WIND RESOURCE

Earth and atmosphere


Atmosphere and Troposphere
The layers of the atmosphere       The earth is surrounded by a blanket of air, which is called atmosphere. It reaches over 560 km from the surface of the earth. The atmosphere around the globe is a very thin layer with respect to the globe that has a diameter of 12,000 km.
         The troposphere, which is the first layer of the atmosphere, starts at the earth's surface and extends to an altitude of 16-18 km over tropical regions, decreasing to less than 10 km over the poles. This layer contains approximately 80% of the atmosphere's total mass.
 









Atmospheric stability
Density variation of the atmosphere         Atmospheric stability is a concept that has large implications for the degree of local mixing of air, and thus for transport of chemicals. It describes the degree of convection in the atmosphere and can be used to explain the degree of turbulence of the troposphere at any given time, as well as the general stability of the stratosphere.
        The stability of the atmosphere at a given time and place is determined by the vertical temperature profile, known as the actual vertical temperature gradient or actual lapse rate. This rate expresses how, on average, the temperature of the atmosphere changes with altitude. For example, the air in the troposphere generally grows cooler with increased altitude (see diagram), since it becomes less dense and because it is primarily warmed only by heat radiated from the earth.






Earth Boundary layer
        The near earth region of atmosphere where the properties of the flow are directly influenced by the earth’s surface is called Earth Boundary Layer. The flow is retarded by surface friction in that layer and thus the wind speed generally increases with height. The Earth Boundary Layer ends at the gradient height, where the motion of wind will be free of the earth’s frictional influence and there is zero turbulence. The gradient height depends on time and on atmospheric conditions. During daytime, the surface of earth is heated and strong thermal mixing has as result an Earth Boundary Layer height of 1000m or more. During night, the surface of earth is relatively cool, resulting in a stable thermal stratification and suppression of turbulence and the Earth Bpoundary Layer height is 100m or less. However, in cloudy conditions, strong winds and neutral flow, it is of the order of 1000m both during day and during night.





Video frames
- Earth Boundary Layer: general notes