The ratio of absolute viscosity--or a fluid's resistance to flow--to the density of that fluid; this is commonly measured by determining the time it takes a liquid to travel through a capillary under the influence of gravity, using an instrument called a capillary viscometer, and can be expressed in units of m² per s. The atmosphere's kinematic viscosity changes with altitude and this affects the rates at which gases exchange between layers. In general, the atmosphere's viscosity is higher at higher altitudes.