The main cause of atmospheric pressure is the weight of air above a given point in the atmosphere. As gravity pulls the air down, the cumulative mass of air molecules creates pressure, pushing down on everything beneath it. Atmospheric pressure decreases with altitude because there is less air above as one moves higher in the atmosphere. This phenomenon is crucial for various processes, including weather patterns, respiration in organisms, and the dynamics of flight for birds and aircraft, highlighting its fundamental role in Earth's systems.