Frigid katabatic winds rake subsequent exposed water, causing sea ice to form. Weather fronts rarely penetrate far into the continent, because of the katabatic winds. Ice and strong katabatic winds often prevent landings, either by boat or by helicopter. At night, the sky is usually clear with katabatic winds blowing down the valleys. In spring, katabatic winds exacerbate fire danger due to the their warm and dry nature. Called by Mawson "the windiest place on Earth", the site experiences fierce katabatic winds. The strong katabatic wind there is the northeastern bora. Katabatic winds are somewhat rare events, and can be extremely violent. The bora is a northern to north-eastern katabatic wind in the Adriatic Sea. Surface temperature inversion is typical of polar environments and leads to the katabatic wind phenomenon. Katabatic winds can be important in this process. The Brookings effect is a katabatic wind that affects the southern coast of Oregon. It is theorised that these canyons were created by katabatic wind. They are not the same as anabatic and katabatic winds, which are larger and stronger. Thus the williwaw is considered a type of katabatic wind.