"For up to an hour after sunset or before dawn they glow like eerie electric discharges or gas jets in the darkening sky, their filmy shapes slowly curling and uncurling with intense shifting colors. They are composed of tiny ice crystals more than twice as high as ordinary clouds, 9-16 miles up, in the stratosphere and form at temperatures of minus 85 Celsius and below. The crystals are all of similar size and they diffract the high altitude sunlight to make the colours."
"Search for nacreous clouds at high latitudes (e.g., Scotland, Scandinavia, Iceland, Northern US) in winter and preferably downwind of mountains," Cowley advises. "They like stormy weather that perhaps creates gravity waves to loft the necessary moisture to make them upwards across the tropopause into the stratosphere. Once seen they are never forgotten!"