This ocean of air must exert some downward pressure on top of you because the air mass above you is constantly being pulled down to the surface due to gravity . Atmospheric pressure measured with an instrument called a barometer . Think of a straw through which you're drinking. You actually create a difference in pressure with your mouth as you draw up liquid from the glass. You have created a difference in pressure or a pressure gradient by creating a partial vacuum in your mouth.
Barometers work on the same principle, of which the most common is called a mercury barometer .
A mercury barometer is constructed using a long glass tube which is then filled with mercury completely. The tube is then inverted and placed over a dish. Some of the mercury will drain out into the dish. Atmospheric pressure will push down on the mercury, until a balance is reached. Atmospheric pressure at any given location on a day to day basis will vary little.