How does Helen know that storm is coming while she sits in the tree?

From the text:

Suddenly a change passed over the tree. All the sun's warmth left the air. I knew the sky was black, because all the heat, which meant light to me, had died out of the atmosphere. A strange odour came up from the earth. I knew it, it was the odour that always precedes a thunderstorm, and a nameless fear clutched at my heart.