Cloud Statistics from Spaceborne Lidar Systems - Earth Science: New Methods and Studies - page 35

The higher frequency of occurrence of high clouds is mainly due to the presence of
optically thin cirrus clouds.
Figure 7. Two-dimensional occurrences (latitude versus altitude) of the cloud top height from LITE
(September 10-19, 1994), GLAS (last week of September and the first 2 weeks of October, 2003),
and CALIOP (September, 2006) measurements.