Pressure Lapse Rate MET

Static pressure is caused by the weight of the column of air that is above a particular point. As you go up in the atmosphere, there is less "stuff" above you, so the weight of it is less, so pressure reduces. Because most of the "stuff" is in the lowest parts of the atmosphere (close the the ground) moving up through these lower layers will result in a big change in pressure, whereas when you move the same distance up though the upper atmosphere, there is less "stuff" so there is a smaller pressure change. Hence, pressure decreases at a decreasing rate.