Hallo SevenSamurai,

Zur Messung...

Das ist in der Nähe des Äquators an der Obergrenze von Cumiulonimben in 14 bis 17 km Höhe.

https://de.wikipedia.org/wiki/Cumulonimbus

Zum alten Rekord: A detailed analysis is made of the development of a region of cold cloud-top temperatures in Tropical Cyclone Hilda (1990) in the Coral Sea off eastern Australia. Observed temperatures of approximately 173 K (−100°C) from two independent satellite sources indicate that the convective turrets penetrated well into the stratosphere to an estimated height of around 19.2 km.

Gruss,
S.