7 PM | Why this winter is extra cold| 28th December 2019

Context: Cold wave in Northern India.

More in news:

• India Meteorological Department (IMD) issued warning of cold day to severe cold day conditions in many pockets are very likely over Punjab, Haryana, Chandigarh, Delhi and Uttar Pradesh.
• Dense fog is very likely to engulf Uttar Pradesh, some pockets over Punjab and Haryana, Chandigarh, Delhi and also a few parts of north Rajasthan.

Definitions:

• Cold Day: A cold-day condition is said to prevail when the maximum temperature during the day is at least 4.5°C below normal.
• Severe Cold Day: If the maximum temperature is at least 6.5°C below normal, it is classified as a severe cold day.
• Cold wave: Cold wave is characterized by a rapid and marked fall of temperature. The term ‘cold’ describes an unusual fall in temperature that is triggered by the transport of cold air masses into a specific area.  The “wave” in cold wave is apparent in the upper-air flow (the jetstream), which is usually amplified into a strong ridge-trough pattern during a major cold outbreak. Cold waves affect much larger areas than blizzards, ice storms, and other winter hazards. 

Formation of Cold Waves: 

• The core requirement of a cold wave at the surface is a strong high pressure center that forms during winter in high latitudes.
• Cold polar or Arctic air masses are relatively shallow, extending one to several kilometre above the surface.

The Fact-Sheet of normal winters:

• In December, the maximum daily temperature does not rise beyond 16-18°C in most of Punjab, Haryana, Himachal Pradesh and western Uttar Pradesh.
• In Delhi and northern Rajasthan, daily maximum temperatures are usually not over 20-22°C for most of December.

The ‘Unusual Winters’:

• The northern region is expected to record its second-coldest December since 1901.
• As per IMD, the mean maximum temperature for December was less than 20 degrees Celsius only in 1919, 1929, 1961 and 1997.
• The most intense cold day when maximum temperatures fell 7°to over 12°C below normal was on December 25 over Punjab, Haryana, Chandigarh, Delhi, Uttar Pradesh, north Rajasthan and some isolated areas in Bihar and Madhya Pradesh.

India Meteorological Department (IMD):

• The IMD is the national meteorological service of the country and it is the chief government agency dealing in everything related to meteorology, seismology and associated subjects.
• It was formed in 1875.
• The IMD is under the Ministry of Earth Sciences, Government of India. 
• IMD mandate:
  • To take meteorological observations and provide current information and forecasting information for the most favorable operation of weather-dependent activities such as irrigation, agriculture, aviation, shipping, offshore oil exploration and so on.
  • To warn against severe weather phenomenon such as tropical cyclones, dust storms, norwesters, heat waves, cold waves, heavy rains, heavy snow, etc.
  • To provide meteorological related statistics needed for agriculture, industries, water resources management, oil exploration, and any other strategically important activities for the country.
  • To engage in research in meteorology and allied subjects.
  • To detect and locate earthquakes and evaluate seismicity in various parts of the country for developmental projects.

Probable causes of such extreme weather:

• Climate Change: The unusually cold December this year could just be another instance of extreme climates becoming more and more frequent, a result of climate change. 
  • Across the world, the frequency and intensity of both heatwaves and cold waves have increased in the last few years, and are predicted to increase further.
  • The same is the case with extreme rainfall and drought.
  • Just this year, India experienced an unusually wet August and September. The amount of rainfall that September produced was a once-in-a-century event.
  • Other such extreme events across world:
    • Australia is witnessing the hottest december. The heat wave resulted in early bush fires. The average maximum temperature across the country hit 41.9 degrees Celsius.
    • A cold wave conditions prevails in northern Afghanistan, Tajikistan, Uzbekistan, Turkmenistan and parts of northern Iran. 
    • An unusual very warm period develops across east-central and southern  Europe.
    • In an another unusual event, a massive patch of water (called as Heat Blob) heating up off the eastern coast of New Zealand is attracting the attention of weather watchers, and is one of the hottest sea spots on the planet right now.
• Western Disturbances: Frequent western disturbances varying from moderate to intense have mainly contributed to the severe cold over all of north India this year.
• In addition, the flow of northwesterly winds over northwest India, that too over much lower levels, further fuelled the chill factor, making the days much colder than normal during December.
• This December also witnessed haze, fog and rainfall after the passing of each western disturbance, triggering cold weather conditions over north India.
• Low Clouds: This extended cold spell has been triggered due to low stratus clouds that are blanketed over a large geographical area between Pakistan, cutting across India and running up to Bangladesh. Similarly, it is prevailing over a stretch of 500 km to 800 km north-south, affecting the entire north India.

Western Disturbances:

• Western Disturbance is a frequently used terminology to describe weather in the Indian sub-continent.
• Western Disturbance can be defined as “a low pressure area or a trough over surface or the upper-air in the westerly winds regime, north of 20°N, causing changes in pressure, wind pattern and temperature fields. It is accompanied by cloudiness, with or without precipitation.”
• Origin: Western Disturbances originate in the Caspian Sea or the Mediterranean Sea as extra-tropical cyclones. They gradually travel across the middle-east from Iran, Afghanistan and Pakistan to enter the Indian sub-continent.

• Time of strong Western disturbances: Though WDs move across the Indian region throughout the year, they are in their peak during winter months of January and February. Their effect is minimal during the monsoon months in India.
• Western Disturbances are the principle rain producing systems during non-monsoonal months over Northwest India including Punjab, Haryana, Uttar Pradesh and Delhi. Their effect sometime extends up to Gangetic plains and Northeast India.
• They are also responsible for bringing snowfall in the higher reaches of Jammu & Kashmir, Himachal Pradesh and Uttarakhand.
• Rabi Crop: Western Disturbance brings winter and pre-monsoon rain and is important for the development of the Rabi crop in the Northern subcontinent. Considering that wheat is one of the most important Rabi crops, which is the staple diet of people in this region, winter showers contribute to meet India’s food security.
• Weak western disturbances are associated with crop failure and water problems across north India.

Source: https://indianexpress.com/article/explained/why-this-winter-is-extra-cold-6188129/