Several radiometric parameters viz. direct solar irradiance, long
wave radiation, Angstrom turbidity and transmission coefficient for Nagpur have
been analyzed and discussed based on data for the period of 1989–1998.
Solar irradiance at Nagpur is the highest in winter and amongst the winter
months in January due to cloudless sky and good transparency of the atmosphere.
It is the least during the monsoons due to increased moisture content of the
atmosphere and extended cloudiness. Diurnal variation in direct solar
irradiance shows that for all the months the peak is reached between optical
air masses 1.5 FN and 1.5 AN with a maximum value of about 700
W/m
$^2$
in January. Turbidity values show fluctuations on
yearly basis. During the study period, the lowest value was observed to be
0.082 in 1990 and the highest was 0.108 in 1996. In a year, winter season
experiences lowest turbidity while the post-monsoon season witnesses the
highest. Diurnal variation of turbidity indicates that optical air mass 3.0 FN
is less turbid for all the months in a year while the peak is reached in
between air masses 1.5 FN and 1.5 AN. It is seen that transparency of the
atmosphere over Nagpur decreases from winter till May. This is to be attributed
to the increasing dust content of the atmosphere over Nagpur. The atmosphere
transmits only 65% of the incident energy. At early morning and late afternoon
hours, transparency is found to be higher when compared to higher solar
elevations. Rather, the transparency decreases from morning till noon and later
increases. This could be attributed to convective activity during the maximum
temperature epoch of the day. Net long wave radiation is found to be the
highest in pre-monsoon (64–65 W/m
$^2$
). With the monsoon
onset, it goes on decreasing, with the lowest value of 42.6
W/m
$^2$
in August. This is mainly due to the extensive cloud
cover and high humidity conditions. After the monsoon withdrawal again it
starts rising.
