An aneroid barometer measures atmospheric pressure. It consists of a sealed metal canister that is partially evacuated and has a flexible metal top. When the atmospheric pressure increases, the canister compresses slightly, causing the flexible metal top to move inward. Conversely, when the atmospheric pressure decreases, the canister expands slightly, causing the flexible metal top to move outward. The movement of the top is recorded on a dial and can be used to determine the current atmospheric pressure. Aneroid barometers are often used to measure changes in atmospheric pressure to help predict weather patterns.