How can the WS425 be protected from lightning? 

The WS425 can be protected with a  lightning rod.  The rod must be installed with the tip at least one meter above the wind sensor and must be properly grounded, compliant with all local applicable safety regulations.  The lightning rod is recommended for systems that are installed high above the ground. Without the rod, a bolt of lightning may travel along the WS425's grounded housing and may induce a high voltage peak potentially damaging the device's electronics .

For more details consult the WS425 users manual. Learn about the surge protector for all Vaisala wind and weather instruments, WSP150.