Noctilucent clouds are extremely high ice-crystal clouds that form up in the mesosphere, at altitudes of around 50 miles / 80 km. Their Latin name roughly translates as ‘night shining’. This is because these ghostly rippling clouds only become visible when the Sun is below the horizon for the observer, so that the sky is dark by the sunlight still catches their gossamer forms. Most commonly viewed between 50° and 70° north and south of the equator, noctilucent clouds are made of ice crystals and can only be observed in either hemispheres during the summer months, which is when the mesosphere is at its coldest. ForNorthern-Hemisphere cloudspotters, therefore, the noctilucent cloud has just begun. Keep an eye out towards the northern horizon in the few hours before sunrise and after sunset for the best chance of spotting these mysterious and ghostly formations. Here is a taster video beautifully filmed over Denmark by Adrien Mauduit of what noctilucent cloud spotters at high enough latitudes might be lucky enough to observe.