Hair curls due to humidity in the climate, dryness within the hair, rain and even as a result of hair length. During the day, humidity in the air or rain causes the hair shaft to absorb the moisture, and revert to its natural curly state. Similarly, dry hair has less oil along the shaft, which can weigh it down and keep it straight.
When hair is short, it is also more likely to lose its straightness during the day. When hair is longer, the shaft carries a heavier weight, which, in turn, weighs it down and keeps it straighter. Anti-frizz and moisturizing hair products can help to resolve the problem of straight hair that curls over the course of the day.