# Natural Coordinate System

The natural coordinate system is one defined not based upon the geography and/or geometry of the Earth but, rather, based upon the local air flow (e.g., wind). In a natural coordinate system, there exist two horizontal coordinates: *s*, following the flow (**streamwise**), and *n*, perpendicular (**normal**) to the flow and* z*, referring to the up-down/vertical direction.The positive *s*-axis is defined in the direction to which the wind is blowing, while the positive *n *axis is defined perpendicular and to the left of the positive *s*-axis. The positive *z*-axis points up so that upward motion is positive and downward motion is negative.

The natural coordinate system is often used "heuristically" (for teaching concepts). It can't be used in numerical problems involving the horizontal wind that require knowing the true wind direciton.