On my visit to Rothiemurchus forest I had recorded very early in the morning in the forest on the opposite side of Loch Morlich from the passing road. I was amazed at just how obtrusive the noise of an occasional passing car or truck could be to the entire southeastern side of the loch and I remembered as a child enjoying an illustration that I thought might explain this phenomena. After a nostalgic flick through the yellowing pages of the Life Science Library book of Sound and Hearing from my grandpa's Life Science Library collection (the book that inspired my interest in all things relating to sound) I found the pages I remembered so well.
The pages below explain that in the morning when the air just above the ground is cold (due to the extraction of heat from the air into the ground at night) and warms as the altitude increases sound waves can be reflected by the warmer air mass above and focused back towards the ground. The reason that this warm air reflection effect occurs is because sound travels through warm air faster than it does through cold air. Furthermore calm water, as was the state of Loch Morlich on this particular morning, is a very good reflector of sound waves and therefore when both reflection effects work together sound is focused across the water in a tunnel that disappears later in the day when the lower altitude air warms in the sun.