Observations of high-altitude cirrus clouds are reported from
measurements made during routine monitoring of cloud properties on
commercial aircraft as part of In-Service Aircraft for a Global
Observing System. The global scale of the measurements is revealed, with
7 years of in-situ data producing a unique and rapidly growing dataset.
We find cloud fractions measured >=10km at aircraft cruise
altitude are representative of seasonal trends associated with the mid
latitude jet stream in the northern hemisphere, and the relatively
higher cloud fractions found in tropical regions such as the
Inter-Tropical Convergence Zone and South East Asia. The characteristics
of these clouds are discussed and the potential different formation
mechanisms in different regions assessed.