  From 1998 to 2017, neutron monitors located at an altitude of 4300 m on the
Tibetan plateau detected 127 long-duration bursts of high-energy radiation in
association with thunderclouds. These bursts typically lasted for 10 to 40
minutes, and 89\% of them occurred between 10:00 and 24:00 local time. They
were also found to be more likely to occur at night, especially during
18:00$-$06:00 local time period. The observed diurnal and seasonal variations
in burst frequency were consistent with the frequencies of lightning and
precipitation on the Tibetan plateau. Based on 19 years of data, the present
study suggests that an annual variation in burst frequency has a periodicity of
$\sim$16 years and a lag of $\sim$3 years relative to solar activity.
