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Cyclones and western disturbances By Lohit Saini

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SUBMITTED BY: LOHIT SAINI
SUBMITTED TO: Dr. SHILPI VERMA
What are Cyclones ?
"Cyclone" is an intense whirl in the atmosphere with very
strong winds circulating around it in anti-clockwise
direction in the Northern Hemisphere and in clockwise
direction in the Southern Hemisphere.
Cyclones are intense low pressure areas – from the centre of
which pressure increases outwards.
The amount of the pressure drop in the centre and the rate at
which it increases outwards gives the intensity of the cyclones
and the strength of winds.
Cyclones are classified as:
(i) Extra tropical cyclones (temperate cyclones)
(ii) Tropical cyclones.
•Extra Tropical cyclones occur in temperate zones and high
latitude regions, though they are known to originate in the polar
regions.
•Tropical Cyclones that develop in the regions between the tropics
of Capricorn and Cancer.
•Tropical cyclones are large-scale weather systems developing over
tropical or subtropical waters, where they get organized into surface
wind circulation.
•Tropical cyclones are the progeny of ocean and atmosphere, powered
by the heat from the sea, driven by the easterly trades and temperate
westerlies, the high planetary winds and their own fierce energy.
•Tropical cyclones are typically between 100 and 2,000 km in diameter.
Global Tropical Cyclone tracks
Cyclones and western disturbances By Lohit Saini
Cyclones are given many names in different regions of
the world – they are known as :
•Typhoons in the China Sea and Pacific Ocean
• Hurricanes in the West Indian islands in the Caribbean Sea
and Atlantic Ocean
• Tornados in the Guinea lands of West Africa and the
southern USA
• Willy-willies in north-western Australia
•Tropical cyclones in the Indian Ocean.

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Cyclones and western disturbances By Lohit Saini

