sufficient variation in brightness, this may indicate the direction of the sun, and be used as a cue to orientation. The effect is greater if the sun is relatively low in the sky, the water is clean, the depth fairly shallow and the surface fairly smooth. In some circumstances the diver can look up at the surface, to see in which direction the land lies. These cues will not give any precise information about position, but will allow the diver to keep a mental picture of where he or she is and is going. Current, wave and surge direction Current direction can be useful as an orientation cue as long as the direction of the current is known. In rivers it tends to be fairly consistent and reliable, though localised eddies may occur. In the sea it may depend on weather conditions and local topography, as well as the state of the tide. In estuaries and harbours the currents will usually be predominantly tidal, so the state of the tide must be known, as the difference in direction between ebb and flow is usually about 180°. Wave surge direction is essentially the same as wave direction, but may be felt at depths where the wave direction is no longer visible. It is useful if the offshore wave direction relative to the shore is known and does not change appreciably during the dive. In shallow water the wave crests will often be parallel to the shore. The important difference is that waves can be seen to travel in a definite direction, whereas surge is a back and forth motion, allowing a possible 180° error. Ripple patterns on a sand bottom A regular and distinct ripple pattern on a sand, mud or gravel bottom is an indication that it has been affected by wave action. The surge of the wave at depth causes the particles to be moved backwards and forwards in the direction the waves are travelling. This movement produces a ripple pattern on the bottom which is an indication of the wave direction on the surface. The ripple crests will be approximately parallel to the crests of the waves that formed them. It is however possible for the surface waves to change direction, and due to shorter wavelength, not reach the bottom to change the ripple pattern. When this is the case there will be no surge at the bottom.