A good understanding of the diverse mechanisms in the atmosphere is required in modelling the climate. In this study, the diurnal and seasonal patterns of all-sky downward longwave radiation (DLR) at Ilorin (8o 32l N, 4o 34l E), Nigeria were investigated alongside relative humidity (RH) and temperature. The average diurnal pattern of DLR gives an arc that begins by increasing gradually with some inherent fluctuations from 01: 00 hour to a maximum at 14: 00 hour local time, and then gradually decreases to a minimum at 00: 00 hour. However, sometimes erratic and double peak arc diurnal DLR patterns occur around the mid and the end of the year periods respectively. The seasonal, diurnal peak of temperature and the minimum of relative humidity (RH) occur approximately two hours after the peak of DLR. Besides, the seasonal trends of both DLR and RH match each other, except sometimes in June, which could be due to the midyear DLR erratic diurnal effect. Possibly, the mechanisms of the inter-tropical discontinuity (ITD) influence the particular diurnal patterns of DLR at the midyear and year-end periods. Moreover, monthly dispersion of DLR is high during known months of high atmospheric aerosols, and annual maximum temperature occurs after the Harmattan season. Hence, variations in DLR are influenced mostly by mineral dust in the atmosphere, mechanisms of ITD and changes in the sun-earth distance, which reflects on the different seasons in Ilorin.