TROPICAL CYCLONES.

  • Tropical cyclones originate over oceans in tropical areas in late summers.
  • They are rapidly rotating violent storms characterised by
    • a closed low-pressure centre with steep pressure gradients
    • a closed low-level atmospheric circulation (winds converging from all directions ― cyclonic circulation),
    • strong winds (squalls ― a sudden violent gust of wind), and
    • a spiral arrangement of thunderstorms that produce very heavy rain (torrential rainfall).
  • The low-pressure at the centre is responsible for the wind speeds.
  • The closed air circulation (cyclonic circulation) is a result of rapid upward movement of hot moist air which is subjected to Coriolis force.

CONDITIONS NECESSARY FOR TROPICAL CYCLONES

  • Large sea surface with temperature higher than 27° C.
  • Presence of the Coriolis force enough to create a cyclonic vortex.
  • A pre-existing weak low-pressure area or low-level-cyclonic circulation.
  • Low wind shear.
  • Upper-level

 

 

  1.  Good Source of Latent Heat
    • Ocean waters having temperatures of 27° C and depth of warm water extending for 60-70 m deep supply enough moisture, and hence latent heat of condensation, to generate and drive a tropical storm.
    • Thick layer of warm water ensures that the deep convection currents within the water do not churn and mix the cooler water below with the warmer water near the surface.
    • tropical cyclones form mostly on the western margins of the oceans Because of warm ocean currents (easterly trade winds drag ocean waters towards west) that flow from east towards west forming a thick layer of warm water with temperatures greater than 27°C.
    • tropical cyclones are very rare on the eastern margins of the oceans because the cold currents lower the surface temperatures of the eastern parts of the tropical oceans making them unfit for the breeding of cyclonic storms.
    • Tropical cyclones weaken on landfall because On landfall, the storm is cut-off from adequate moisture supply and hence it is deprived of latent heat of condensation. Thus, the storm dissipates (weakens or dies off) on landfall.

 

  1. Coriolis Force
    • The Coriolis force is zero at the equator, but it increases with latitude.
    • Coriolis force at 5° latitude is significant enough to create a storm (cyclonic vortex).
    • About 65 per cent of cyclonic activity occurs between 10° and 20° latitude.
    • The cyclonic circulation is anti-clockwise (counter clockwise) in the northern hemisphereand clockwise in the southern hemisphere.

 

 

    • cyclones occur mostly in late summers because :
      • Due to high specific heat of water, and mixing, the ocean waters in northern hemisphere attain maximum temperatures in August (in contrast continents attain maximum temperatures in June-July).
      • Whirling motion (cyclonic vortex) is enhanced when the doldrums (region within ITCZ) over oceans are farthest from the equator (Coriolis force increases with distance from the equator).
    • ‘tropical cyclones’ winds rotate counter-clockwise in the Northern Hemisphere because of following reasons:
      • 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 start 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.
  1. Low-level Disturbances
    • Low-level disturbance is a low-pressure trough (an extended region of low-pressure) that moves from east to west in the form of easterly wave disturbances in the Inter-Tropical Convergence Zone (ITCZ).
    • Easterly wave disturbances act as seedling circulations (birthplace) for a large number of tropical cyclones. However, not all disturbances develop into cyclones.
  1. Temperature Contrast Between Air Masses
    • The convergence of air masses of different temperatures results in instability causing low-level disturbances which are a prerequisite for the origin and growth of violent tropical storms.
    • Trade winds from both the hemispheres meet along the inter-tropical front (ITCZ). Temperature contrasts between these air masses must exist when the ITCZ is farthest from the equator so that the low-level disturbances can intensify into a depression (intensifying low-pressure cell).
  1. Wind Shear
    • Wind Shear is the difference between wind speeds at different altitudes.
    • Tropical cyclones develop when the wind is uniform.
    • Why is convective cyclogenesis (tropical cyclogenesis) confined to tropics?
      • Because of weak vertical wind shear, cyclone formation processes are limited to latitude equatorward of the subtropical jet stream.
      • In the temperate regions, wind shear is high due to westerlies, and this inhibits convective cyclogenesis.
  1. Upper Air Disturbance
    • The remains of an upper tropospheric cyclone from the Westerlies move deep into the tropical latitude regions. As divergence prevails on the eastern side of the troughs, a rising motion occurs; this leads to the development of thunderstorms.
    • Further, these old abandoned troughs (remnants of temperate cyclones) usually have cold cores, suggesting that the environmental lapse rate is steeper and unstable below these troughs. Such instability encourages thunderstorms (child cyclones).

