This movement of air we call wind. To measure winds, you have to consider the speed of the wind and its direction. Wind speed is measured with an anemometer. There are many types, but the most common consists of three small cups that catch the wind and spin. The faster they spin, the greater the wind speed.
Wind direction is determined using a wind vane.
Because of the variable nature of wind, the usual convention is to average wind speed and direction over two minutes. The winds that are felt many afternoons from late spring and into autumn result from the aforementioned temperature differential. In this case, the interior regions of California heat up during the afternoon, sometimes by as much as 40 degrees warmer than the cool Pacific waters. This results in lower barometric pressure inland and an afternoon breeze that flows from the sea toward the land. These winds, which start out as westerly winds blowing through the Golden Gate and San Bruno Gap, are channeled by the terrain in the South Bay and are primarily north-northwest as they move through Santa Clara County.