Kevin Kilty

Some time back I provided a brief essay on acoustic issues with regard to wind turbines, and why I think methods of calculating acoustic levels and then making adequate allowance is deficient. In this related essay I tackle the strange issue of shadow flicker.

Shadowing is not a new issue in planning and zoning. Most jurisdictions at least acknowledge the need for windows on existing structures to not have light and air cut off by planned development on adjacent lots. Unfortunately the language used is filled with subjectivity such as “unreasonable”.  And practically all jurisdictions state that not all shadowing is detrimental, but may at times provide comfort from heat of the sun.  Moreover, in many cities provisions are made to make sure that such light sensitive features as gardens and solar water heating or solar PV systems are not negatively impacted. Some jurisdictions have found a way to ensure no interference with installed solar energy systems is by way of solar easements or permitting ( See for example the City of El Paso ordinances chapter 20.12.030 – Supplemental height regulations.)

However, wind turbines present unusual considerations. First, their shadows present no relief from heat and glare. Second, the shadows of the blades are moving in repeating patterns. I do not find complaints that the repeating shadow patterns are a health hazard particularly convincing, especially the claims that they can induce seizures [1], but they certainly fall under the category of a nuisance that a planning commission or regulatory agency may be required to consider or should be required to consider means of reducing to an acceptable level.

What piqued my interest in shadow flicker was actually observing the shadow of a wind turbine near sunset along the old Lincoln highway near Medicine Bow, Wyoming in mid-June. It was an odd observation — a shadow of very strange appearance circulating on a field south of the highway. I intended to return at some point in the summer prepared to capture some video, but we have had so much smoke from fires in the air that there is no sun visible near the horizon, nor sometimes any distinct shadows cast at midday. This goal will have to await clearer skies in autumn, perhaps.

The modeling or estimation of shadow flicker

With my new interest I decided to read that part of the ISC application dealing with shadow flicker. Several statements caught my attention. Both concerned rules about the circumstances under which shadow flick will occur. For example, background from Section 3 of the ISC application reads:

“However, when the sun angle is very low (less than 3 degrees), sunlight passes through more atmosphere and becomes too diffused to form a coherent shadow. Shadow flicker does not occur when the sun is obscured by clouds or fog, at night, or when the source turbine(s) are not operating. In addition, shadow flicker occurs only when at least 20 percent of the sun’s disc is covered by the turbine blades.”

There are two “rules” being announced here that are not obvious. Let’s take this statement apart starting with the second rule about shadow flicker occurring only when 20% or more of the sun is covered by an object. This statement is difficult to label as obviously true or false because of the confounding of a shadow, the source of shadow flicker, with the psychological perception of flicker. It is not true that a shadow occurs only when the blocking object is nearer than some fixed distance relative to size of the obstruction. A geometrical shadow is a very simplified model of what happens when an object blocks a beam of illumination.

What actually occurs when an opaque object obstructs light is that the edges of the object serve as secondary, or even tertiary, point sources of scattering. Interference of this scattered energy in the illuminated field beyond the obstruction produces what we observe — shadows are a diffraction pattern.[2] In the near field diffraction produces a shadow with sharp edges the  geometry of the obstruction, but very quickly with increasing distance the edges of shadows blur into a more complex pattern of illumination. Figure 2 shows this. The chain link casts a distinct, sharp edged shadow on the sidewalk on the right side of the photo. Here the wires subtend a large angle from the surface of the walk. On the left side the wires subtend a small angle that is less than 20% the angle subtended by the sun. Even though the shadows are not sharp edged, they are certainly not gone. In fact, the shadows of the top chord or beam of the fence have diffused edges even though in this instance at twice the subtended angle of the sun, a person might think this pipe should cast a geometrical shadow. Shadows grow in width to become broader than the object causing them. The actual cross section of an obstruction is some two times larger than its geometrical cross section — a result known as the extinction paradox.[3]  Shadows are even stranger than this, as a shadows cast by a solid object becomes in the far field the same as a shadow cast by its complement object, an aperture in a baffle — Babinet’s principle. There even develops a bright region at the center of the shadow.

Let’s now analyze that first rule in the background information dealing with sun elevation. In places where the atmosphere is nearly saturated with water vapor, and filled with clouds, haze or smoke, then sun reddens near the horizon and the illumination is so diffuse that no sharp shadows result. However, this is not a universal occurrence, and one ought to be obliged to actually address what goes on locally. At all Wyoming localities the air is dry and very clear at most times. Figure 3 shows a shadowed scene with low angle sun. The date and time stamp (July 7, 8:25 pm MDT) I calculate as pertaining to a sun angle only 1.4 degrees above the horizon, and this is verified by the distance down the wall a shadow from another portion of the building casts on the brick. It is obvious, however, that distinct shadows are cast. I believe sharp shadows will be cast by the sun setting right to the horizon on this day, which is not out of the ordinary in Wyoming.

Figure 3. Shadows still have sharp contrast even with the sun only 1.4 degrees above the horizon when the air is clear.

Having stated two rules of shadowing with dubious validity we should ask “do the consultants actually employ these rules in making an EIS or agency application?” The answer is yes. From the application once again:

“Sun angles less than 3 degrees above the horizon were excluded for the reasons identified earlier in Section 3. Since shadow flicker is only an issue when at least 20 percent of the sun’s disc is covered by the blades, WindPro uses blade dimension data to calculate the maximum distance from the turbine where shadow flicker must be calculated.”

Thus, we have solid reason to believe that both the duration of shadow flicker and the area affected are likely underestimated. This employment of rules or standards of dubious pertinence also formed the basis of my criticisms of acoustic studies.

Is flicker a real or imagined nuisance?

