The mesosphere is an interesting region for investigation of atmospheric trends, which are expected to be larger there compared to the troposphere. Mesospheric ice layers, observed as noctilucent clouds (NLC) from ground, are the visible manifestation of an extreme temperature state in the polar summer mesopause region. Temperature measurements in the middle atmosphere are very limited in time coverage and accuracy. Hence, indirect information, as provided by mesospheric ice particles, is valuable as it is known that they are very sensitive to background temperature as well as water vapor content.

Ground-based lidar instruments provide NLC occurrence frequency, brightness and altitude. The longest continuous NLC data set is acquired with our lidar at the Arctic station ALOMAR (69°N). The time series covers 22 years and contains about 2860 hours of NLC detections. NLC properties are impacted differently by solar and atmospheric parameters. In general, Lyman-alpha and stratospheric ozone impact all NLC properties, temperature at 83 km impacts mainly the NLC altitude. Strong NLC show increasing occurrence frequency and brightness from 1998 to 2015. Amongst the ice particle parameters, altitude is the longest measured property, first determined by optical triangulation in 1890. Since the early days of such investigations, the accuracy and data basis of NLC altitude measurements has been substantially improved. The overall mean altitude determined from our lidar observations is 83.3 km, which is surprisingly close to the historic values, and differs by less than the geophysical variability of 1.3 km. Since 1994 the altitude of strong clouds increased by only few meters per year.