  Present paper studies the $\gamma$-ray spectrum observed using NaI(Tl)
scintillation detector over a year. The paper reports the presence of a
distinct diurnal pattern in the total number of $\gamma$-ray counts detected by
the NaI detector. The counts start decreasing after sunrise and show gradual
recovery after sunset. The amplitude of this variation is quite significant ( >
10%) at the observation site Tirunelveli, South India. However, further
investigation based on different energy ranges reveals that the mentioned
diurnal pattern is actually present only in the energies related to the
terrestrial background radioactivity. The study demonstrates that the pattern
is associated with the radioactivity of isotopes of radon ($^{222}$Rn and
$^{220}$Rn) and their daughter radionuclides. The amplitude of the diurnal
variation is found to have seasonal dependence, with the lowest amplitude
during June-October (~ 2%), and highest in April-May months (~ 14%). The
dependence of the amplitude of the diurnal variation on various atmospheric
parameters, namely temperature, relative humidity, and vertical wind is
examined. The observed diurnal pattern is attributed to the generation of the
inversion layer. The distribution of concentration of radon and thoron
progenies in an air column changes with the vertical mixing and atmospheric
boundary layer (ABL), resulting in the diurnal variation of $\gamma$-rays.
