do not need as much moisture as tropical cyclones these systems can develop over land or over cool water. 
  • These systems form along fronts, or boundaries between air masses of different temperatures and dew points (a measure of absolute humidity). 
  • As a mid-latitude cyclone develops, a warm front will usually develop on the western side of the low while a cold front develops on the equator ward side (this is the southern side in the northern hemisphere and northern side in the southern hemisphere). 
These systems are usually not as intense as tropical cyclones but are typically larger in geographic extent, especially if you include the fronts they connect with. They do sometimes produce gale force and even hurricane force winds, especially at sea. Unlike tropical 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, flooding, or outbreaks of severe weather and tornadoes

There are also subtropical cyclones, which have characteristics of both tropical and extratropical cyclones. These typically follow the same naming conventions as tropical cyclones. A cyclone can transition between these three types.


  • A mesocyclone is a dense, swirling pack of 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 from 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 of these turning into tornadoes.

Polar or Arctic Cyclones
  • Arctic or polar cyclones occur in Antarctic regions and can reach up to 1,200 miles wide. 
  • Polar cyclones differ with others because they are not seasonal. 
  • They can occur at any time of the year, unlike in the Gulf of Mexico when during late summer, the risk of a hurricane increases. 
  • Polar cyclones can also form quickly (sometimes less than 24 hours), and their direction or movement cannot be predicted. 
  • Plus, they can last from a day up to several weeks. 
  • Most frequently, polar cyclones develop above northern Russia and Siberia.

Some terms related to Tropical understand it furthur !!!

What is a "CDO"?
  • "CDO" is an acronym that stands for "central dense overcast"
  • This is the cirrus cloud shield that results from the thunderstorms in the eyewall of a tropical cyclone and its rainbands. 
  • Before the tropical cyclone reaches very severe cyclonic storm (64 knots,), typically the CDO is uniformly showing the cold cloud tops of 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 of the satellites. 
  • Tropical cyclones that have nearly circular CDO's are indicative of favourable, low vertical shear environments.

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.
1.     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). 
2.     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. 

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.

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 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 magnitudev2/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.

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).
  • 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. 

Pressure deficient
Associated wind speed
Knots (Kmph)
Low pressure area
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)
48-63 (90-119)
Very Severe Cyclonic Storm
64-119 (119-220)
Super Cyclonic Storm

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.

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