From Wind to Pressure Field
An interesting property of vector fields, as on the foregoing map, is that they may be inverted.
If you think of a vector field as having been derived from the topography of some surface this assertion is that the topography can be calculated when only the slope is known.
At least up to a constant of integration (the absolute elevation) and if the data are curl free.
In the particular instance here, this says that the barometric pressure could be estimated from the mail delivery times.