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

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.