  • 1. SUBMITTED BY: LOHIT SAINI SUBMITTED TO: Dr. SHILPI VERMA
  • 2. What are Cyclones ? "Cyclone" is an intense whirl in the atmosphere with very strong winds circulating around it in anti-clockwise direction in the Northern Hemisphere and in clockwise direction in the Southern Hemisphere. Cyclones are intense low pressure areas – from the centre of which pressure increases outwards. The amount of the pressure drop in the centre and the rate at which it increases outwards gives the intensity of the cyclones and the strength of winds.
  • 3. Cyclones are classified as: (i) Extra tropical cyclones (temperate cyclones) (ii) Tropical cyclones. •Extra Tropical cyclones occur in temperate zones and high latitude regions, though they are known to originate in the polar regions. •Tropical Cyclones that develop in the regions between the tropics of Capricorn and Cancer. •Tropical cyclones are large-scale weather systems developing over tropical or subtropical waters, where they get organized into surface wind circulation. •Tropical cyclones are the progeny of ocean and atmosphere, powered by the heat from the sea, driven by the easterly trades and temperate westerlies, the high planetary winds and their own fierce energy. •Tropical cyclones are typically between 100 and 2,000 km in diameter.
  • 6. Cyclones are given many names in different regions of the world – they are known as : •Typhoons in the China Sea and Pacific Ocean • Hurricanes in the West Indian islands in the Caribbean Sea and Atlantic Ocean • Tornados in the Guinea lands of West Africa and the southern USA • Willy-willies in north-western Australia •Tropical cyclones in the Indian Ocean.
  • 8. Structure There are a number of structural characteristics common to all cyclones. •A cyclone is a low-pressure area. A cyclone's center (often known in a mature tropical cyclone as the eye), is the area of lowest atmospheric pressure in the region. • Near the center, the pressure gradient force and the force from the Coriolis effect must be in an approximate balance, or the cyclone would collapse on itself as a result of the difference in pressure. •Because of the Coriolis effect, the wind flow around a cyclone is anticlockwise in the NH and clockwise in the SH.
  • 10. Cyclone in Northern Hemisphere Counter clockwise Direction Cyclone in Sothern Hemisphere Clockwise Direction
  • 11. •Eye – A region 30-65 km in diameter found at the center where skies are often clear, winds are light, and the storm's lowest pressure readings are obtained. •Eye Wall – Dense cloud surrounding eye having maximum wind speed. It causes maximum destruction. •Spiral rain bands – Band of clouds spiraling into eye wall and it contains heavy rains. Thunderstorms are observed here.
  • 12. Conditions for the formation of Tropical Cyclone 1. Warm ocean waters at least (26.5°C) to provide the heat. 2. Availability of moisture. Moisture is necessary for providing latent heat of condensation. 3. Coriolis force to provide the rotation 4. A seedling or cyclonic weather disturbance with extra cloud cover. 5. Almost uniform large scale wind with little vertical shear to allow heat to accumulate (less than 10m/s). 6. An exhaust system consisting of a divergent upper level disturbance.
  • 13. Stages Of Development Tropical Disturbance: The birth of a hurricane, having only a slight circulation with no closed isobars around an area of low pressure. Tropical disturbances commonly exist in the tropical trade winds at any one time and are often accompanied by clouds and precipitation. Tropical Depression: A tropical cyclone in which the maximum sustained wind speed less than 64 KMPH. Depressions have a closed circulation.
  • 14. Tropical Storm A tropical storm has a maximum sustained surface wind speed between 64 KPH and 117 KPH . The convection in tropical storms is usually more concentrated near the center with outer rainfall organizing into distinct bands. Typhoon or Hurricane Typhoon has a maximum winds exceed 117 KPH.
  • 16. A Western Disturbance is an extra-tropical storm originating in the Mediterranean region that brings sudden winter rain to the north-western parts of the Indian subcontinent. •It is a non-monsoonal precipitation pattern driven by the westerlies. The moisture in these storms usually originates over the Mediterranean Sea and the Atlantic Ocean. •Extra-tropical storms are a global phenomena with moisture usually carried in the upper atmosphere. •In the case of the Indian subcontinent, moisture is sometimes shed as rain when the storm system encounters the Himalayas.
  • 17. •Although the western disturbances cause meagre rainfall, even this little amount of rainfall is highly beneficial to the rabi crops, especially wheat. •The precipitation is in the form of light rains in the plains and heavy snowfall in the western Himalayas, It is this snow that sustains the flow of water in’ the Himalayan rivers during the summer months.
  • 18. Formation Western disturbances originate in the Mediterranean region. A high-pressure area over Ukraine and neighbourhood consolidates, causing the intrusion of cold air from polar regions towards an area of relatively warmer air with high moisture. This generates favorable conditions for cyclogenesis in the upper atmosphere, which promotes the formation of an eastward-moving extratropical depression. Traveling at speeds up to 12 m/s (43 km/h), the disturbance moves towards the Indian subcontinent until the Himalayas inhibits its development, upon which the depression rapidly weakens. The Western Disturbances are embedded in the mid-latitude Subtropical Westerly Jet.
  • 20. Significance and Impact •Western disturbances, specifically the ones in winter, bring moderate to heavy rain in low-lying areas and heavy snow to mountainous areas of the Indian Subcontinent. •They are the cause of most winter and pre-monsoon season rainfall across northwest India. •Precipitation during the winter season has great importance in agriculture, particularly for the rabi crops. • Wheat among them is one of the most important crops, which helps to meet India's food security. •An average of four to five western disturbances form during the winter season. •The rainfall distribution and amount varies with every western disturbance.
  • 21. •Western disturbances are usually associated with cloudy sky, higher night temperatures and unusual rain. Excessive precipitation due to western disturbances can cause crop damage, landslides, floods and avalanches. •Over the Indo-Gangetic plains, they occasionally bring cold wave conditions and dense fog. • These conditions remain stable until disturbed by another western disturbance. •When western disturbances move across northwest India before the onset of monsoon, a temporary advancement of monsoon current appears over the region. •The strongest western disturbances usually occur in the northern parts of Pakistan, where flooding is reported number of times during the winter season.
  • 22. The main difference between the Western Disturbances and Tropical cyclone are as follows: •Western disturbance are caused by Westerlies wind whereas Tropical Cyclone is caused by low pressure influx of strong wind which are tropically present anywhere over or nearby large water body. •Western disturbance is non-monsoon phenomena of precipitation observed whereas the tropical cyclone is monsoon phenomena of precipitation. • The Western disturbance is an extra tropical phenomenon whereas the tropical cyclone is intra tropical phenomena. •Sometimes, the rain is shed when struck over mountains in Western disturbance whereas rain is a usual act in tropical cyclone