This is the 14177th most frequent Russian word.
опускаться
To descend, sink, or lower oneself.
Here, 'опускается' refers to the motion of the sun, gradually moving down and disappearing into the horizon.
Солнце опускается за горизонт.
The sun is setting below the horizon.
In this context, 'опустилась' indicates a decrease in temperature, figuratively moving to a lower value or state.
Температура опустилась ниже нуля.
The temperature dropped below zero.
Here, 'опустилось' metaphorically conveys a lowering or diminishing of his mood.