Geography and Wind

If all of Iowa 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. The effect of trees and buildings is discussed later.

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 sheltered, 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 airflowing 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.

Surface Roughness
The surface over which the wind blows affects its speed. Rough surfaces, such as areas with trees and buildings, will produce more friction and turbulence than smooth surfaces such as lakes or open cropland. The greater friction means the wind speed near the ground is reduced.

The approximate increase of speed with height for different surfaces can be calculated from the following equation:
v2 = v1 x (h2/h1)n where v1 is the known (reference) wind speed at height h1 above ground, v2 is the speed at a second height h2, and n is the exponent determining the wind change. Values for n are listed in the following table for different types of wind cover. If the wind comes across a fallow crop field, you do not have to reach as high for greater wind speeds as you would in a forest or suburb.

Ground Cover n
smooth surface ocean, sand .10
low grass or fallow ground .16
high grass or low row crops .18
tall row crops or low woods .20
high woods with many trees suburbs, small towns .30

Here is an example of how this method is used. Suppose you are interested in buying a wind turbine and have taken measurements for a year with a wind speed instrument on a 30 ft. tower in an area of low woods. The average speed is 10 mph. You want to estimate the speed at the planned 100 ft. height of the wind turbine.

In your calculation v is equal to 10 mph, h1 is 30 ft and h2 is 100 ft. Since your surroundings consist of low woods, the correct value for n is .20. Plugging these values into the formula, the average wind speed at 100 feet is:

v2 = 10 mph x (100 ft/30ft).20
v2 = 10 x (3.33).20
v2 = 10 x 1.27
v2 = 12.7 mph

Again, this method only provides a rough estimate of wind speeds, not a precise value. it is most useful when using average and not instantaneous wind speeds. In addition, this formula should only be used for relatively flat terrain because hills and mountains often have unpredictable influences on wind characteristics.

Lastly, within dense vegetation, such as a forest or an orchard, a new effective ground level is established at approximately the height where the branches of adjacent trees touch. Below this level there is little wind. in a dense cornfield, this height would be the average corn height. In a forest, it would be the average height ofthe tree canopy, and so on. When using the wind speed equation all heights should be expressed above the effective ground level.

Trees and Buildings
Trees and buildings are the most common obstacles to wind in the vicinity of a potential wind turbine site. They act to disturb the air both upwind and downwind of the obstruction by reducing wind speed and increasing turbulence.

If it is impossible to avoid all obstacles entirely you may want to use these siting rules of thumb:

    1. Site the wind turbine upwind at a distance of more than two times the height of the obstruction.
    2. Site the wind turbine downwind a minimum distance of 10 times, and preferably 20 times, the height of the obstruction.
    3. Site the wind turbine hub at least twice the height of the obstruction above ground, if the wind turbine is immediately downwind of the obstruction.

The upwind and downwind directions can be defined as being aligned with the prevailing wind direction.

Buildings that are wider than they are tall have an influence on air flow at greater distances downwind than do taller buildings. For example, at a distance 20 times the building height downwind, only very wide buildings produce more than a 10% decrease in the available wind power (those in which width divided by height = 3 or more). On the other hand, tall, narrow buildings will create power reductions of less than 10% at distances as close as 5 building heights downwind. Most residential structures, such as houses, barns and garages, are as wide or wider than they are tall so the previously mentioned rules of thumb should be followed.

Vegetative Indicators of Wind
Trees growing in an area of high winds are often permanently deformed. Severe deformation, such as when the tree trunk is bent away from the prevailing wind direction, occurs at wind speeds of 15 to 18 mph. However, “brushing” or “flagging” can be seen in a tree exposed to average speeds as low as 8 to 10 mph which prevail from one dominant direction. An examination of the vegetation in an area can be a rough indicator of the wind strength there.

We see brushing commonly in deciduous trees, like maple, oak and elm, where the branches and twigs bend downward like the fur of a pelt that has been brushed in one direction.

Flagging is common in coniferous trees, like pine and spruce. It is indicated by branches that stream downwind and by short or missing upwind branches.

The absence of deformation does not necessarily imply that the wind resource is weak. Some tree species are more sensitive to the wind than others. Trees within a continuous forest, for example, are too sheltered, and strong winds may blow from more than one major direction. Therefore, use tree deformation only as a rough guide and not as a primary tool in selecting a wind turbine site.

