Sastrugi Sastrugi Sastrugi, or zastrugi, are sharp irregular grooves or ridges formed on a snow surface by wind erosion, saltation of snow particles, and deposition, and found in polar and open sites such as frozen lakes in cold temperate regions. The ridges are usually perpendicular to the prevailing winds; they are steep on the windward side and sloping to the leeward side. Smaller irregularities of this type are known as ripples (small, ~10 mm high) or wind ridges. Larger features are especially troublesome to skiers and snowboarders. Traveling on the irregular surface of sastrugi can be very tiring, and can risk
Surface wave detection by animals to test certain wave properties. Wave characteristics are measured optically. This is done by shining a laser (often helium-neon) at the water surface. Reflections and distortions of the laser’s beam are picked up by a photodiode. Fishes' movements are videotaped with a high-speed camera from above the testing tank so that the precise timing and nature of their responses’ to stimuli can be reviewed. "Aplocheilus lineatus" assumes a hunting posture at the water surface in which it holds the top of its head within 200 μm of the water surface. On perception of appropriate wave stimuli, fish first turn towards