Seasons are the divisions of the year into months or days according to the changes in weather, ecology and the intensity of sunlight in a given region. The temperature cycle plays a major role in defining the meteorological seasons of the year. This study aims at investigating seasonal boundaries applying harmonic analysis in daily temperature for the duration of 30 years, recorded at six stations from 1988 to 2017, in northwest part of Bangladesh. Year by year harmonic analyses of daily temperature data in each station have been carried out to observe temporal and spatial variations in seasonal lengths. Periodic nature of daily temperature has been investigated employing spectral analysis, and it has been found that the estimated periodicities have higher power densities of the frequencies at 0.0027 and 0.0053 cycles/day. Some other minor periodic natures have also been observed in the analyses. Using the frequencies between 0.0027 to 0.0278 cycles/day, the observed periodicities in spectral analysis, harmonic analyses of minimum and maximum temperatures have found four seasonal boundaries every year in each of the stations. The estimated seasonal boundaries for the region fall between 19-25 February, 19-23 May, 18-20 August and 17-22 November. Since seasonal variability results in imbalance in water, moisture and heat, it has the potential to significantly affect agricultural production. Hence, the seasons and seasonal lengths presented in this research may help the concerned authorities take measures to reduce the risks for crop productivity to face the challenges arise from changing climate. Moreover, the results obtained are likely to contribute in introducing local climate calendar.

Insufficient rainfall results in water shortage and eventually leads to drought. This research has investigated drought by utilizing standardize precipitation index in monthly mean rainfall data for 30 years from 1988 to 2017 in Rajshahi division, a region in the northwestern part of Bangladesh. Estimated indices have identified that the years 1992, 1994, 2006, and 2012 experienced moderate to severe droughts, and the year 2010 suffered from extreme drought. Non-parametric and linear trend analyses have shown that the number of draughts in the study area has been growing. The study area is thus judged as moving forward to experience more droughts from lack of water due to rainfall deficit, especially during monsoon. This region has already started to experience a shortage of rainwater, approximately 18%, in the monsoon season. This shortage is likely to affect the volume of surface water and thus the groundwater recharging, which would distort irrigation for agriculture in the region. This work would therefore assist in policy-making addressing the watering system of the region to ensure smooth agricultural production.

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This work has presented yearly dry and wet seasons in the analysis of 28 years daily recorded temperature, relative humidity and rainfall data from 1988 to 2015 in Rajshahi division, Bangladesh using Hilbert frequency analysis. Analysis has estimated the seasonal boundaries in time according to the instantaneous frequency in cycles/day and the estimations are verified with studying power spectrum of the time series. Two boundaries are obtained in each analysis over the average of yearly analysis of four years. Obtained seasonal boundaries on 16 March and 20 October are indicated as the differentiator of wet season comprises of pre-monsoon and rain in each year. Results have also shown that the length of the wet season is varying ±11days. Estimations have further justified with average rainfall distribution as shown in this work. It is even difficult to differentiate rainy season in rainfall data, however, the estimated wet season using Hilbert analysis well supported the rainy season over temperature and humidity. The presented analysis may assist further to learn more about the seasonal variability in climate dynamics.