The level of mercury in a tube will decrease when the air pressure decreases. This is because the weight of the air pressing down on the surface of the mercury is what causes the mercury to rise in the tube. When the air pressure decreases, the weight of the air pushing down on the mercury decreases, and so the mercury level drops.

The amount that the mercury level decreases is directly proportional to the amount that the air pressure decreases. So, for example, if the air pressure decreases by 10%, the mercury level will also decrease by 10%.

This relationship between air pressure and mercury levels is important for understanding how barometers work. Barometers are used to measure air pressure, and they work by measuring the height of a column of mercury in a tube. The higher the air pressure, the higher the mercury column will rise. Conversely, the lower the air pressure, the lower the mercury column will drop.