Apart from a skydiving rig containing two parachutes, most skydivers wear a jumpsuit, a helmet and an altimeter of some kind. The jumpsuit keeps you warm at the high altitudes and protects your clothing. Many skydivers do not wear a jumpsuit when the weather is warm. The helmet protects your head in case you hit the aircraft on exit or other jumpers in freefall, or if you land in obstacles like trees. In the UK, all skydivers are required to wear a helmet, but in the USA it is the jumper’s choice. A barometric or electronic altimeter worn on the wrist or chest, or an audible altimeter in the helmet lets the skydiver know how high they are and when to deploy their canopy.