Over the last couple of days, towards sunrise and sunset, our skies have contained something that I never really hoped to see, because it was so rare:   a phenomenon known as nacreous clouds. This morning, from our front windows, we were gazing at a kaleidoscope of colour, obviously very high in the atmosphere, dazzlingly brilliant.  These clouds have been sighted all over Scotland, and social media are buzzing with them. What are they? Nacreous clouds form in the stratosphere, between nine and 16 miles above the Earth, at temperatures of around -85ºC.  They are composed of ice particles which are of a consistent size throughout, dispersing rays of sunlight into…