ἀναθυμίασις

Under specific conditions, there are gases or aerosols (gas mixed with solid particles) coming out of the ground. This happens for instance:
• craters and vaults at active volcanos
• geysirs and other hot springs when the ground breaks
• above moors and marshes

Regardless of their different causes, for a distant observer all these fogs might be summarized as "exhalations" of the Earth.

With his a extraordinarily broad knowledge, Aristotle described different types of these exhalations (warm/ cold, dry/ humid) and speculated that some of them could possibly be inflamable in the upper atmosphere. He thought, that this way possibly comets are caused because they stand above the landscape like flames in the sky that die after a while.