At these times, there is an increase in meteors and noctilucent clouds. His notes provide evidence that noctilucent clouds first appeared in 1885. Noctilucent clouds are located in the mesosphere. Observed from the ground, this phenomenon is known as noctilucent clouds. The occurrence of noctilucent clouds appears to be increasing in frequency, brightness and extent. At higher altitudes, NASA studied inducing noctilucent clouds in 1960 and 2009. Noctilucent clouds have four major types based on physical structure and appearance. Noctilucent clouds may occasionally take on more of a red or orange hue. Noctilucent clouds are not fully understood and are a recently discovered meteorological phenomenon. They compared the noctilucent cloud phenomenon to the exhaust plume from NASA's "Endeavour" space shuttle. Noctilucent clouds may be confused with cirrus clouds, but appear sharper under magnification. Noctilucent clouds form mostly near the polar regions, because the mesosphere is coldest there. People also saw lavender suns and, for the first time, recorded noctilucent clouds.