Rise and fall of the water level in a particular place. Tides have an approx cycle of 12 hours and are affected by the Moon's phase and position relative to the Sun and Earth. Certain areas around the world have no appreciable rise and fall of tide, eg Mediterranean Sea. Spring tides bring about the highest high tides and the lowest low tides. Neap tides have smaller high tides and low tides compared with spring tides. All charts are reduced to the chart datum (CD), which is usually the lowest spring tide (LLWS or LAT). Drying heights are the distance an object projects out of the water at CD. Charted vertical clearances are measured from mean high water springs (MHWS) and the height of a light is also measured from this level.