Cloud tops reaching to a level where the temperature is at least: minus 18Â°C. (but see 4, below) A large amount of energy available to be released (CAPE). If the CAPE is large, then the cloud top temperature can be as warm as minus 15Â°C, and thunder will occur - however, these are the exception rather than the rule.
Eratosthenes calculated the circumference of the Earth without leaving Egypt. Eratosthenes knew that at local noon on the summer solstice in the Ancient Egyptian city of Swenet (known in ancient Greek as Syene, and now as Aswan) on the Tropic of Cancer, the Sun would appear at the zenith, directly overhead.
The actual polar circumference of Earth is just a bit over 40 thousand km (about 24,860 miles). Eratosthenes was a talented mathematician and geographer as well as an astronomer. He made several other important contributions to science.