The simple answer is the wind. The more complicated answer involves physics. Simply put, Newtons first law describes how an object that is at rest will remain at rest unless acted upon by a force. Heavy items require heavy forces. Light items, lighter forces. People can produce force by lifting or pushing. Water can produce force by falling or flowing, and in the circumstances of fly away beach umbrellas, the blowing wind is a moving force. Beach umbrellas do not require that much force to move not because they are light, but because of the way they are designed. Umbrellas are constructed and designed to resist the downward forces of falling rain. Lets look at the simple design. The ribs and stretchers, which are essentially the skeleton of support are arranged like an up pointing arrow. The perfect design to deflect a downward force. All umbrellas use that design to keep you dry. There are unique modifications but the fundamentals remain the same. The curved surface redirects the water down and to the side, keeping the occupant dry. Trouble arises when the wind blows. The acting force is no longer from the top but from the side. The umbrella shape becomes a disadvantage to the umbrella in windy conditions. Their curved design makes them extremely lift-friendly. Its a unintended design flaw. Why is that important? The curved shape of an umbrella is quite similar to the rounded curve found on an airplane wing. Moving air that’s redirected over a curved surface produces both a high and low pressure area around the shape. In the airplane wing example, air moving across the wing produces a high pressure area under the airplane wing and low pressure areas above the wing. The gradients between the two pressures will produce lift and the airplane can take off. Of course this phenomenon is much more complicated but simplified here for the course of discussion. Curved umbrellas create the same conditions when the wind is blowing. Similar to an airplane wing, wind that blows at an umbrella will create areas of turbulent, high pressure air, that gets trapped under the umbrella while smooth, low-pressured, laminar flowing air courses over the top. With high pressure air below and low pressure air above…guess which direction that umbrella wants to go?…UP! The umbrella creates its own lift in the wind and if the wind is blowing hard enough, your umbrella goes looking for the next exotic beach destination without you.
Woman Studying Physics