Weather patterns such as La Nina and their impact on India’s weather

Unusually colder and wetter winter and some deep dive into reasons

Weather patterns such as La Nina and their impact on India’s weather

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• The story: In North India, the winter of 2021-22 has been unusually cold and long. Since December 2021, maximum temperatures across the North, Northwest and Central India regions have persistently remained below normal, resulting in “cold day” conditions. Technically, this means more than just a day that is cold. Time to dive in and see what's happening.
• Key points: A "cold day" is one in which the maximum temperature falls below 16 degrees Celsius, a phenomenon that is commonly seen during the winter months in the northern plains of India. Light to moderate intensity rainfall is also commonly seen during winters in neighboring regions of North India. The January of 2022 has seen widespread rain across the central, northwestern, northern, eastern, and northeastern regions of India. As many as 24 states or Union Territories have recorded rainfall varying from excess to large excess this month. December and January are known for the formation of dense fog across North India. In January 2022, the national capital remained affected by fog for 252 hours against a normal of 292 hours. IMD officials said the ongoing winter has recorded the lowest fog hours since 1991-92 over Delhi.
• Reasons: There are many -

1. Cold Winds from Far North - Cold winds from the far north of the country penetrate to lower latitudes, and can reach up to even Telangana and Maharashtra, leading to colder weather, and sometimes to cold wave conditions.
2. Western Disturbances - Until 25th January 2022, seven western disturbances had passed over India — nearly all of them strong enough to cause widespread rain, snowfall, and turbulent weather across large geographical areas between Pakistan and Northeast India. These systems caused hailstorms in northern Maharashtra, and heavy rainfall in Tamil Nadu.
3. La Niña - Frequent and higher numbers of western disturbances are associated with La Niña. At present, moderate intensity La Niña conditions — which manifests itself as cooler than normal sea surface temperatures in the equatorial Pacific Ocean — are prevailing.
4. Low-lying Clouds and Moisture - The presence of low-lying clouds and the availability of moisture along the Indo-Gangetic plains made it favourable for cold day conditions and the additional chill factor experienced during the day time. This was the longest and most intense spell of the season so far.

• Western Disturbance (reduced air pressure): The Western Disturbance (WD), labelled as an extra-tropical storm originating in the Mediterranean, is an area of low pressure that brings sudden showers, snow and fog in northwest India. The disturbance travels from the ‘western’ to the ‘eastern’ direction. These travel eastwards on high-altitude westerly jet streams - massive ribbons of fast winds traversing the earth from west to east. Equilibrium exists in nature due to which the air in a region tries to normalise its pressure. In the term “extra-tropical (outside the tropic) storm”, storm refers to low pressure.
• La Niña: The La Niña events represent periods of below-average sea surface temperatures across the east-central Equatorial Pacific. It is indicated by sea-surface temperature decreased by more than 0.9℉ for at least five successive three-month seasons. The La Nina event is observed when the water temperature in the Eastern Pacific gets comparatively colder than normal, as a consequence of which, there is a strong high pressure over the eastern equatorial Pacific.
• Summary: The unusual cold winter of 2021-22 in North India was due to geographical factors like Western Disturbance and La Niña.  
• EXAM QUESTIONS: (1) What do you understand by ‘Western Disturbances’ and how are they formed? Also explain their impact on India. (2) What do you understand by ‘La Niña’ and how is this effect created? Explain the impact on India.

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