In addition, from Figure 2, it can be seen that there are two L-shaped tubes attached to the top of the Perspex called pitot-static tubes. The labels H2 and H4 represent tube openings directly facing the air flow direction (0 degrees). The pressure measure here is called stagnation pressure. It accounts for the increase in pressure when air is blowing. The label H3 represents an opening on the tube that does not face the air flow and can be considered as the static pressure at Plane 3. Finally, the difference between the static pressure and the stagnation pressure is called the dynamic pressure. The dynamic pressure accounts for the air velocity and can be calculated using equation (2) in the section below.