Barometer model (model used to measure the amount of air pressure)

Barometers are used to monitor changes in air pressure. This home-made barometer uses a balloon to trap air in a glass jar.When air pressure drops, the air in the balloon expands, so the pointer moves down.When air pressure increases, the air in the balloon contracts, so the pointer moves up.

Although air is invisible, it still has weight (1m³of air weighs about 1kg). Air pressure varies daily, and is due to the weight of air in the atmosphere above us. High pressure is usually associated with warm dry weather and low pressure with wet and cloudy weather.