 

ORIGIN AND DEVELOPMENT OF TROPICAL CYCLONES

  • The tropical cyclones have a thermal origin, and they develop over tropical seas during late summers (August to mid-November).
  • At these locations, the strong local convectional currents acquire a whirling motion because of the Coriolis force.
  • After developing, these cyclones advance till they find a weak spot in the trade wind belt.

 

 

 

A)Origin

  • Under favourable conditions, multiple thunderstorms originate over the oceans. These thunderstorms merge and create an intense low pressure system (wind is warm and lighter).

B)Early stage

  • In the thunderstorm, air is uplifted as it is warm and light. At certain height, due to lapse rate and adiabatic lapse rate, the temperature of air falls and moisture in the air undergoes condensation.
  • Condensation releases latent heat of condensation making the air more warmer. It becomes much lighter and is further uplifted.
  • The space is filled by fresh moisture laden air. Condensation occurs in this air and the cycle is repeated as long as the moisture is supplied.
  • Due to excess moisture over oceans, the thunderstorm intensifies and sucks in air at much faster rate. The air from surroundings rushes in and undergoes deflection due to Coriolis force creating a cyclonic vortex (spiralling air column)
  • Due to centripetal acceleration (centripetal force pulling towards the center is countered by an opposing force called centrifugal force), the air in the vortex is forced to form a region of calmness called an eyeat the center of the cyclone. The inner surface of the vortex forms the eye wall, the most violent region of the cyclone.
  • All the wind that is carried upwards loses its moisture and becomes cold and dense. It descends to the surface through the cylindrical eye region and at the edges of the cyclone.
  • Continuous supply of moisturefrom the sea is the major driving force behind every cyclone. On reaching the land the moisture supply is cut off and the storm dissipates.
  • If ocean can supply more moisture, the storm will reach a mature stage.

C)Mature stage

  • It is characterised by multiple spiralling columns or multiple convective cells.
  • The regions with cumulonimbus cloud (rising limbs of convective cell) formation are called rain bands below which intense rainfall occurs.
  • The ascending air throws out moisture at some point and descends back to surface through the calm regions that exist between two rain bands.
  • Cloud formation is dense at the centre. The cloud size decreases from centre to periphery.
  • Rain bands are mostly made up of cumulonimbus clouds. The ones at the periphery are made up of nimbostratus and cumulus clouds.
  • The dense overcast at the upper levels of troposphere is due to cirrus cloud which are  made up of hexagonal ice crystals.

 

 

 

 

STRUCTURE OF TROPICAL CYCLONE

A) EYE

  • The “eye” is a roughly circular area of comparatively light winds and fair weather found at the centre of a severe tropical cyclone.
  • This is the region of no precipitation with clear blue sky.
  • The eye is the region of lowest surface pressure and warmest temperatures (in the upper levels) – 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 kms.

B) EYE WALL

  • The eye is surrounded by the “eye wall”, the roughly circular ring of deep convection, which is the area of highest surface winds in the tropical cyclone. Eye Wall region is the region of fastest winds
  • The eye is composed of air that is slowly sinking and the eye wall 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 (adiabatic) 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.

