Brocken, the highest mountain in Northern Germany (1143 m), is famous for its harsh climate and represents a rigorous test environment for wind sensors. Our new sonic versions were operated here successfully in winter 2011/2012. The main change with regard to former models is the replacement of brazen material by stainless steel. Due to the limited thermal conductivity of stainless steel the implementation of an efficient and sufficient heating power turned out to be a challenge and called for verification in field tests.