Tidal phenomena are not limited to the oceans, but can occur in other systems whenever a gravitational field that varies in time and space is present. While tides are usually the largest source of short-term sea-level fluctuations, sea levels are also subject to forces such as wind and barometric pressure changes, resulting in storm surges, especially in shallow seas and near coasts. These data are compared to the reference (or datum) level usually called mean sea level. Gauges ignore variations caused by waves with periods shorter than minutes. To make accurate records, tide gauges at fixed stations measure water level over time. Tides vary on timescales ranging from hours to years due to a number of factors, which determine the lunitidal interval. A "mixed tide"-two uneven magnitude tides a day-is a third regular category. Other locations have a diurnal tide-one high and low tide each day. Many shorelines experience semi-diurnal tides-two nearly equal high and low tides each day. They are however only predictions, the actual time and height of the tide is affected by wind and atmospheric pressure. The predictions are influenced by many factors including the alignment of the Sun and Moon, the phase and amplitude of the tide (pattern of tides in the deep ocean), the amphidromic systems of the oceans, and the shape of the coastline and near-shore bathymetry (see Timing). Tide tables can be used for any given locale to find the predicted times and amplitude (or " tidal range"). Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun, and the rotation of the Earth. Tide coming in, video stops about 1 + 1⁄ 2 hours before high tide
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