C) SPIRAL BANDS

  • Another feature of tropical cyclones that probably plays a role in forming and maintaining the eye is the eye wall convection.
  • Convection in tropical cyclones is organized into long, narrow rain bands which are oriented in the same direction as the horizontal wind.
  • Because these bands seem to spiral into the center 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 rain band 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 center and encircles it and the eye and eye wall form.
  • Thus, the cloud-free eye may be due to a combination of dynamically forced centrifuging of mass out of the eye into the eye wall and to a forced descent caused by the moist convection of the eye wall.

D) VERTICAL STRUCTURE OF CYCLONE

There are three divisions in the vertical structure of tropical cyclones.

  • The lowest layer, extending up to 3 km and known as the inflow layer, is responsible for driving the storm.
  • The middle layer, extending from 3 km to 7 km, is where the main cyclonic storm takes place.
  • The outflow layer lies above 7 km. The maximum outflow is found at 12 km and above. The movement of air is anticyclonic in nature.

 

CATAGORIES OF TROPICAL CYCLONE

 

 

BREEDING GROUNDS OF TROPICAL CYCLONES

 

 

  • South-east Caribbean region where they are called
  • Philippines islands, eastern China and Japan where they are called typhoons.
  • Bay of Bengal and Arabian Sea where they are called cyclones, similarly in the south-east African coast and Madagascar-Mauritius islands and in North-west Australia they are called as willy walleyes.

 

WARNING OF TROPICAL CYCLONES

  • Detection of any unusual phenomena in the weather leading to cyclones has three main parameters: fall in air pressure,
    increase in wind velocity, and
    the direction and movement (track) of storm.
  • There is a vast network of weather stations monitoring pressure fall and wind velocities in all countries of the world.
  • The islands attain special significance as they facilitate monitoring of these developments.
  • In India, there are weather radars along both the coasts, although monitoring is also done by aircrafts carrying instruments like weather radar.
  • Cyclone monitoring by satellites is done through very high-resolution radiometers, working in the visual and infra-red regions (for night view) of the spectrum to obtain an image of the cloud cover and its structure.
  • Remote sensing by radars, aircraft and satellites helps predict where exactly the cyclone is going to strike. It helps in taking advance steps in the following areas:
    1. closing of ports and harbours,
    2. suspension of fishing activities,
    3. evacuation of population,
    4. stocking of food and drinking water, and
    5. provision of shelter with sanitation facilities (safety homes).
  • Today, it is possible to detect a cyclone right from its genesis in the high seas and follow its course, giving a warning at least 48 hours prior to a cyclone strike.
  • However, the predictions of a storm course made only 12 hours in advance do not have a very high rate of precision.

 

WHY ONLY FEW CYCLONES OVER ARABIAN SEA THAN BAY OF BENGAL?

  • The average annual frequency of tropical cyclones in the north Indian Ocean (Bay of Bengal and Arabian Sea) is about 5-6 % of the global annual average. (Most of them occur in Western Pacific and Western Atlantic)
  • The frequency is more in the Bay of Bengal than in the Arabian Sea and the ratio being 4:1…….WHY ??

A) More low-level disturbances in Bay of Bengal

  • Cyclones that form over the Bay of Bengal are either those that develop in-situ over southeast Bay of Bengal or remnants of typhoons over Northwest Pacific that 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 higher.
  • The cyclones over the Arabian Sea either originate in-situ over southeast Arabian Sea 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.

B) The surface temperature of Bay of Bengal is higher

  • Surface temperature in the Bay of Bengal is usually between 22°C and 31°C. It is cooler by 1-2°C in the Arabian Sea because of the monsoon winds.

C) Arabian Sea surface has higher salinity

  • Salinity near the surface in Bay of Bengal is as low as 31 ppt as the bay receives lots of freshwater from the Ganga, Brahmaputra, Irrawaddy, Godavari rivers.
  • Salinity near the surface in the Arabian Sea is much higher than in the Bay of Bengal because evaporation over the Arabian Sea is much greater than precipitation and river runoff (it loses more freshwater than it receives).

