viscosity has very interesting units - stress (force / area) divided by rate (1 / time). viscosity is measured (a field known as rheology) by, in some way, moving a thing through a fluid at increasingly fast accelerations, or equivalently, at increasingly high frequencies. that is, imagine moving your hand back and forth in a fluid - the faster you do so (the number of back and forth motions per second), the more resistance you will feel from the fluid. for newtonian fluids, the resistance you feel (measured in force / area, ie the area of your hand), is proportional to the frequency of your hand moving back and forth in the liquid, so, the graph is a line. non newtonion fluids do not have a linear relationship between shear stress and shear rate. air is also a fluid - all gasses are, and thus possess rheological properties. air, however, at stp, is essentially an ideal gas, that is, it is non-interactive, and thus, has 0 viscosity. the point here, is that viscosity is a consequence of the interactions of particles. as gases become denser, their viscosity increases. liquids, for comparison, is ~1000x as dense as air. the details of how molecular interactions lead to viscosity is actually quite complicated.