The atomspheric pressure to first order is a measure of the amount of material contained in the atmosphere of a planet. The higher the pressure the larger the amount of mass in the atmosphere. Why is this so? Well, imagine you are standing on the surface of a planet.

This weight determines the pressure you feel due to the atmosphere.

• On Earth at sea level, you feel around 14.7 pounds per square inch pushing on you because of the weight of the overlying atmosphere.

• On Venus the atmospheric pressure is 90 bars or roughly 90 times that of the Earth, you would feel a pressure of 1,300 pounds per square inch!!

The atmospheric pressure on Venus is roughly the pressure you would feel if you lived at a depth of ~ 900 meters in the ocean. The mass of Venus's atmosphere is thus much greater than ours (a point we will consider later is, why is this so and where does the mass come from or, more to the point for the Earth, why is our atmospheric mass so low?).