#atozchallenge/ B is for barometer.

B is for barometer.
A barometer is a device which measures atmospheric pressure. It is used by meteorologists to detect changes in pressure which are caused by weather. The two most common types are water-based barometers and mercury barometers. 

The concept that falling atmospheric pressure indicates stormy weather is the is the principle behind the water-based barometer. It is composed of a glass container filled halfway with water. A narrow tube is connected to the water body and ends above the water level which is open to the air. When the pressure drops the water level in the tube rises higher than the water level in the body. When the pressure rises, the water level in the tube falls below the water level in the body.

A mercury barometer is composed of a glass tube which is 84 centimeters long, which is closed on one end. An open mercury-filled body is at the base. The mercury's weight creates a vacuum at the top of the tube. When the weight of the mercury tube equals the atmospheric pressure of the mercury-filled body, High pressure causes mercury to rise in the tube lower pressure cause the mercury to fall in the tube