|I've heard that, technically speaking, our atmosphere is a fluid. Can you discuss this?
Since both gases and liquids are fluids, the earth's atmosphere is certainly a fluid. Any material that flows in response to sheer stress (tearing) is considered a fluid. The earth's atmosphere flows in responses to sheer stress—for example when you drive your car past another car, the air in between experiences this tearing and it flows in a complicated fashion. Winds are another important example of fluid flow in the earth's atmosphere.