D) Higher stratification in Bay of Bengal

  • If all the freshwater that the bay receives during a year is accumulated and spread uniformly over its entire surface, it would form a layer over a metre thick.
  • Freshwater is less dense compared to saline water. Hence vertical mixing is inhibited in Bay of Bengal.
  • On the other hand, high evapouration and low inflow of fresh water increases salinity (water becomes denser) at the surface in the Arabian Sea, and this increases vertical mixing.

E) Monsoon winds drive away moisture

  • Though the monsoon winds increase evaporation from the Arabian Sea, the moisture is constantly driven away by the winds towards India.

 

POSITIVE EFFECTS OF CYCLONES

A) Tropical cyclones bring rainfall to rain shadow and other parched regions

  • Rainshadow regions of Western Ghats ad semi-arid regions in south India (Telangana, Rayalaseema, Hyderabad-Karnataka, Vidarbha) sometimes receive copious rain during the cyclone season.

B)  Red Tide in water bodies

  • Red tide is a phenomenon which involves discolouration of coastal waters caused by algal blooms.
  • The algal bloom not only deplete oxygen in the waters, release harmful toxins but also destroys water bodies.
  • As tropical cyclones move across the ocean, winds and waves mix and dilutes the red tide causing bacteria.

C) Replenish Barrier Islands

  • Tropical cyclones take along with them substantial amounts of sand, nutrients and sediment from the ocean’s bottom and bring it towards barrier islands.
  • Storm surge, wind and waves will often move these islands closer to the mainland as sand is pushed or pulled in that direction.

D) Seed dispersal to faraway locations

  • Tropical cyclone wind blow spores and seeds further inland from where they would normally fall; this effect can be seen a thousand miles inland as storms move away from the shoreline. These seeds can replenish lost growth after fires and urbanisation.

 

NAMING OF CYCLONES (UPSC MAINS QUESTION)

  • WMO (World meteorological organisation) divided the world Oceans into Basins and assigned the responsibility of naming the Cyclones to the respective regional bodies.
  • Each regional body has its own rules in naming cyclones.
  • In most regions, pre-determined alphabetic lists of alternating male and female names are used.

A) Why to name them?

  • Since the storms can often last for weeks and more than one cyclone can be occurring in the same region at the same time, names can reduce the confusion about which storm is being described in mass media for the population to get alert.
  • Naming them after a person/flower/animal etc. makes it easier for quick information exchange.

B) Northern Indian Ocean Region

  • The names of cyclones in Indian Seas are not allocated in alphabetical order but are arranged by the name of the country which contributed the name.
  • It is usual practice for a storm to be named when it reaches tropical storm strength (63 kmph).
  • The Indian Meteorological Department (IMD) which issues cyclone advisories to eight countries has a list of names contributed by each of them.
  • Every time a cyclone occurs, a name is picked in the order of the names that are already submitted.
  • Each country gets a chance to name a cyclone. After all the countries get their turn, the next list of names are followed.

C) Regional names of tropical cyclones

Regions

What they are called

Indian Ocean Cyclones
Atlantic Hurricanes
Western Pacific and South China Sea Typhoons
Western Australia Willy-willies

 

TEMPERATE CYCLONES

AIR MASSES

  • An air mass is a large body of air having little horizontal variation in temperature and moisture.
  • They are an integral part of the planetary wind system and are associated with one or other wind belt.
  • They extend from surface to lower stratosphere and are spread across thousands of kilometres.

A) Conditions for the formation of Air Masses

  • Source region should be extensive with gentle, divergent air circulation (gentle anticyclonic circulation).
  • Areas with high-pressure but little pressure difference or pressure gradient are ideal source regions.
  • There are no major source regions in the mid-latitudes as these regions are dominated by frontal cyclones and other disturbances.

