The year 2015 has seen a delayed winter and warm weather conditions in December all over northern and eastern India, with unseasonal rains thrown in. However, a year before in December 2014, a severe cold wave wreaked havoc across North India claiming 16 lives in Uttar Pradesh alone, taking the national death toll to 140. Most casualties were related to cold-related deaths and accidents due to fog.
Criteria for cold waves
Weather scientists claim that extreme weather events will be increasingly palpable, throwing life out of gear, more often now than ever 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.
Word Meteorological Organisation (WMO) has determined certain criteria for declaring a cold wave, which is followed by Indian Meteorological Department (IMD). Although cold wave conditions may be understood differently for varied geographical locations, only coastal locations have been meted out a separate parameter by WMO. Temperate, mountainous or coastal areas may have wind chill factor playing an important role in bringing down the actual minimum temperature. Thus, the actual minimum temperature of a station is reduced to ‘wind chill effective minimum temperature’ (WCTn) based on this factor. Thus cold conditions are not measured only in terms of temperature but also in terms of human comfort level.
In the plains of north India, foggy conditions prevail during winter for several days or weeks. The minimum temperature on these days remains above normal, while maximum temperature remains much below normal. This creates cold conditions for a prolonged period. When maximum temperature is less than or equal to 16°C in plains, a ‘cold day’ is declared. When declaring a cold wave, however, wind chill factor is taken into account. When WCTn is 0°C or less, cold wave is declared irrespective of normal minimum temperature of the station. However, this criterion is not applicable for those stations whose normal minimum temperature is below 0°C. For coastal stations the threshold value of minimum temperature of 10oC is rarely reached. However, local people feel discomfort due to wind chill factor, which reduces the minimum temperature by a few degrees depending upon the wind speed. Here WCTn is used to declare cold wave or cold day.
Western disturbances (WD) and cold waves
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 experience a fall and the prevailing winds remain southwesterlies. The following two or three days are characteristically marked by the arrival of multi-layered clouds and a nearly overcast sky, with or without rains. The day temperatures fall further while night temperatures remain steady and the prevailing winds shift to southeasterlies and easterlies. Day four onwards with the passage of WD the cloud cover moves away leaving skies clear, with rising day temperatures but plummeting night temperatures with cold dry north westerly winds giving rise to cold wave conditions.
Cold waves (CW) and their frequency
A cold wave or severe cold wave (SCW) spell is the duration where such conditions are continuously experienced, varying from one day to several days, by a sub-division. A study by D S Pai, et al., 2004, ‘Decadal variation in the heat and cold waves over India during 1971-2000’, published in Mausam, provides insightful information about the prevalence and distribution of cold waves in India (Fig. 1a, b, c).
The duration of most frequent CW/SCW spells in most sub-divisions is generally less than 1-2 days. Only a rare occasion saw a CW spell last more than 10 days, and there were only two occasions when a SCW lasted beyond 5 days. The data also indicates that the longest CW/SCW spells were experienced post-1989. During the 1981-90 period, a 10-day cold wave spell was experienced in the Bihar plains, and the longest (7 day) SCW was experienced in Jharkhand. In the period 1991-2000, Jammu and Kashmir experienced the longest 11-day CW spell, while the longest 4 day SCW spell was experienced by western Rajasthan, Madhya Maharashtra and Marathwada.
As D S Pai’s study points out, a significant increase in the frequency, persistency and spatial coverage of CW/SCW may be observed during 1991-2000 in comparison to the two earlier decades. In 1971-80, the most frequent spells and highest number of CW/SCW days were experienced in western Madhya Pradesh, in 1981-90 and 1991-2000, the maximum CW activity was in Jammu and Kashmir and Punjab respectively, while the maximum SCW activity was in Rajasthan and Punjab. It is thus noted that CW activity in northern India showed an increase from one decade to the next. However, how this is linked to broader issues of climate change, falls within the ambit of further research.