Noctilucent clouds (NLC) or sometimes called Polar Mesospheric Clouds (PMC) are located 80-85 km above the Earth in the atmosphere. They can be seen in latitudes 50 and 70 on both hemispheres circa one hour after sunset and one hour before sunrise and shines white, unlike other clouds that are darker. There are 4 different types and some of them has even its subtypes in common classification.
The NLC need to satisfy 3 conditions: cool temperature (-140 °C to -160 °C), a lot of water and a few particles of dust. Little drops of water concentrate on particles of dust and begin to crystalize immediately. Little crystals of ice are created (radius of 30 - 100 nm) and they reflect the sunbeam when the sun is just beyond the horizon.
The NLC can be observed from May to September in the northern hemisphere and from November to February in the southern hemisphere. But are there any changes in this phenomenon? See the next article! (Little hint: You can see them in Australia and South Africa now, so prepare to hunt!)