38km link, low rssi, low transmit gain


We have installed a link on 38km with BH10 units, one side about 250m
(210m above sea level+40m watertower) other side 150m+30m.

We got about Rssi 700, jitter 1-2, -75dB power level.
Link set to 50%, thruput 100% each direction.

The strange, rssi is low, but both unit said 19-20 of transmit gain.
It seems to me, 10dB fade margin is so low for a 38km link, but radios are not pushing full power right now. Do they will power up if a heavy rain comes in?