Answer:
This is due to a relative decrease in atmospheric pressure in high places.
Explanation:
Given that atmospheric pressure decreases at the higher point or ground, this reduced atmospheric pressure, however, will be unable to contain the Mercury in the barometer tube.
Therefore, at the top of the mountain where the air pressure is low, the barometer reading ultimately goes down.
Hence, the level of mercury falls in a barometer while taking it to a mountain "due to a relative decrease in atmospheric pressure in high places."
