# A Percentile Method to Determine Cold Days and Spells in Bangladesh
## 1 Introduction
Relative methods, such as (i) standard deviation (SD), and (ii) percentile methods, are contemporary and commonly used in the literature to identify cold days using long-term temperature data. These approaches have significant advantages over the fixed threshold method of detecting cold days. For instance, (i) they identify extreme events more accurately and (ii) they provide proper analysis and comparison of the results [29]. In the SD method, long-term temperature data are used to calculate the daily normal and deviation (\(\sigma\)). Different SD values (e.g., 3, 2, 1.5, 1.25, and 1) are used to define cold days in different regions of the world. Furthermore, researchers have used the 1 st, 3 rd, 5 th, and 10 th percentiles of \(T_{max}\), \(T_{min}\), and/or \(T_{avg}\) over the long-term temperature data to identify cold days and spells in different regions and countries (see Table 1). \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Reference** & **Period** & **Region/County** & **Temperature** & **Percentile** & **Data Used** & **Defining a Cold Spell** \\ \hline
[17] & 2006-2009 & Guangdong, China & 5 th & 3 cities & Cold days continue for \(\geq\)5 consecutive days. \\
[11] & 1975-2003 & Castile–La Mancha, Spain & \(T_{\rm{crit}}\) & 5 th & 5 weather stations & \(T_{\rm{min}}<\) 5 th percentile, differs from station to station. \\
[30] & 1961-2018 & China & 5 th & 1629 weather stations & Cold days continue for more than 3 days. \\
[31] & 1961-2018 & Haui River Basin, China & 10 th & 134 weather stations & Cold days continue for at least 3 consecutive days. \\
[32] & 1981-2014 & South America & 10 th & 851 GSD0 station data & Cold days continue for 3 consecutive days. \\
[12] & 2007-2009 & Shanghai, China & 5 th & 1 weather station & Cold days continue for at least 4 consecutive days. \\
[33] & 2006-2011 & China & 5 th & 66 communities & Cold days continue for 2 consecutive days. \\
[22] & 2007-2013 & China & 5 th & 31 provincial capitals & Cold days continue at least 2 consecutive days. \\
[34] & 1996-2004 & Brisbane, Australia & 1 st & 1 weather station & Cold days continue for 2-4 consecutive days. \\
[35] & 1994-2007 & Taiwan & \(T_{\rm{avg}}\) & 1 st or 5 th or 10 th & 4 cities & Cold days for 2-3 and 3-days days for 1 st percentile; and 3-8 days and 5-8 days for 5 th and 100 percentiles. \\
[8] & 2001-2009 & Shanghai, China & 3 rd & 1 weather station & Two categories: cold days continue for at least 5 and 7 consecutive days. \\
[36] & 1999-2007 & Yakutsk, East Siberia & 1 st and 3 rd & 1 weather station & Cold days continue for 2-9 and 23 consecutive days with 3 rd and 1 st percentiles. \\
[37] & 2000-2006 & Moscow, Russia & 1 st and 3 rd & 1 weather station & Cold days continue for 2-9 and 25 consecutive days with 3 rd and 1 st percentiles. \\
[38] & 1987-2000 & USA & 1 st to 5 th & 99 cities & Cold days continue for 2 or more consecutive days. \\
[39] & 1962-2006 & USA & 3 rd and 5 th & 209 cities in 9 climatic regions & Cold days continue for 2, 3, or at least 4 consecutive days. \\
[40] & 1992-2015 & Korea and Japan & 1 st or 3 rd or 5 th & 47 preentries (Japan) and & 6 cities (Korea) & Cold days for 2 or more consecutive days. \\ \hline
[22] & 2007-2013 & China & \(T_{\rm{crit}}\) or \(T_{\rm{avg}}\) & 3 rd or 5 th & 31 capital cities & Cold days for 2-5 consecutive days; 5 th percentile and 2 consecutive days were more suitable. \\ \hline
[41] & 1995-2015 & Europe & \(T_{\rm{crit}}\) and \(T_{\rm{max}}\) & 10 th &
\begin{tabular}{c} Extended ensembebsystem \\ of ECMWF; model \\ resolution 32 km (T639). \\ \end{tabular} & Cold days continue for at least 3 consecutive days. \\ \end{tabular}
\end{table} 
Table 1: Example studies that used the percentile method to define cold spell.