Shorter hair won't necessarily keep you cool. Shorter hair cuts can be varying in length from the collarbone to the chin to a pixie cut. People tend to think that this cut is a lighter and cooler option for summer, but they often forget that you can’t simply put it up. With a bob it is often too short to pull back into the ole fallback: the ponytail.
Be sure to pick the right length for your face shape and hair texture. It is important to have a realistic expectation of the style you're going for depending on what kind of hair and texture you have. If you have curly hair, it is important to remember curls shrink up when you cut them, so your style will often look shorter once you leave the salon. Before making the plunge, you should talk to your stylist about creating a shape that works with your specific hair type.