Noctilucent clouds (NLCs), are rare, electric-blue streaks of ice crystals that form in the mesosphere between 31-53 miles high.  Making them the highest clouds in Earth's atmosphere, the temp Drops to - 90c. Best seen during summer twilight (late May–August), they are illuminated by the sun from below while the surface is in darkness. Seen Best between Latitudes 50 and 60 deg N.  Lincolnshire sits nicely within this range.