Ask an Explainer
On the air in motion page, it says that at any point in an airstream at any given time, the amount of air passing by is the same. But if I pour water into a funnel, the water enters the funnel at a much faster rate than it exits. How is it different?
When air moves through an airstream (or water through a funnel), it has to move the same amount of air, which has mass, through the same time period. So for the air to move the same amount through a smaller opening, it has to speed up as it moves through. This is due to conservation of mass as applied to the mass flow rate of the fluid. Also, the pressure decreases as the speed increases - this is Bernoulli's principle.