Question

1) What is the cause of atmospheric pressure

Answer 1

Atmospheric pressure or Air pressure

The atmospheric pressure at any place is just the weight of the atmosphere above that spot.

Though the surrounding air seems like a lot of nothing, the column of air above people's heads is hundreds of miles tall.

Because of that height, it weighs enough to exert a pressure of about 1,013.25 millibars (14.7 pounds per square inch) at sea level.