B) Air masses based on Source Regions (not important for exams)

There are five major source regions. These are:

  1. Warm tropical and subtropical oceans;
  2. The subtropical hot deserts;
  3. The relatively cold high latitude oceans;
  4. The very cold snow-covered continents in high latitudes;
  5. Permanently ice-covered continents in the Arctic and Antarctica.

Accordingly, following types of airmasses are recognised:

  1. Maritime tropical (mT) ;
  2. Continental tropical (cT);
  3. Maritime polar (mP);
  4. Continental polar (cP);
  5. Continental arctic (cA).

 

 

C) Influence of Air Masses on World Weather

  • The properties of an air mass which influence the accompanying weather are vertical temperature distribution (indicating its stability and coldness or warmness) and the moisture content.
  • The air masses carry atmospheric moisture from oceans to continents.
  • They transport latent heat, thus contributing to latitudinal heat balance.
  • Most of the migratory atmospheric disturbances such as cyclones and storms originate at the contact zone between different air masses called as fronts.
  • Characteristics of the air masses involved determine the weather associated with the disturbances.

FRONTS

  • Understanding front formation and types of fronts is important to understand the formation of mid-latitude cyclones and the dominant weather patterns of mid-latitudes.
  • Fronts are the typical features of mid-latitudes weather (temperate region – 30° – 65° N and S). They are uncommon (unusual) in tropical and polar regions.
  • Front is a three-dimensional boundary zone formed between two converging air masses with different physical properties (temperature, humidity, density).
  • The two air masses don’t merge readily due to the effect of the converging atmospheric circulation, different physical properties, relatively low diffusion coefficient and a low thermal conductivity.

 

 

A) Front Formation

  • The process of formation of a front is known as frontogenesis (war between two air masses), and dissipation of a front is known as frontolysis (one of the air masses win against the other).
  • Frontogenesis involves convergence of two distinct air masses and Frontolysis involves overriding of one of the air masses by another.
  • In northern hemisphere frontogenesis (convergence of air masses) happens in anti-clockwise direction and southern hemisphere, clockwise direction. This is due to Coriolis force.
  • Mid-latitude cyclones (temperate cyclones or extra-tropical cyclones) occur due to frontogenesis.

B) Classification of Fronts

  • Based on the mechanism of frontogenesis and the associated weather, the fronts can be studied under the following types.

 

C) Stationary Front

  • When the surface position of a front on the earth surface does not change a stationary front is formed.
  • The wind motion on both sides of the front is parallel to the front. Warm or cold front doesn’t move towards each other.
  • Once this boundary resumes its forward motion, it becomes a warm front or cold front.

 

  • Weather along a stationary front
    • Cumulonimbus clouds are formed.
    • Overrunning (uplifted air) of warm air along such a front causes frontal precipitation.
    • Frontal cyclones migrating along a stationary front can cause heavy amounts of precipitation, resulting in significant flooding along the front.

D) Cold Front

  • Such a front is formed when a cold air mass replaces a warm air mass by advancing into it or that the warm air mass retreats and cold air mass advances.
  • In such a situation, the transition zone between the two is a steep sloped cold front.
  • Cold front moves up to twice as quickly as warm fronts.
  • Frontolysis begins when the warm air mass is completely uplifted by the cold air mass.

 

  • Weather along a cold front
    • The weather depends on a narrow band of cloudiness and precipitation (because the slope is steep).
    • Severe storms can occur. During the summer months, thunderstorms are common in warm sector.
    • In some regions, tornadoes occur in warm sector.
    • Cold fronts produce sharper changes in weather (because upliftment of warm air is quite rapid).
    • Temperatures can drop more than 15 degrees within the first hour.

