Both pressure and wind characteristics change with height. Above the level of surface frictional effects (about 500 to 1000 m), the wind increases in speed and becomes more or less geostrophic. With further height increase, the reduction of air density leads to a general increase in wind speed (see Chapter 6A.1). At 45°N, a geostrophic wind of 14 m s-1 at 3 km is equivalent to one of 10 m s-1 at the surface for the same pressure gradient. There is also a seasonal variation in wind speeds aloft, these being much greater in the northern hemisphere during winter months, when the meridional temperature gradients are at a maximum. Such seasonal variation is absent in the southern hemisphere. In addition, the persistence of these gradients tends to cause the upper winds to be more constant in direction. A history of upper air observations is given in Box 7.1.
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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.