As the air is compressed by the force of the wind against the buildings or mountains, its speed increases in proportion to how much it is compressed. This means that the speed of the air between obstacles to the wind can be considerably more than in the surrounding area. This is known as a "tunnel effect".

So, in an area where the average wind speed over the open terrain is, for example, six metres per second, a natural tunnel in the area can easily experience an average of nine metres per second.

By using the benefits of such a natural tunnel for a wind turbine it is quite possible to obtain greater output. The higher wind speeds through the tunnel can be exploited to good effect. There are other considerations to be aware of, a good tunnel effect can only be achieved if the the tunnel has "soft" walls. Where a valley or pass is set is hard ground it is likely that the sides would be very rough and uneven. This would negate any benefit of the tunnel effect by producing lots of turbulence. The rough edges cause the wind to whirl in many different (and rapidly changing) directions rather than in a predictable direction.