The definition of air flow is the movement of air through a space or across a surface. Air flow is a crucial element in the functioning of many systems, from ventilation in buildings to the propulsion of aircraft. Air flow is caused by differences in air pressure, which can be created by a variety of factors, such as the heating and cooling of air, the movement of air masses, and the shape of the space or surface. Air flow is also affected by the presence of obstacles, such as walls or trees, which can create turbulence and eddies. Understanding air flow is important for designing efficient systems, such as air conditioning systems, and for understanding the behavior of smoke and dust particles in the atmosphere. Air flow can also be used to measure the speed of wind, which is important for predicting weather patterns.