The zero-pressure plastic (usually polyethylene) balloon is used to make measurements of the atmosphere as the balloon travels horizontally. The plastic balloon is filled only partly with gas while on the ground. As the balloon ascends, the expanding gas fills the envelope. This type of balloon has a valve that automatically discharges excess gas when the balloon reaches a certain altitude, so that the balloon can maintain this altitude as it travels horizontally. When the Sun sets, the gas cools, the volume decreases, and the balloon falls to the ground, unless ballast is released.