Final answer:
Air temperature cannot be depicted on a flow-line map, as it does not have a direction of movement, which is necessary for the flow representation used in such maps.
Step-by-step explanation:
The variable that would NOT be suitable for depiction on a flow-line map is B) Air temperature. Flow-line maps are designed to represent movements or flows, such as river direction, ocean currents, and economic trade routes. The direction and volume of flow can be effectively depicted using lines of varying thickness. However, air temperature is a scalar value and does not have a direction of movement, making it incompatible with a flow-line map which is used to represent vector quantities like currents and directional movements.