Does the shadow of wind turbine blades produce a real nuisance sensation or not? At some distance it surely does not, but one has to keep in mind several facts. First, our sensory system is tuned to rates of changes in sensation as much if not more than magnitude. Second, people vary enormously in sensitivity to practically every chemical or sensory stimulation one can name. Third, there is often an interaction among several sensory inputs which will magnify responses to one or the other or both. A clear example of this is Weber’s phenomenon where a cold to the touch object feels heavier than an identical object that is warm to the touch — an interaction between the pressure and heat flow sensors of the skin. In the case of shadow flicker there appears to be an interaction between wind turbine noise and annoyance from shadow flicker. What an engineer concerned with shadow flicker would do is to consult a table of ergonomic data indicating the variation of illumination which 99%, or some other proportion, of the population will not notice in various circumstances.[4] I know of no such data available for things like flicker or wind turbine noise, however.

A good video illustrating what shadow flicker looks like is available, currently, on YouTube here.  One might contemplate that some of the visual effect is contributed by response of the video camera itself to the changing illumination level, but nonetheless it shows clearly the complex nature of the shadow itself, and how one’s impression of the flicker changes depending on whether one’s current focus is toward or away from the obstacle turbine; i.e. whether one is viewing in forward scattered or back scattered light. Another interesting video showing a simulation of the problem, with a presenter who progresses a bit too slowly, is here.

How do the wind turbine developers respond to this complaint?

In researching this topic I discovered some interesting things. First, a number of video productions exist where employees of wind turbine developers or consultants talk about shadow flicker, and mention solutions to this problem. One of the most common suggestions is to plant trees. I don’t know how to classify such a silly suggestion. Waiting for trees to grow and obscure a nearby nuisance is a solution worthy of parody. Even when I lived in the Pacific Northwest I never observed hybrid trees farmed for pulp to grow fast enough to do this. A pertinent question is “why should one property owner have to screen their view in order to ameliorate a nuisance originating on a neighboring property?”

Discounting unworkable suggestions like planting trees between the turbine and the complainant, or adding window coverings, one useful solution mentioned in these videos is that wind developers have the technical ability to calculate when shadow flicker will potentially produce a nuisance on any neighboring property, and they can shut the turbine off during the time this occurs. [One example involving experts, another ]. This would obviously work to eliminate the problem. So would proper placement of the wind turbines in the first place.

However, what wind developers do in fact rather than what they might do hypothetically is best illustrated by replies to public comments in the final EIS of the Campo Wind Project in San Diego County, California.

“The Project design has been revised slightly to address potential nuisance-level shadow flicker effects. The Draft EIS explained that ‘all turbine software would include programming to reduce or shut off turbines during times of shadow flicker potential to avoid any concerns regarding adverse effects on nearby receptors due to flicker from turbine blades.’ Upon further consideration, it was determined that this design feature would significantly impact the economic benefits of the Project to the Tribe, thereby undermining the key purpose of the Project. It is also not a cost-effective measure to address effects that would only amount to a temporary visual disruption for certain receptors, if at all.”

Instead the developer suggests working with affected owners for vegetative or window screenings. This is also suggested for properties affected by the Fountain Wind project in Northern California in its final EIS. I already mocked the idea of planting trees, and the video of what flicker looks like should put to rest the idea of “window coverings” doing the job.

There is a sort of bait and switch strategy being employed here. Suggest something that will obviously work, but then be willing in reality to apply something cheaper but less effective instead. Indeed, promoters of the local wind farm proposal that has occupied my time lately sent out a fact sheet last year explaining, among other things, that 90% of a wind turbine is recyclable. This was in response to opponents’ concern that much of the decommissioned plant would eventually end up in some local landfill. They didn’t say they would recycle 90%; they just said that 90% could be. No doubt many people receiving this flyer in their mail inferred “can” to mean “would”. Yet, at just about the time they were suggesting the extent to which wind turbines might be recycled, the developers were also asking for a variance that would allow them to recover less during decommissioning than specified in statute.[5]


The environmental impact statements I have read this summer are pro forma. They look like cut outs of earlier successful EISs that are sometimes not even edited to be consistent with the specific site. Some are abbreviated versions of earlier reports which don’t contain the same suite of basic data that earlier versions contained. No problem. I know of no EIS that isn’t found to be acceptable by permitting agencies. Renewable energy is so important that careful work just gets in the way of progress. There are elected officials who have actually said as much.  A friend of mine who spent decades in mining operations and in the exploration for mineral deposits told me “The kinds of flaws you noted would have been a slam-dunk to crush a proposal for a new mine.” Yet, renewable energy is viewed differently.

There is no point insisting, as proponents do, that renewable energy assets don’t present nuisances and occasional dangers. At a minimum they present conflicts over views, property values, nuisances from sound and flicker, blinking red lights at night, and so on. Do they present a fire hazard? Well done EISs and applications can alert people to potential problems. Then, proper siting can mitigate many nuisances in the short term. In the long term however renewable energy is so land greedy that serious conflicts are inevitable.


[1]-Seizures are induced by light fluctuations more rapid than three per second. What wind turbines generate is far slower.

[2]-In fact, all light propagation is a diffraction phenomenon.

[3]-Charles Adams and Ifan Hughes, Optics f2f: From Fourier to Fresnel, Oxford University Press, 2019.

[4]-For example, in deciding where to place controls on the dashboard of a vehicle being designed engineers would consult ergonomic tables regarding the comfortable reach of various percentiles of the population. The various student contests run by the Society of Automotive Engineers (SAE) specify vehicle dimensions to accommodate the 95 percentile in most cases.

[5]-Asking for a variance allowing them to decommission all structures within three feet of ground surface rather than four feet. This is a big variance considering the massive foundation involved.

via Watts Up With That?

August 14, 2021