These are a rather rare atmospheric phenomenon, which was visible from Amsterdam on June 21st (see pictures below, many prettier exist online). They form in the mesosphere above ~80km of altitude, where the density is comparable to Mars’ atmosphere. Typically, they are only visible from ~50-75 degrees in the summer towards north right after sunset/before sunrise.
This is because the temperature needs to be low enough that water ice cubes of less than 0.1micrometers can form and the Sun needs to be at the right angle. The water nucleates around meteorituc dust to form the cubes. These clouds are not red like you would expect by analogy with sunset/rise because these icecubes scatter blue light into the line of sight.
Man-made global warming has a double effect on these clouds: CO2 cools the upper atmosphere because of the reflection of sunlight (while heating the bottom by greenhouse), which helps crystallization. CH4 (methane, cow burps) instead oxydates (I think CH4+2O2->2H2O+CO2) producing water, which can crystalize.
The low Sun activity decreases the UV background which would destroy the molecules (not sure which is easier to photodisintegrate). All these ingredients made these clouds visible as south as Italy last Friday, there are awesome pictures online.
[disclaimer: I didn’t know these even existed just a few days ago and the above is my current basic understanding on a little bit of shallow reading. Thanks to PPM for discussions.]