Iowa Winds

Iowa is fortunate to lie at the eastern edge of the Great Plains where winds blow strongly and steadily, particularly in the winter and spring. The relative flatness of the terrain means that most areas of the state are well exposed to the wind. In addition, most of the state consists of cropland with few trees to reduce wind speeds near the ground. Iowa is, consequently, an especially attractive state for wind power development, and several projects are already in progress or under consideration.

Parts of Iowa fall into a major area of the United States which is very suitable for wind power generation. Of Iowa’s 144,950 km2 of total land area, 39.1%, or 56,700 km2 is potentially available windy land. According to studies prior to actual measurements performed by a grantee of the Iowa Energy Center, the state’s wind electric potential was estimated at 62,900 Megawatts, at a hub height of 50m and assuming 25% efficiency and 25% energy loss. This estimate assumes the availability of land and transmission line access.

The prevailing winds in Iowa come out of the southeast and northwest, but significant winds also come from the south, southwest, west and north directions. It is the northwest winds which provide the greatest wind power potential. East and northeast winds, when they occur, provide the lowest wind power levels.

There are average variations in windspeeds daily and annually which are valuable to consider when trying to assess the load capacity of wind power. Ultimately, wind is an intermittent power source since there is no definite way to predict whether there will be wind when there is a corresponding high power demand. However, as part of a mix of power sources, wind can be quite reliable and inexpensive. In Iowa, average hourly year-round wind speed varies between 11.5 mph at low wind sites, to 17.5 mph at high wind sites. Keep in mind that these are average wind speeds. Actual wind speeds at any given time may be much higher or lower. There is a slight rise in average wind speeds between 1 pm and 6 pm daily. Afternoon heating of the earth with respect to the air tends to increase air currents, and therefore wind. There is usually a temporary drop in wind speeds at sunrise and sunset when the temperature difference between the air and the earth is lower.

On an annual basis, July and August are the lowest wind months. November through March are the highest wind months. The wind is more powerful during the winter because of the cold temperatures. When the temperature is low, air density is high. At high air density, the same wind speed delivers more air, and thus more power. In the winter time, much of Iowa has Class 4 winds, meaning that this time of year is very good for wind power generation.

In general, wind speeds in Iowa increase from southeast to northwest. But there are high wind areas in central and north central Iowa that rival the more famous Buffalo Ridge sites in northwest Iowa. Exposed cropland and elevated sites in each region will have the highest winds in that region. Lower and more sheltered locations will tend to have less wind power. This is especially true in the autumn and winter when vertical air mixing is restricted, causing wind speeds to be less than at more elevated locations. Estimated wind speeds for each of 2,000 towns and cities and wind maps of Iowa can be found on the Wind Index page. (must use BACK button to return to this page)

Usually there are layers of faster moving air at higher elevations above the ground. Wind towers are made as tall as possible to take advantage of these higher wind speeds. The change in wind speeds at different heights is called wind shear. Iowa has a higher than average wind shear, especially in locations that are hilly or forested. The wake effects of hills, trees and buildings can cause turbulence which lowers wind speeds up to 50 meters above the ground. Wind shears are less on hot and windy days where there is more vertical mixing of the air. It is often reversed in the early morning hours before dawn. But, in general, the taller the tower, the greater the wind power.

The daily and seasonal variations in wind speed have a great effect on planning for a distributed generation system. For example, if you were interconnecting a wind turbine to a manufacturing company which uses more energy in the morning or during the summer, and shuts down at night, you might only be able to use directly 25% of the electricity the turbine generates. If you were interconnecting the turbine to a school which uses little power while classes are out of session in the summer, your ability to use the available power would be higher. Farmers often need a lot of electricity to run grain dryers in the fall when the winds are high.

When wind farms are connected to the grid, all of the low cost wind power can be utilized. Now that electricity can be bought and sold on the spot market, the intermittency of wind is less of a factor in determining the suitability of investing in wind power. As the market for green energy emerges, wind power will be increasingly sought after. Part of the cost of wind energy is the “wheeling charge” to move the power from its source to an urban center. Wind power produced in Iowa will have increased value for midwest urban centers.

The intermittency of wind can be further reduced by distributing wind farms over a larger geographic region. Winds are produced as weather fronts move through the state. As a front leaves a northwestern Iowa wind farm, it may just be starting to produce maximum power at a central Iowa wind farm.