A storm is predicted if atmospheric pressure –

Atmospheric pressure is basically the weight of air in the atmosphere above the reservoir, so the level of mercury continues to change until the weight of mercury in the glass tube is exactly equal to the weight of air above the reservoir. In areas of low pressure, air is rising away from the surface of the earth more quidkly than it can be replaced by air flowing in from surrounding areas. This reduces the weight of air above the reservoir so the mercury level drops to a lower level.