The reply was:At what wind speed causes switch.
Kind of hard to quantify that but I guess we can assume that the actual wind speed as measured with for an anemometer is not really necessary, only if there is any wind (a breeze) present or not. While I liked just suspending a magnet over a magnetic reed switch I have seen methods as simple as gluing a small cardboard tab to a toothpick and gluing that to a lever actuated micro-switch.Any speed does not matter, however we have to consider that it is not much wind speed.
I guess if the thread starter wants to return with more information there can be more ideas and methods suggested. Beats me?