On Tuesday, August 8, 2017 at 12:29:58 PM UTC+1, Caecilius wrote:
On Mon, 7 Aug 2017 13:56:27 -0700 (PDT), misterroy
wrote:

Am I correct in thinking that large wind turbines do not produce power when the wind speed is below 12mph?
Why on some days do they stop altogether and others they keep turning, even though there is hardly any wind at all?

It could be the wind gradient: the wind speed drops off quite a bit as
you get close to the ground. So depending on how high the blades are,
the wind velocity could be higher than at ground level.

there are calculators on the web for the wind gradient, and the effect is quite small, at low speeds anyway.