# A Percentile Method to Determine Cold Days and Spells in Bangladesh
## 1 Introduction  
Cold days and spells in the winter season are considered a natural disaster in many subtropical countries of the globe, including Bangladesh. Such a cold phenomenon is often accompanied by a sudden drop in air temperature, sometimes within a single day. The cold severity in Bangladesh during winter depends on several factors, which vary with space and time [1, 2], including (i) high wind speed, (ii) high relative humidity, (iii) less solar radiation in winter, and (iv) cold waves from the north and northwest from the Himalayas. In fact, the country suffers from cold severity every winter, which has a devastating effect on the country's economy. It disrupts the transportation sectors, especially waterways; causes extensive damage to agricultural crops; and impacts tourism [3]. Moreover, every winter, cold-related diseases such as pneumonia, asthma, diarrhea, influenza, respiratory complications, and cardiovascular diseases threaten public health [4, 5, 6]. In many cases,these diseases are the cause of death (mortality), especially for children, the elderly, and homeless people [4; 7; 8; 9]. The mortality increases dramatically (two to three times) when the temperature exceeds a threshold value, for example, 34 \({}^{\circ}\)C in Shanghai [10]. Cold weather becomes more detrimental at the beginning of winter season because people are not well prepared and do not take any preventive measures against the cold at that time [11]. Recently, extreme weather events, including cold spells, are experiencing an increase, not only in intensity, but also in frequency, because of climate change [12; 13; 14; 15]. Residents of altitudes and higher latitudes, with longer winters, are generally well prepared to deal with cold and their related effects [16]. However, countries located in the subtropical regions, such as Bangladesh, with a shorter winter season, are relatively less prepared to deal with the cold [17]. In addition, research related to cold weather phenomena has been carried out in developed countries more than in developing countries [17]. Thus, it is critical to conduct research on cold weather events in Bangladesh to address the adverse effects of cold days/spells that would facilitate policy development and adaptation strategies in minimizing mortality during the winter season. To define cold days/spells, one of the simplified methods is to use a single threshold of a specific air temperature corresponding to the daily maximum (\(T_{max}\)), minimum (\(T_{min}\)), or average (\(T_{avg}\)) temperature [6; 18; 19]. Using such a single threshold of temperature for a country creates some issues, which are not appropriate for the following instances: (i) large waterbodies being present in some parts of a country [20], as areas adjacent to waterbodies usually have mild temperature; (ii) countries spanning over a larger range of latitudes [21; 22], as higher latitudes predominate relatively cooler temperatures [23; 24]; and (iii) diverse topography, which produces temperature variations, even in cases of narrow latitude [25]. In Bangladesh, a single \(T_{min}\) threshold of \(\leq\)10 \({}^{\circ}\)C is used for the whole country to define a cold day, and a cold spell is declared when at least two nearby weather stations show three and more consecutive cold days [26; 27]. Generally, northwestern and western regions of Bangladesh experience relatively low temperatures in the winter season compared to the rest of the country, especially the southeastern and southern regions close to the Bay of Bengal [4; 28]. A fixed air temperature threshold is used to define cold days/spells in the case of long-term temperature data unavailability; however, the data are available now. Therefore, it is necessary to change the conventional method and introduce a contemporary method (e.g., relative approach) to determine the cold days/spells and their severity in the winter season in Bangladesh.