Four Obstacles That Reduce Wind Speed

Description

• Solar energy creates wind. When the atmosphere is unevenly heated, the warm air molecules are more active than those in the cooler air. These active molecules cause the warm air to rise farther away from the surface of the Earth. The cooler air rushes in to replace the warm air, and it is this movement that causes wind.
  
  

The Beaufort Scale

• In 1806, British Navy Admiral Sir Francis Beaufort created the Beaufort Scale which is used to estimate wind speed using visual clues. Originally used by sailors, the device was later adapted to be used on land. The chart starts at 0, indicating calm conditions, and ends at 12 for hurricane-force winds. Each value has a description of how the wind would affect surface features at a given speed. For example, a value of 2 indicating winds of between 4 and 7 miles per hour would read "Feel wind on your face. Weathervane moves. Leaves rustle. Light wind."
  
  

Wind Measurement

• Meteorologists use wind monitors to get a more precise wind speed measurement. This is particularly useful if an area is being considered for installation of wind turbines. By taking several measurements during the course of a year it is possible to determine an area's wind potential. Wind speed measurements are normally taken at about 33 feet or 10 meters above ground level. Measurements taken at ground level are considered inaccurate because of obstacles that affect wind speeds as well as direction.
  
  

Obstacles

• Obstacles decrease wind speed by either deflecting the wind around the object or by letting only a portion of the wind through the object. The more porous, or open, an object is, the less of an effect on wind speed. Buildings are solid and the wind is forced around them, decreasing the speed on the downstream side of the obstacle. Trees are more porous obstacles that let some of the wind pass through. In summer a densely foliated tree will slow wind speed more than a barren tree in winter. Mountains, hills and plateaus can also cause a reduction in wind speed as the wind is forced around them. As the wind flows into narrow passages it then speeds up, causing turbulence.
  
  

Frictional Drag

• Frictional drag can also cause a reduction in wind speed, as well as changes in wind direction. An example of this would be a wind flowing over a relatively calm ocean with a smooth surface and then hitting a shoreline. Even if the shoreline is relatively flat, like a beach, the uneven surface with its thatches of grass and piles of driftwood can decrease the speed of surface winds. It can be compared to driving down a well maintained highway at high speed and then turning off to a smaller road that has a less than pristine surface. The bumps and other obstacles force you to slow down.