Appearance of night-shining clouds has increased

First spotted in 1885, silvery blue clouds sometimes hover in the night sky near the poles, appearing to give off their own glowing light. Known as noctilucent clouds, this phenomenon began to be sighted at lower and lower latitudes -- between the 40th and 50th parallel -- during the 20th century, causing scientists to wonder if the region these clouds inhabit had indeed changed -- information that would tie in with understanding the weather and climate of all Earth.
For the full article: http://www.sciencedaily.com/releases/2014/04/140411091939.htm