In basic fluid mechanics for atmospheric motion, the primary cause of air flowing from one place to another is attributed to which head/driver?

Introduction / Context:
Air moves in response to forces arising from spatial variations in pressure. While elevation and temperature influence pressure, the immediate driver of motion is the pressure gradient force, which accelerates air from high to low pressure until balanced by other forces (e.g., Coriolis, friction) in larger-scale flows.

Given Data / Assumptions:

• Steady basic flow concept; no need for full meteorological balance equations.
• Bernoulli terms (elevation head, velocity head) are considered for explanation only.
• We seek the fundamental cause that initiates air movement.

Concept / Approach:
Pressure gradient force per unit mass is proportional to the spatial pressure difference. Air accelerates toward lower pressure; elevation affects pressure via hydrostatics, but it is the resulting pressure difference—not elevation head itself—that drives flow. Velocity head is an outcome (kinetic energy of flow), not a root cause.

Step-by-Step Solution:
Recognize that motion starts where pressure varies in space.Identify the driver: pressure head (pressure gradient).Select the option explicitly naming pressure head.

Verification / Alternative check:
From momentum equations, acceleration is proportional to negative pressure gradient; in absence of gradients, no flow develops (ignoring transient disturbances).

Why Other Options Are Wrong:

• Elevation head: influences pressure distribution but does not alone cause lateral flow without a pressure gradient.
• Velocity head: a consequence of flow, not its cause.
• All of the above: incorrect because only pressure gradient directly initiates motion.
• Temperature alone: it modulates density/pressure but again acts via pressure differences.

Common Pitfalls:
Equating temperature differences with immediate motion; misreading Bernoulli terms as independent drivers.

Final Answer:
Pressure head (pressure gradient) differences