This is a stroboscopic effect which happens when you are seing repeated images of the tire. This can happen if you see the tire on film (often 24 frames a second) or on a TV or lit by an AC electric light or any process which regularly breaks up your image of the tire, such as looking through a fan at the tire. Normally the eye will interpret the sucessesion of rotated images of the tire as the tire rotating because each image has rotated a little further than the previous one. If the tire is rotating quickly enough that by the time you see the next image of the tire, it has rotated so much that patterns on the tire are slightly backward from the previous image, (for example, maybe it made 99% of a turn), then what you are seeing is exactly the same as it would look if the tire really was moving backwards and so that is what you see.

Let's take an example of a fan with three blades to represent the wheel. Let the blades be labelled A, B, and C. Our eyes transmit images to the brain in "pulses", in very short time intervals. Let's say the brain "sees" blade A at a certain position. The fan or wheel is moving very fast. If, by the time the eye sends the next pulse to the brain, blade B is exactly at the position originally occupied by A, and for the next pulse, C is at that position, then the wheel or blades of the fan will appear to be stationary. (The brain won't be able to record images in between the pulses.) Now if at the time of the next pulse, B is just ahead of the postion originally occupied be A, and the same to C, then the wheel or blades will appear to move forward slower than their actual speed. But if A(after completing a whole circle) ,B, or C, is not yet in the position originally occupied by A, then the wheel or blades will appear to move backward slowly. This effect may be prevented to an extent by painting each of the blades with a different colour, so that the brain can differentiate between them in consecutive images.

The effect requires "pulsing" of the light, or some of the light, coming from the moving object, agreed, but it doesn't need a strobe light or the like. It can be seen in ordinary daylight under a variety of circumstances. The spokes of wire wheels will reflect a flash of light at a particular moment, for example, and create the effect. Or something with spokes or the like moving in and out of shadow. Even a circulating desk fan can give the effect of moving forward then backward then forward as it slows down.

I believe this effect in the human eye is called "Temporal Aliasing". A good resource for information is: http://en.wikipedia.org/wiki/Temporal_aliasing IRT Bob Blaylock: If you read the article, and the other related links, you'll see that scientists have observed the aliasing effect with no strobe lighting. You'll also see their hypotheses as to why this happens. [Edit] Basically this comes down to the eye and brain. Even though they are organic 'real time' organs, they are still sensitive to frequency. Photosensitive epilepsy is proof of this. If you stare at a spinning ceiling fan, or blink while looking at one, you'll see that occasionally your eyes will catch a snapshot of the blades frozen. When you see the wheels of a car seeming to slow down or even move in reverse while the vehicle is moving forward, it is simply because the frequency of rotation is at the point that your eyes' sensitivity to frequency pick up snapshots of the wheel (similar to the fan example). What you are seeing is a series of freeze-frames all put together very quickly. The wheel might be spinning fast enough that at each freeze frame the wheel appears to be in the same position, making it look like it's standing still. Or, the wheel spokes or tyre labelling etc. might be just a bit back from where it was in each previous snapshot. This makes it look like the wheel is turning backwards. A really good example of this is to watch F1 or indy cars from the in-cabin camera view. Looking out the front you can see the wheels appear to speed up, then slow down, then stop, then go backwards, all as the car is accelerating - all from the position of the writing on the tyres. Obviously this will be exaggerated a bit by the camera filming it, but your eyes still interpret it the same way. As for what this has to do with an anemometer, I have no idea. An anemometer is a device for measuring windspeed and pressure. For more info on anemometers, see: http://clcgi.msu.edu/~rtsmith/anemomet.html http://en.wikipedia.org/wiki/Anemometer