Geographical Variation in the Wind Resource

Ultimately the winds are driven almost entirely by the sun’s energy, causing differ ensential surface heating .The heating is most intense on land masses closer to the equator,and obviously the greatest heating occurs in the daytime, which means that the region of greatest heating moves around the earth’s surface as it spins on its axis. Warm air rises and circulates in the atmosphere to sink back to the surface in cooler areas. The resulting large-scale motion of the air is strongly influenced by coriolis forces due to the earth’s rotation. The result is a large-scale global circulation pattern. Certain identifiable features of this such as the trade winds and the ‘roaring forties’ are well known. The non-uniformity of the earth’s surface, with its pattern of land masses and oceans, ensures that this global circulation pattern is disturbed by smaller cal variations on continental scales. These ariations interact in a highly complex and non linear fashion to produce a somewhat chaotic result, which is at the root of the day to day unpredictability of the weather in particular locations. Clearly though, underlying tendencies remain which lead to clear climatic differences between regions.

These differences are tempered by more local topographical and thermal effects.Hill sand mountains result in local regions of increased wind speed. This is partly a result of altitude the earth’s boundary layer means that wind speed generally increases with height above ground, and hill tops and mountain peaks may' project into the higher wind-speed layers. It is also partly a result of the acceleration of the wind flow over and around hills and mountains, and funneling through passes or along valleys aligned with the flow. Equally, topography may produce areas of reduced wind speed, such as sheltered valleys, areas in the lee of a mountain ridge or where the flow patterns result in stagnation points.

If all of landscape geographic consisted of flat and smooth land, there would be little wind variation from place to place. But with the addition of hills, valleys, river bluffs and lakes,a complex and highly variable wind regime is created. Trees and buildings add to the complexity of the wind on a smaller scale. Each geographical feature influences wind flow in certain ways, as detailed below

Hills, plateaus and bluffs provide high ground on which to raise a wind turbine into a region of higher wind speeds. Valleys, which are lower and heltered, generally have lower wind speeds. However, all valleys are not necessarily poor wind sites. When oriented parallel to the wind flow, valleys may channel and improve the wind resource. A constriction to the valley may further enhance wind flow by funneling the air through a smaller area. This is often the case in narrow mountain passes or gaps that face the wind.

Valleys often experience calm conditions at night even when adjacent hilltops are windy. Cool, heavy air drains from the hillsides and collects in the valleys. The resulting layer of cool air is removed from the general wind flow above it to produce the calm conditions in the lowlands. Because of this, a wind turbine located on a hill may produce power all night, while one located at a lower elevation stands idle. This phenomenon is more likely to occur on high terrain features that reach at least several hundred feet above the surrounding land.

High terrain features can accelerate the flow of wind. An approaching air mass is often squeezed into a thinner layer so it speeds up as it crosses the summit. Over a ridge, maximum acceleration occurs when the wind blows perpendicular to the ridge line. Isolated hills and mountains may accelerate the wind less than ridges because more of the air tends to flow around the sides. The downward, or "lee," side of high terrain features should be avoided because of the presence of high wind turbulence.

Land areas adjacent to large bodies of water may be good wind sites for two reasons. First, a water surface is much smoother than a land surface, so air flowing over water encounters little friction. The best shoreline site is one where the prevailing wind direction is "on-shore." Second, when regional winds are light, as on a sunny summer day, local winds known as sea or lake breezes can develop because the land and water surfaces heat up at different rates.Because land heats more quickly than water, the warm rising air over the land is replaced by the cooler air from over the water. This produces an on-shore breeze of typically 8 to 12 mph or more. At night the breeze stops or reverses direction, as the land cools more quickly

Source :
Wind energy handbook,1987