Every aircraft over the UK, drawing a trail only where the physics says one forms: the air at its height must be cold enough, and it lingers only where the air is saturated with ice. The glow shows where lingering trails would form at the chosen height.
Aircraft: adsb.lol (ODbL). Upper-air weather: Open-Meteo (CC BY 4.0). Physics: Schmidt–Appleman criterion (Schumann 1996). Coastline: Natural Earth. A forecast from model humidity, not a photograph of the sky.