# A Percentile Method to Determine Cold Days and Spells in Bangladesh
## 4 Discussion  
The primary estimation showed a general pattern of having a higher number of cold days in the western and northwestern parts of the country, which decreased gradually towards the northeast and southeast, and some coastal stations in the south having a higher number of cold days (see Figure 3). However, coastal stations were supposed to have a smaller number of cold days compared to other parts of the country due to their close presence to a larger waterbody, the Bay of Bengal. This was discussed in detail by [PERSON] et al. [4; 28], including the relationship with water depth for any variations observed in the coastal stations [28]. The study also indicated that the occurrence of more cold days had a relationship with higher elevation. Nevertheless, the final estimation, using a \(T_{threshold}\) (17 \({}^{\circ}\)C) approach to remove the discrepancies (between observed and calculated cold days), was able to minimize the number of cold days in the coastal stations, where appropriate. In fact, a similar approach was also reported in other studies when methods such as SD [28] and absolute threshold [53] were applied. In this study, the application of the 10-percentile (10P) method was applied to calculate the final estimation of cold days and spells during the winter season in Bangladesh to understand its performance in comparison to the SD method and the traditional BMD threshold. The results were validated by [PERSON] et al. [53], who determined cold days and spells from the daily temperature dynamics during 1988-2017, but the used method of the absolute \(T_{min}\) threshold was different than our proposed 10P. At present, BMD uses 10 \({}^{\circ}\)C thresholds of \(T_{min}\) for calculating cold days in Bangladesh. This threshold of \(T_{min}\) is applicable all over the country (e.g., coastal stations, hilly stations, and plain land), but the dynamics of temperature are not uniform all through. The long-term average \(T_{min}\) observed \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Period** & **Station** & **Days/Decade** & **CL** & **Period** & **Station** & **Days/Decade** & **CL** \\ \hline \multirow{8}{*}{11–20 December} & Dinajpur & 1.88 & 99 & Dinajpur & 2.86 & 95 \\  & Rangpur & 0.45 & 95 & Rangpur & 2.86 & 90 \\  & Rajshahi & 1.14 & 90 & & Rajshahi & 3.33 & 90 \\  & Bogura & 0.57 & 95 & & Jessore & 2.68 & 90 \\  & Ishwardi & 1.43 & 95 & & Khulna & 0.91 & 90 \\  & Mymensingh & 1.18 & 99 & & Sattkhira & 2.40 & 90 \\ \hline \multirow{2}{*}{21–31 December} & Madaripur & 1.60 & 90 & \multirow{2}{*}{December} & Madaripur & 3.33 & 95 \\  & Bbola & 1.85 & 95 & & Mymensingh & 3.25 & 95 \\  & Feni & 1.43 & 90 & & Barisal & 1.72 & 90 \\ \hline \multirow{4}{*}{1–10 January} & Sattkhira & \(-\)0.67 & 90 & & Bbola & 3.04 & 95 \\  & Chandpur & \(-\)1.03 & 90 & & Khepupara & 1.34 & 90 \\  & M. Court & \(-\)0.29 & 90 & & Feni & 2.00 & 95 \\  & & & & Sitakunda & 1.21 & 90 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Significant trends of cold days/decade in different meteorological stations of Bangladesh for the periods of 11–20 December, 21–31 December, 1–10 January, and the month of December, during 2000–2001 to 2020–2021. CL is the confidence level. in the month of January at northeastern Sreemangal station for the periods of 1961-1990 and 1991-2020 is 8.89 \({}^{\circ}\)C and 9.84 \({}^{\circ}\)C, and at southeastern Cox's Bazaar station is 14.80 \({}^{\circ}\)C and 15.54 \({}^{\circ}\)C, respectively. The long-term average minimum temperature at Sreemangal is found below 10 \({}^{\circ}\)C, but the threshold of minimum temperature for identification of cold is 10 \({}^{\circ}\)C, which is not justifiable. The threshold of minimum temperature should be a few degrees lower than that of the average minimum temperature. The 10P of temperature depends on the station itself, and it varies from station to station. In the case of the threshold of minimum temperature, the diurnal temperature range (DTR) is not considered. [PERSON] et al.