When the sun is 30 minutes to 60 minutes after sunset, that is, when the sun is 6 degrees below the horizon to 16 degrees and faces west, if you see white and blue wave-like noctilucent clouds in the sky, you are likely to see rare noctilucent clouds.
Noctilucent clouds are also called polar mesospheric clouds. When they are seen from space, they are called "night clouds". Seen from the earth, it is called "noctilucent cloud". It is a cloud formed in the middle layer, about 85 kilometers above the ground, and its position is very high. This rare cloud can only be seen in summer at high latitudes of 50 to 70 degrees.
The noctilucent cloud looks a bit like cirrus cloud, but it is much thinner, and its color is bright silvery white or blue. It appears when the angle between the sun and the horizon is between 6 degrees and 16 degrees after sunset. There is no doubt about this angle itself, because the noctilucent cloud is too weak to be seen too early and will sink into the shadow of the earth too late.