In the 《
尔雅》 (Erya, the earliest Chinese dictionary), each season has its own word for 'sky':
春为苍天 (spring = blue heaven),
夏为昊天 (summer = vast heaven),
秋为旻天 (autumn = compassionate heaven),
冬为上天 (winter = high heaven).
旻 is the autumn-specific term, often glossed as 'merciful' because of harvest abundance.