An instrument used for measuring the change in atmospheric pressure . It uses a long glass tube, open at one end and closed at the other. After first filling the open end with mercury, it is then temporarily sealed and placed into a cistern of mercury. A nearly perfect vacuum is established at the closed end after the mercury descends. The height of the column of mercury in the tube is a measurement of air pressure .
Measurement is taken in inches of mercury .
Although mercurial barometers are very accurate, practicality has led observers to use aneroid barometers . This weakens the zonal flow . Its circulation can be larger than the tornado that may develop within it, but not necessarily. Originally a radar term for a rotation signature that met certain criteria, it is best seen on Doppler radar . A small, concentrated area of high pressure that may be created by the cold outflow and rain -cooled air from thunderstorms .