E) Warm Front

  • It is a sloping frontal surface along which active movement of warm air over cold air takes place (warm air mass is too weak to beat the cold air mass).
  • Frontolysis (front dissipation) begins when the warm air mass makes way for cold air mass on the ground, i.e. when the warm air mass completely sits over the cold air mass.
  • Weather along a warm front
    • As the warm air moves up the slope, it condenses and causes precipitation but, unlike a cold front, the temperature and wind direction changes are
    • Such fronts cause moderate to gentle precipitation over a large area, over several hours.
    • The passage of warm front is marked by rise in temperature and pressure.
    • There is no cumulonimbus cloud formation along a warm front

F) Occluded Front

  • Occlusion: a process by which the cold front of a rotating low-pressure system catches up the warm front so that the warm air between them is forced
  • Such a front is formed when a cold air mass overtakes a warm air mass and goes underneath it.
  • Thus, a long and backward swinging occluded front is formed which could be a warm front type or cold front type occlusion

 

 

  • Weather along an occluded front
    • Weather along an occluded front is complex—a mixture of cold front type and warm front type weather. Such fronts are common in western
    • The formation mid-latitude cyclones involve the formation of occluded front.

ORIGIN AND DEVELOPMENT OF TEMPARATE CYCLONE

 

 

A) Polar Front Theory

 

  • According to this theory, the warm-humid air masses from the tropics meet the dry-cold air masses from the poles and thus a polar front is formed as a surface of discontinuity.
  • Such conditions occur over sub-tropical high, sub-polar low pressure beltsand along the Tropopause.
  • The cold air pushes the warm air upwards from underneath. Thus a void is created because of lessening of pressure. The surrounding air rushed in to occupy this void and coupled with the earth’s rotation, a cyclone is formed which advances with the westerlies (Jet Streams).
  • In the northern hemisphere, warm air blows from the south and cold air from the north of the front.
  • When the pressure drops along the front, the warm air moves northwards and the cold air move towards south setting in motion an anticlockwise cyclonic circulation (northern hemisphere). Which is due to Coriolis Force.
  • The cyclonic circulation leads to a well-developed extra tropical cyclone, with a warm front and a cold front.
  • The warm air glides over the cold air and a sequence of clouds appear over the sky ahead of the warm front and causes precipitation.
  • The cold front approaches the warm air from behind and pushes the warm air up. As a result, cumulus clouds develop along the cold front. The cold front moves fasterthan the warm front ultimately overtaking the warm front. The warm air is completely lifted up and the front is occluded (occluded front) and the cyclone dissipates.
  • The processes of wind circulation both at the surface and aloft are closely interlinked.
  • So temperate cyclone is intense frontogenesisinvolving mainly occlusion type front.
  • Normally, individual frontal cyclones exist for about 3 to 10 days moving in a generally west to east direction.
  • Precise movement of this weather system is controlled by the orientation of the polar jet stream in the upper troposphere.

 

 

B) Seasonal Occurrence of Temperate Cyclones

  • The temperate cyclones occur mostly in winter, late autumn and spring. They are generally associated with rainstorms and cloudy weather.
  • During summer, all the paths of temperate cyclones shift northwards and there are only few temperate cyclones over sub-tropics and the warm temperate zone, although a high concentration of storms occurs over Bering Strait, USA and Russian Arctic and sub-Arctic zone.

C) Distribution of Temperate Cyclones

  • USA and Canada – extend over Sierra Nevada, Colorado, Eastern Canadian Rockies and the Great Lakes region,
  • the belt extending from Iceland to Barents Sea and continuing over Russia and Siberia,
  • winter storms over Baltic Sea,
  • Mediterranean basin extending up to Russia and even up to India in winters (called western disturbances) and the Antarctic frontal zone.

 

 

D) Associated Weather

  • The approach of a temperate cyclone is marked by fall in temperature, fall in the mercury level, wind shifts and a halo around the sun and the moon, and a thin veil of cirrus cloud.
  • A light drizzle follows which turns into a heavy downpour. These conditions change with the arrival of the warm front which halts the fall in mercury level and the rising temperature.
  • Rainfall stops and clear weather prevails until the cold front of an anticyclonic character arrives which causes a fall in temperature, brings cloudiness and rainfall with thunder. After this, once again clear weather is established.
  • The temperate cyclones experience more rainfall when there is slower movement and a marked difference in rainfall and temperature between the front and rear of the cyclone. These cyclones are generally accompanied by anticyclones.

