English Free Article | Weather n Climate | VOL. 12, ISSUE 75, November-December 2012 |

Cold Waves

Weather scientists claim that extreme weather events will be increasingly palpable, throwing life out of gear, more often now than before. Cold waves too fall under such extreme events, which can severely and fatally affect the very young, the elderly and the infirm. Occurrences of extreme low temperature in association with incursion of dry cold winds from north into the sub-continent are defined as cold waves. A region is said to be experiencing a cold wave when, according to the India Meteorological Department (IMD), temperatures dip by more than four degrees below normal. The northern parts of India especially the hilly regions and the adjoining plains are influenced by transient disturbances in the mid latitude westerlies, known as western disturbances (WD). The cold wave mainly affects the areas to the north of 20 degrees – but in association with large amplitude troughs, cold wave conditions are sometimes reported from states like Maharashtra and Karnataka as well. The maximum number of cold waves occur in Jammu and Kashmir followed by Himachal Pradesh, Punjab, Bihar, Haryana and Uttar Pradesh.

In India the Western Himalayas and adjoining northern parts experience cloudiness and precipitation in association with WD during winter. Mostly WD is a double edged sword that on one end plays an important role in bringing relief in extreme weather conditions, while on the other can also be responsible for that very same extreme climate. These are aptly named western disturbances as they enter India through the western region in the form of a trough in the upper and middle tropospheric westerlies and join the monsoons, cyclones and severe thunderstorms in modulating weather over the Indian mainland. They occasionally deepen after entering the Indo-Pak area, particularly over Rajasthan and Punjab. The intensification is the combined effect of incursion of moist air from the Arabian Sea (or sometimes from the Bay of Bengal) and the orography of the region. A WD originates usually over the Mediterranean/Black Sea as an extra-tropical frontal system, but its frontal characteristics are lost while moving eastward towards India across Afghanistan/Pakistan. As they re-organise and intensify upon approaching India, their moist air streams slow down and start ascending over the southern slopes of the Himalayas. About two to three systems are known to annually traverse northwest India. All weather patterns, however, have a self-controlling mechanism, which ensures that they normalise after some time.

Fig 1. Typical western disturbance onset (end December) Fig 2. Western disturbance intensifying in 4-6 days

Fig 1. Typical western disturbance onset (end December)
Fig 2. Western disturbance intensifying in 4-6 days

An intense WD is capable of producing widespread heavy snowfall over the Western Himalayan region and rains over northern plains for a day or two and may lead to snow avalanches. A generalised pattern of day to day changes during a WD is as follows – it starts with the build-up of low and medium clouds making for partly cloudy skies; a slight warming of the air is accompanied during nights and early mornings; day temperatures experiences a fall and the prevailing winds remain southwesterlies (Fig 1 and 2). The following two or three days are characteristically marked by the arrival of multi-layer clouds and a nearly overcast sky, with or without rains. The day temperatures fall further while night temperatures remain steady and the prevailing winds shifts to southeasterlies and easterlies. Day four onwards the cloud cover moves away (northwest wind) leaving skies clear, with rising day temperatures but plummeting night temperatures. Immediately after the passage of a WD, lot of moisture is available in the atmosphere and the prevailing local conditions provide the trigger for the formation of fog.

India has experienced two cold waves in 2012 – one in the mid-week of November and the second in the last week of December when temperatures fell appreciably. The November wave affected parts of Karnataka, Maharashtra and Gujarat with temperatures plummeting 6 to 7 degrees below normal in many areas, breaking a 60-year record. The IMD termed it as a severe cold wave condition. The December wave gripped whole of north and east India with maximum temperatures recorded 6 to 10 degrees below normal and would by the beginning of the new year spread over peninsula India, according to the IMD predictions.

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