How a forward-facing tube turns airflow into a speed reading
As an aircraft moves, air flows around it. A small forward-facing opening—part of a pitot tube—catches some of that flow and brings it nearly to a stop inside the tube. The air presses harder there than it does in the surrounding atmosphere. That extra pressure is called dynamic pressure: it reflects the motion of the air relative to the aircraft.
The tube does not measure speed by counting how much air enters. Instead, an instrument compares the pressure at its opening with the surrounding static pressure, the ordinary atmospheric pressure around the aircraft. The pressure difference grows as the aircraft moves faster through the air, allowing the instrument to calculate airspeed.
Why the comparison matters
Static pressure changes with altitude and weather, so the pressure inside the forward tube alone would not give a reliable speed. A separate opening, usually called a static port, senses the surrounding air pressure. Using both readings lets the system distinguish pressure caused by motion from pressure caused by the atmosphere.
The result is airspeed through the air mass—not speed over the ground. A headwind or tailwind can make those two speeds differ. That distinction matters because the wings respond to airflow over them: an aircraft can move quickly over the ground in a strong headwind while experiencing less airflow, and therefore less lift, than its ground speed suggests.
