and other Antarctic coastal waters. McMurdo Sound experiences katabatic winds spilling down from the Antarctic polar plateau. McMurdo Sound freezes over with sea ice about thick during the winter. During the Antarctic summer when the pack ice breaks up, wind and currents may push the ice northward into the Ross Sea, stirring up cold bottom currents that spill into the ocean basins of the world. Temperatures during the winter months at McMurdo Station have dropped as low as . December and January are the warmest months, with average highs at . Effects of wind Polar winds are a driving force behind weather systems arising from three surface zones that converge at McMurdo Sound: the polar plateau and Transantarctic Mountains, the Ross Ice Shelf, and the Ross Sea. These surface zones create a range of dynamic weather systems. Cold, heavy air descending rapidly from the polar plateau at elevations of or more spawns fierce katabatic winds. These dry winds can reach hurricane force by the time they reach the Antarctic coast. Wind instruments recorded Antarctica's highest wind velocity at the coastal station Dumont d'Urville in July 1972 at (Australian Government Antarctic Division). Prevailing winds spilling into McMurdo Sound shoot between mountain passes and other land formations, stirring up blizzards known locally as "Herbies". Such blizzards can occur any time of year. Residents of McMurdo Station and Scott Base have dubbed the nearby White Island and Black Island as Herbie Alley due to winds that funnel blizzards between the islands (Field Manual for the U.S. Antarctic Program). Overall the continent's extreme cold air does not hold enough moisture for significant snowfall. The water equivalent from annual snowfall on Ross Island averages only . Snowfall in Antarctica's interior is far less at . Snow seldom accumulates on the McMurdo Dry Valleys on the western shores of McMurdo Sound. McMurdo Sound provides an important component in Antarctica's global effects upon climate. A key factor is the polar winds that can drive the sound's pack ice into the Ross Sea summer or winter. Frigid katabatic winds rake subsequent exposed water, causing sea ice to form. Freezing surface water excludes salt from the water below; leaving behind heavy, cold water that sinks to the ocean floor. This process repeats itself along Antarctica's coastal areas, spreading cold sea water outward into the world's ocean basins. According to an interview with a climatologist Gerd Wendler, published in the