I think it is a metaphor (or a simile) one of those thingymajigs.
In the song Kiss From a Rose by Seal, he intones...
There used to be a greying tower alone on the sea.
You became the light on the dark side of me.
Love remained a drug that's the high and not the pill.
But did you know,
That when it snows,
My eyes become large and
The light that you shine can be seen.
Thus he is the (dull / uncheery) grey castle alone in the sea has become a lighthouse for others to see in time of storm when it "snows". Apparently the worst storms on the sea usually occur when it snows.