Lixy

I would think that the leaves would have to be green, the heat evaporates the eucalytus oil in certain gum trees and creates a lovely eucalpyt scent in the air.

In very hot places with a high population of gums, the air turns a bluish colour which means extreme fire danger as a fire and a flammable atmosphere (full of eucalypt oil) can cause explosive fires of the worst kind. Naturally for this type of atmosphere to accumulate, it needs to be a still day. Wind disperses the evaporated oils