If your Davis anemometer starts to indicate wind speed zero or seems to work intermittently, most of the time the problem is with the reed switch. The glass is probably cracked and humidity has damaged the contact inside. The switch can be replaced. Follow the easy steps below. Maplin UK sells a reed switch, which I found can be used to replace the faulty one. The code for the product is CL36P.
Remove the cups from the anemometer to expose the sealant holding the reed switch in place. Carefully pull the sealant with a long nose pliers and unsolder the reed switch from the black and red wires.
Take the new reed switch and carefully bend wire on one side as shown without breaking the glass (use a long nose pliers to hold wire before bending). Then solder the reed switch to the black and red wires.
Carefully push the wires and the reed switch inside the anemometer assembly and apply some clear silicone sealant to hold it in place.