High pressure forces the mercury to rise in the column. Another type of barometer is an aneroid barometer . Aneroid barometers use a tightly sealed box with a partial vacuum inside. Small changes in external air pressure cause the cell to expand or contract, which reflects outside atmospheric pressure. Okay, so it's not the actual barometer you're interested in, but barometric pressure .
Barometric pressure is the amount of pressure the air is exerting.
Barometric pressure is an indicator of incoming weather systems. Low pressure systems often bring with them clouds and precipitation, because air rises in low pressure systems. The higher the altitude, the cooler the air gets, causing condensation, clouds and precipitation. High pressure systems usually result in mostly clear days. High pressure systems push air closer to the ground, preventing the formation of clouds.