Mesospheric clouds (MC, also known as noctilucent and polar mesospheric clouds) are layers of sub-micron ice particles formed near the coldest region of the planet. MC are usually found at 82-84 km heights, at high latitudes in the summer As we begin the 21st Century we can look back on more than one hundred years of observations of these “clouds at the edge of space” A unique data base of this ethereal phenomenon is being assembled, composed of many decades of ground-based sightings, plus more than two decades of satellite measurements, especially by SBUV, SBUV/2, and SAGE II...

The two most obvious signals in the MC time series are an apparent anti-correlation with solar activity and an upward trend which is manifested in different data sets as more frequent clouds or more frequent bright clouds. The record is now long enough to clearly separate the two signals. The upward trend has been attributed to increasing water vapor, decreasing temperatures, or both. The solar signal is quite large and emphasizes the role of water vapor and temperature in MC formation. This anti-correlation contains a 1-2 year time lag in MC change that still defies explanation and is apparent in all data sets examined. This implies an incomplete understanding of the dynamics.of the middle atmosphere (or coupling to the lower atmosphere?).