As the Earth continues its path on its elliptical orbit around the sun, high latitudes and higher mid-latitudes of the northern hemisphere are entering the midnight sun season. If you go too far up north, the sun never sets but at lower latitude like southern Scandinavia or central Canada, the sun goes down just a few degrees under the horizon and lingers there for a few hours, gliding unnoticed from the north-west to the north-east. This allows summer nights to remain in a constant twilight and it never gets dark enough to see the milky way or auroras for example. However this situation can give birth to one of the most intriguing and jaw-dropping shows on Earth: noctilucent clouds (NLC) or literally ‘night-shining clouds’ from Latin. While the Earth rotates far away from the sun in the summer, its mesosphere gets cooler, allowing the formation of tiny ice particles. These particles form the highest clouds on our planet (82km in the atmosphere) and wouldn't be visible if it wasn't for the bright nights They’re the highest clouds on Earth at the edge of space, and while all the other the other tropospheric clouds remain plunged in darkness NLC can still catch the sunlight because of their height. NLC only form in particular conditions. For starters the layer of the atmosphere were they grow (the mesosphere) needs to dip below -120°C. Weirdly enough it can only happen during the northern hemisphere summer. While all the hot air in the bottom part of the atmosphere expands, it cools the upper layers allowing temperature drops. The mesosphere is right at the edge of space and it’s a boundary between space and Earth. It’s a misunderstood place where odd chemical reactions happen including the formation of NLC. Rare moisture- thought to come from the reaction of CO2 and CH4, meets meteoritic dust and accretes on it. If it’s cold enough this mixture transforms into minuscule ice crystals through a process called nucleation. When our Sun has sunk from 6 to 12 degrees elevation below the horizon it hits these ice particles with a certain angle and it makes them glow in the dark with a ‘backlit appearance’! We call this period of the night the nautical twilight/dawn when stars appear in the sky but it is still bright enough for you to make things out around. NLC create a transparent shiny veil behind which you can still see the stars. It takes on the colors of the background sky as well going from deep orange to electric blue! NLC also evolve into a lot of shapes like huge billows, waves, trough or bands, making them look like an ocean of ice in the sky.
Here is a compilation of all the NLC displays of 2018 as a trailer for the 2019 season! Everything was recorded in Canada (Alberta) and Denmark. NLC season roughly starts at the end of May and finishes at the end of August. If you are between 45 and 50 degrees N of latitude you will only have the nautical twilight window so make sure to be out by then. For people living between 50 and 58 degrees North you will have more time because the Sun lingers in the ‘twilight zone’ giving you all-night possibilities. If you live between 58 and 65 deg. N you will generally have too bright nights to even see NLC. Your short window will either be in May/start of June or mid-end of August. As a rule though you’re going to have to look North in the ‘twilight arc’- the zone of the sky illuminated by the Sun. Find a cloudless night and try your luck! The moon doesn’t have any effect on how you see the NLC.
