3.2 Annual and seasonal trends of climatic variables
The spatial distribution maps of climatic variables are shown in Fig. 4. The MT increased from the northern to the southern region in Bangladesh with little fluctuating temperature (Fig. 4a). The highest RH was witnessed in the southern part and the lowest in the central part with a complicated spatial pattern (Fig. 4b).
The low SD was detected in the southern and northeastern regions while the high SD was found in northwestern and southeastern with decreasing spatial pattern from north to east during the study period (Fig. 4c). There was an irregular spatial pattern observed in rainfall where the lowest concentration of R was found in the western region and the highest concentration of R was detected in the eastern portion of Bangladesh (Fig. 4d). The spatial variation of wind speed was quite complicated and similar to MT and higher values were found in the southern region and lower in the northern region (Fig. 4e). During the winter season, the northeasterly winds prevail over the country blowing from land to sea except for northern areas where mainly easterly wind prevails.
The annual MMK trend test results for climatic parameters during 1980-2017 showed in Table 2. Except for three stations (Mymensingh, Patuakhali and Rangamati), all the 15 sites showed a noteworthy rising tendency in the MT. A decreasing trend of R was seen almost 50% of the study area. A downward trend of SH and WS also been observed in most of the stations and also in a substantial rate.
The results of linear regression show that the MT significantly increased in most meteorological stations (Fig. 5). Though the MT fluctuated throughout the study period but statistically rising tendency was observed and the rate was 0.12°C/decade that noteworthy. The SD was declining in several locations where the lowest was observed at the Rangamati region at a rate of 2.89 hours/decade. The maximum RH level was located at the Mymensing region (0.27%), while the minimum (-0.15%) was found in Dhaka region. A slightly decreasing trend of rainfall was observed during the period 1980–2017 in the study area in 13 sites. But the all sites neither displayed a major rising nor a declining pattern of rainfall (Fig. 5).