In order to study atmospheric motions, we need to establish spatial coordinates so that the movement of air can be described as a function of space and time. There are many ways to set up such coordinate systems to represent air motions in the atmosphere. The most common one is the classical cartesian coordinate system, the (x,y,z) system, where x, y, and z usually (in meteorology) represent the eastward, northward, and upward (vertical) coordinates.