What causes ‘winterline’ and why is it visible only in a few places in the world?

From mid-October to January, when the sun sets, the western horizon turns a myriad combination of yellow, red, orange and mauve. This is known as the winterline. The phenomenon is unique to Mussoorie and a part of Switzerland. The sun drops behind a false horizon, a grey and mauve strip, with a bright line of yellow and orange colours at its upper end. It occurs due to the refraction of sunlight at a particular angle, visible only from mountain areas with a long, clear valley towards the west.