Wind speed is typically judged as the velocity of wind. Most measurements of air movement are taken of outside air, and there are several factors that can affect it. Average wind speed is often determined by an anemometer and is usually categorized in a standardized measurement scale, called the Beaufort Scale.
Of the major factors that influence wind speed, the most important is called the pressure gradient, created by a graduated disparity in atmospheric pressure that occurs in different places. Some areas have low pressure, while others have higher pressure. For example, a valley may have a higher atmospheric pressure than the peak of a mountain that is only a few miles away. Usually, the pressure increases gradually between both points.
For the most part, air moves along these pressure gradients from high pressure to low pressure. The movement is the major force that creates wind on Earth. The greater the difference in pressure, the greater the wind speed. Therefore, areas that experience a large change in pressure over a short distance will typically have higher wind velocities than those where the change is more gradual.
Another factor that may affect the speed of the wind is local weather conditions. Storm fronts often contribute to air currents, as they can create pressure gradients for the wind to travel along. Also freak storms, like hurricanes or cyclones, can drastically alter wind speed.
Another influence on wind velocity is the presence of Rossby waves. These upper atmospheric air currents manipulate weather patterns in the air below. They are caused by the Coriolis effect. A Rossby wave may influence pressure gradients and stir up faster speeds.
The most common way to measure wind speed is to use an anemometer. The earliest anemometers consisted of a vertical shaft with a horizontal wheel of spokes. Each spoke holds a small cup at its end, and the cups catch the wind to spin the wheel. The speed of wind can be calculated based on how often the wheel rotates in a given amount of time. Many of these devices are still made like this.
Other types of anemometers have been developed as well. Laser Doppler anemometers use lasers to calculate wind speeds. Windmill anemometers operate with a fan pointed into the wind. Hot wire anemometers use the wind-produced friction on an electrically charged wire to determine wind speed.
The Beaufort Scale is a standardized measurement for wind speed. It is an empirical rating system originally based on the appearance and height of waves at sea. The system has developed to also include speed ratings for each level in knots, miles per hour, and kilometers per hour.