Humidity can make it seem like hair has a mind of its own, by affecting its natural curl pattern.
Any change in relative humidity can make the hair fiber assume or exaggerate its natural state.
As each hair fiber is formed in the follicle, the sac it grows from, it emerges with a distinct curl pattern, determined by different protein structures.
In general, if hair is very curly, the cross section tends to be more flat than round; straight hair is more round than flat.
In a normal water set, wet hair is wound around a rod and takes that shape as it dries.
Upon exposure to high humidity, however, water goes into the hair fiber, acts on the proteins, and lets them change back. A change from high humidity to low takes water out of the strand, also allowing change.
The natural curl can naturally change with age, perhaps from straight to curly.
After the loss of hair because of chemotherapy, it may grow back more curly than it was.