# A Percentile Method to Determine Cold Days and Spells in Bangladesh
## 3 Results
### Final Estimation of Cold Days  
The final estimation of cold days across Bangladesh, using the \(T_{threshold}\) of 17 \({}^{\circ}\)C in the winter months from 2000-2001 to 2020-2021, is shown in Figure 4a-d. The average number of cold days were reduced drastically in the southeastern and southern coastal stations in compared to the primary estimation (see Section 3.1) for the winter season (Figure 4d), including December (Figure 4a), January (Figure 4b), and February (Figure 4c). In contrast, when the \(T_{threshold}\) value was used, not much effect was found in the western and northwestern regions. In general, the number of cold days in winter were higher in the northwest and west (Rangpur and Rajshahi divisions), and reduced gradually towards the northeast, east, south, and southeast of Bangladesh (Figure 4d). The highest average numbers of cold days were found in the districts of Rajshahi (6.05 d/yr) and Dinajpur (6.0 d/yr) in December (Figure 4a); Bogura (8.33 d/yr), Rajshahi (7.86 d/yr), Dinajpur (7.86 d/yr), and Ishwardi (7.48 d/yr) in January (Figure 4b); Dinajpur (2.19 d/yr) and Rajshahi (2.05 d/yr) in February (Figure 4c); and Dinajpur (16.05 d/yr), Rajshahi (15.95 d/yr), and Bogura (14.71 d/yr) in the whole winter season (Figure 4d). On the other hand, the lowest average numbers of cold days were noticed in Sylhet (5.14 d/yr), Chattogram (2.0 d/yr), Sandwip (4.86 d/yr), Teknaf (0.05 d/yr), and Cox's Bazar (0.71 d/yr) stations. In the coastal regions, the average cold days ranged from 1-3, 1-5, 0.2-0.8, and 2-8 days/year in December, January, February, and the whole winter season, respectively. In addition, in the case of the entire country, the average numbers of cold days were 2.9, 5.2, 0.7, and 8.8 days/year in December, January, February, and the whole winter season, respectively. Overall, the highest and lowest numbers of cold days in the winter season were noted in January and February, respectively. Figure 4: Spatial distribution of average cold days at different stations using the 17 \({}^{\circ}\)C \(T_{threshold}\) for the winter months—**(a)** December, **(b)** January, **(c)** February, and **(d)** the whole winter season, during the period 2000–2001 to 2020–2021. Black dots are meteorological stations, with their IDs in blue numbers. Spatial distribution of the average mild to extreme categories of cold days during the winter periods (2000-2001 to 2020-2021) across Bangladesh is presented in Figure 5a-e. The category of mild cold days (Figure 5a) was observed mostly in the southern coastal regions, including Khepupara (4.43 d/yr), Feni (4.38 d/yr), Bhola (4.33 d/yr), and Hatiya (3.90 d/yr); the hill district Rangamati (4.76 d/yr); and lower in the central regions. Moderate cold days (Figure 5b) occurred across Bangladesh, in an irregular fashion. Very (Figure 5c) and severe (Figure 5d) cold days were noticed highest in Rajshahi (3.24 d/yr) and Ishwardi (4.14 d/yr) stations. The highest numbers of extreme cold days (Figure 5e) were found in Dinajpur (4.81 d/yr) and Rangpur (3.48 d/yr) stations in the northwest of the country. In contrast, no severe and extreme categories of cold days were reported in the south and southeastern coastal stations, including Teknaf, Cox's Bazar, Chattogram, Hatiya, Sandwip, M Court, and Khepupara (Figure 5e). In terms of administrative divisions, the extreme-category cold days were in Dhaka (0.79 d), Rajshahi (2.79 d), and Rangpur (4.14 d) in each year, on average (Figure 5e). Moreover, Khulna (1.16 d/yr), Mymensingh (1.71 d/yr), Rangpur (2.67 d/yr), and Rajshahi (2.76 d/yr) divisions showed the severe category (Figure 5d). Moreover, Sylhet (1.98 d), Dhaka (2.24 d), Khulna (2.27 d), Rangpur (2.64 d), Mymensingh (2.76 d), and Rajshahi (3.6 d) divisions revealed very cold days each year (Figure 5c). In general, extreme, severe,  
Figure 5: Categories of cold days—(**a**) mild, (**b**) moderate, (**c**) very, (**d**) severe, and (**e**) extreme, during the winter seasons from 2000–2001 to 2020–2021.