NOCTILUCENT CLOUD DISPLAY OF 18-19 JULY 1990

Noctilucent clouds were reported at 11 different sites across western Canada on the evening of 18 July - morning of 19 July 1990, from 0400 to 1100 UTC. The display is thought to have covered some 2.7 x 10(6) km2. Most stations recorded the phenomenon exclusively in evening twilight, which is unusual as such extensive exhibitions are often seen in both evening and morning twilight. Heating of the upper mesosphere by a coincidental display of Aurora Borealis may have prevented a more temporally expansive showing.