What happens to the level of mercury in a mercury barometer when air pressure decreases?

Atmospheric pressure is commonly measured with a barometer.

Explanation:

In a barometer, a column of mercury in a glass tube rises or falls as the weight of the atmosphere changes ... As the pressure decreases, the amount of oxygen available to breathe also decreases.

With an increased weight of air above the reservoir, the mercury level rises to a higher level ... Since the number of air molecules above the area decreases, there are fewer molecules to exert a force on that surface. With a reduced weight of air above the reservoir, the mercury level drops to a lower level