 

COMPARISON OF TROPICAL AND TEMPERATE CYCLONES

Tropical Cyclone Temperate Cyclone
Origin
  • Thermal Origin.
  • Dynamic Origin: Coriolis Force, Movement of air masses.
Latitude
  • Confined to 10-30º N and S of equator.
  • Confined to 35-65º N and S of equator.
  • More pronounced in Northern hemisphere due to greater temperature contrast.
Frontal system
  • Absent.
  • The very cyclone formation is due to frontogenesis. (Occluded Front).
Formation
  • They form only on seas with temperature more than 26-270 C.
  • They dissipate on reaching the land.
  • Can form both on land as well as seas.
Season
  • Seasonal: Late summers (Aug-Nov).
  • Irregular. But few in summers and more in winters.
Size
  • Limited to small area.
  • Typical size: 100 – 500 kms in diameter.
  • Varies with the strength of the cyclone.
  • They cover a larger area.
  • Typical size: 300 – 2000 kms in diameter. Varies from region to region.
Shape
  • Elliptical
  • Inverted ‘V’
Rainfall
  • Heavy but does not last beyond a few hours.
  • If the cyclone stays at a place, the rainfall may continue for a few days.
  • In a temperate cyclone, rainfall is slow and continues for many days, sometimes even weeks.
Wind Velocity and destruction
  • Much greater.
  • 100 – 250 kmph
  • Comparatively low.
  • Typical range: 30-150 kmph.
  • 200 – 1200 kmph in upper troposphere)
  • Greater destruction due to winds, storm surges and torrential rains.
  • Less destruction due to winds but more destruction due to flooding.
Isobars
  • Complete circles and the pressure gradient is steep
  • Isobars are usually ‘V’ shaped and the pressure gradient is low.
Lifetime
  • Doesn’t last for more than a week
  • Lasts for 2-3 weeks.
Path
  • East – West. Turn North at 200 latitude and west at 300 latitude.
  • Move away from equator.
  • The movement of Cyclones in Arabian Sea and Bay of Bengal is a little different.
  • Here, these storms are superimposed upon the monsoon circulation of the summer months, and they move in northerly direction along with the monsoon currents.
  • West – East (Westerlies; Jet Streams).
  • Move away from equator.

 

Temperature distribution
  • The temperature at the centre is almost equally distributed.
  • All the sectors of the cyclone have different temperatures
Calm region
  • The center of a tropical cyclone is known as the eye. The wind is calm at the centre with no rainfall.
  • In a temperate cyclone, there is not a single place where winds and rains are inactive.

Driving force
  • The tropical cyclone derives its energy from the latent heat of condensation, and the difference in densities of the air masses does not contribute to the energy of the cyclone.
  • The energy of a temperate cyclone depends on the temperature, humidity and density differences of air masses.

Influence of Jet streams
  • The relationship between tropical cyclones and the upper level air-flow is not very clear.
  • The temperate cyclones, in contrast, have a distinct relationship with upper level air flow (jet streams, Rossby waves etc.)
Clouds
  • The tropical cyclones exhibit fewer varieties of clouds – cumulonimbus, nimbostratus, etc.
  • The temperate cyclones show a variety of cloud development at various elevations.
Surface anti-cyclones
  • The tropical cyclones are not associated with surface anticyclones and they have a greater destructive capacity.
  • The temperate cyclones are associated with anticyclones which precede and succeed a cyclone.
  • These cyclones are not very destructive.
Influence on India
  • Both coasts affected. But east coast is the hot spot.
  • Bring rains to North-West India.
  • The associated instability is called ‘Western Disturbances’.
Weather Prediction
  • Tough as the movement can be erratic due to a lot of factors.
  • Easy because of the general westerly path of the cyclone, less variable jet stream path and simple frontal system.

 

 

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