Noctilucent Clouds

I've been keeping an eye out for this phenomenon which can happen on summer nights during astronomical twilight in the 50-70 degree North/South latitudes.
This was about 1:15 am looking North. They are caused by high altitude ice crystals in the upper atmosphere around 50 miles up illuminated from below the horizon.

If you are up late and it's clear, take a peek outside and you might see them too!