Wikipedia: Noctilucent clouds, or night clouds, are tenuous cloud-like phenomena in the upper atmosphere of Earth. They consist of ice crystals and are only visible during astronomical twilight. Noctilucent roughly means "night shining" in Latin. They are most often observed during the summer months from latitudes between 50° and 70° north and south of the Equator. They are visible only during local summer months and when the Sun is below the observer's horizon, but while the clouds are still in sunlight.
They are the highest clouds in Earth's atmosphere, located in the mesosphere at altitudes of around 76 to 85 km (47 to 53 mi). They are too faint to be seen in daylight, and are visible only when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in Earth's shadow.
Yesterday visible from Pland. They show up just after midnight, and I shooted them to 1:30am.
View from 16mm (and APS-C sensor). You can see how ,,big'' they was.