Why can't contour lines ever cross each other?

Contour lines do not cross each other because of the variance in elevation between the lines. However, an instance where two contour lines intersect occurs when the elevations are simultaneously crossed at an identical point through a vertical ledge.

Contour lines are irregularly shaped patterns that typically form waves, concentric rings, loops or other oscillations that identify a meteorological quantity on a contour map. An "isobar" is associated with pressure, an "isotherm" with temperature, an "isotach" with the speed of wind and an "isodrosotherm" with dew point. The value of each atmospheric quantity is uniform along a contour line, hence, no two contour lines intersect.