## 23 Variation of Wind Speed with Height

Wind blows slower near the surface of the ground than it does higher up. This is because the wind is slowed down by friction with the ground and other features attached to the ground, like trees, bushes, dunes, tall grass, and buildings. Because of this, wind speeds measured at, say 50 feet from the ground, are usually higher than wind speeds measured at only 20 feet from the ground. In fact, the wind speed measured at 50 feet will usually be 14% higher than the speed at 20 feet, assuming clear, level ground, and even wind flow. As a general rule, the wind speed over clear ground will vary with 1/7th the power of the height from the ground. This is called the "1/7th power rule."

where v = wind speed measurement, h = height from ground, and k = units conversion and proportionality constant.

For this reason, when wind data from a local weather station is being compared to a specific site, it is well to note that most standard wind measurements are made at a height of 10 meters or 32.8 feet. If, for example, it is necessary to know what the wind speed was at a height of 15 feet, then by applying the 1/7th power rule, it is found that the wind speed at 15 feet would have been about 11% less than that measured at 32.8 feet, all other things being the same.

If a wind speed is measured to be 81 mph at a standard weather reporting station, that does not automatically mean that a nearby building was also subject to winds that exceeded the code threshold. If the building was only 10 feet high, then the wind at that height would likely have been about 16% less or 68 mph, which is well below the code threshold. If there were also nearby windbreaks or other wind-obstructing barriers, it could have been even less.

Local geography can significantly influence wind speed. Some geographic features, such as long gradual inclines, can speed up the wind. This is why wind turbines are usually sited at the crests of hills that have long inclines on the windward side. The arrangement of buildings in a downtown area can also increase or decrease wind speed at various locations by either blocking the wind or funneling it. Thus, the wind speed recorded at a weather station does not automatically mean that it was the same at another location, even if the two sites are relatively close. The relative elevations, the placement of wind obstructing or funneling structures, and the local geography have to be considered.

## Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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