is called slack water or slack tide. The tide then reverses direction and is said to be turning. Slack water usually occurs near high water and low water, but there are locations where the moments of slack tide differ significantly from those of high and low water. Tides are commonly semi-diurnal (two high waters and two low waters each day), or diurnal (one tidal cycle per day). The two high waters on a given day are typically not the same height (the daily inequality); these are the higher high water and the lower high water in tide tables. Similarly, the two low waters each day are the higher low water and the lower low water. The daily inequality is not consistent and is generally small when the Moon is over the Equator. Reference levels The following reference tide levels can be defined, from the highest level to the lowest: Highest astronomical tide (HAT) – The highest tide which can be predicted to occur. Note that meteorological conditions may add extra height to the HAT. Mean high water springs (MHWS) – The average of the two high tides on the days of spring tides. Mean high water neaps (MHWN) – The average of the two high tides on the days of neap tides. Mean sea level (MSL) – This is the average sea level. The MSL is constant for any location over a long period. Mean low water neaps (MLWN) – The average of the two low tides on the days of neap tides. Mean low water springs (MLWS) – The average of the two low tides on the days of spring tides. Lowest astronomical tide (LAT) – The lowest tide which can be predicted to occur. Tidal constituents Tidal constituents are the net result of multiple influences impacting tidal changes over certain periods of time. Primary constituents include the Earth's rotation, the position of the Moon and Sun relative to the Earth, the Moon's altitude (elevation) above the Earth's Equator, and bathymetry. Variations with periods of less than half a day are called harmonic constituents. Conversely, cycles of days, months, or years are referred to as long period constituents. Tidal forces affect the entire earth, but the movement of solid Earth occurs by mere centimeters. In contrast, the atmosphere is much more fluid and compressible so its surface moves by kilometers, in the sense of the contour level of a