## Lagrangian coordinates

- (
*Also called*material coordinates.) 1. A system of coordinates by which fluid parcels are identified for all time by assigning them coordinates that do not vary with time.

Examples of such coordinates are 1) the values of any properties of the fluid conserved in the motion; or 2) more generally, the positions in space of the parcels at some arbitrarily selected moment. Subsequent positions in space of the parcels are then the dependent variables, functions of time and of the Lagrangian coordinates. Few observations in meteorology are Lagrangian; this would require successive observations in time of the same air parcel. Exceptions are the constant- pressure balloon observation, which attempts to follow a parcel under the assumption that its pressure is conserved, and certain small-scale observations of diffusing particles.*Compare*Eulerian coordinates;*see also*Lagrangian equations.