Information AboutTide |
| CATEGORIES ABOUT TIDE | |
| tide | |
| oceanography | |
| physical oceanographytide | |
| oceanography | |
| physical oceanography | |
| bodies of water | |
| physical oceanography | |
| navigation | |
| moon | |
|
Tides are the cyclic rising and falling of Earth 's Ocean surface caused by the Tidal Force s of the Moon and the Sun acting on the oceans. Tides cause changes in the depth of the marine and Estuarine water bodies and produce oscillating currents known as tidal streams, making prediction of tides important for coastal navigation (see '' Tides And Navigation ''). The strip of seashore that is submerged at high tide and exposed at low tide, the Intertidal Zone , is an important ecological product of ocean tides (see '' Intertidal Ecology ''). The changing tide produced at a given location is the result of the changing positions of the Moon and Sun relative to the Earth coupled with the Effects Of Earth Rotation and the local Bathymetry . The orientation and geometry of the coast affects the phase, direction, and amplitude of coastal Kelvin Wave s as well as resonant Seiche s in bays. In estuaries seasonal river outflows influence tidal flow. Sea level measured by coastal Tide Gauge s may also be strongly affected by wind. More generally, tidal phenomena can occur in other systems besides the ocean, whenever a gravitational field that varies in time and space is present (see '' Other Tides ''). INTRODUCTION AND TIDAL TERMINOLOGY A tide is a repeated cycle of sea level changes in the following stages:
Tides may be semidiurnal (two high tides and two low tides each day), or diurnal (one tidal cycle per day). In most locations, tides are ''semidiurnal''. Because of the ''diurnal'' contribution, there is a difference in height (the daily inequality) between the two high tides on a given day; these are differentiated as the ''higher high water'' and the ''lower high water'' in Tide Table s. Similarly, the two low tides each day are referred to as the ''higher low water'' and the ''lower low water''. The daily inequality changes with time and is generally small when the Moon is over the equator. Tide tables usually list ''mean lower low water'' (mllw, the 19 year average of mean lower low waters), ''mean higher low water'' (mhlw), ''mean lower high water'' (mlhw), ''mean higher high water'' (mhhw), as well as ''perigean tides''. These are ''mean'' in the sense that they are predicted from mean data. Glossary of Coastal Terminology: H – M , Washington Department of Ecology, State of Washington (checked 5 April 2007 ). The various frequencies of astronomical forcing which contribute to tidal variations are called ''constituents''. In most locations, the largest is the "principal lunar semidiurnal" constituent, also known as the ''M2'' (or ''M''2) tidal constituent. Its period is about 12 hours and 24 minutes, exactly half a ''tidal lunar day'', the average time separating one lunar Zenith from the next, and thus the time required for the Earth to rotate once relative to the Moon. This is the constituent tracked by simple Tide Clock s. The Moon orbits in the same direction the Earth spins. Compare this to the minute hand crossing the hour hand at 12:00 and then again at about 1:05 (not at 1:00). Tides vary on timescales ranging from hours to years, so to make accurate records Tide Gauge s measure the water level over time at fixed stations which are screened from variations caused by waves shorter than minutes in period. This data is compared to the reference (or datum) level usually called Mean Sea Level . Tidal lunar day , NOAA . Do not confuse with the astronomical Lunar Day on the Moon. A lunar zenith is the Moon's highest point in the sky. Constituents other than ''M''2 arise from factors such as the gravitational influence of the Sun, the tilt of the Earth's rotation axis, the inclination of the lunar orbit and the ellipticity of the orbits of the Moon about the Earth and the Earth about the Sun. Variations with periods of less than half a day are called harmonic constituents. Long period constituents have periods of days, months, or years. Tidal range variation: springs and neaps The semidiurnal tidal range (the difference in height between high and low tides over about a half day) varies in a two-week or fortnightly cycle. Around New and Full Moon when the Sun, Moon and Earth form a line (a condition known as Syzygy ), the Tidal Force s due to the Sun reinforce those of the Moon. The tide's range is then maximum: this is called the ''spring tide'', or just ''springs'' and is derived not from the season of spring but rather from the verb meaning "to jump" or "to leap up". When the Moon is at first quarter or third quarter, the Sun and Moon are separated by 90° when viewed from the earth, and the forces due to the Sun partially cancel those of the Moon. At these points in the lunar cycle, the tide's range is minimum: this is called the ''neap tide'', or ''neaps''. Spring tides result in high waters that are higher than average, low waters that are lower than average, ''slack water'' time that is shorter than average and stronger tidal currents than average. Neaps result in less extreme tidal conditions. There is about a seven day interval between springs and neaps. The changing distance of the Moon from the Earth also affects tide heights. When the Moon is at Perigee the range is increased and when it is at Apogee the range is reduced. Every 7½ Lunation s, perigee coincides with either a new or full moon causing ''perigean tides'' with the largest ''tidal range''. If a storm happens to be moving onshore at this time, the consequences (in the form of property damage, etc.) can be especially severe. Tidal phase and amplitude When oscillating tidal currents in the stratified ocean flow over uneven bottom topography, they generate Internal Waves with tidal frequencies. Such waves are called ''internal tides''. MISAPPLICATIONS Tsunamis , the large waves that occur after earthquakes, are sometimes called ''tidal waves'', but this name is due to their ''resemblance'' to the tide, rather than any actual link to the tide itself. Other phenomena unrelated to tides but using the word ''tide'' are Rip Tide , Storm Tide , Hurricane Tide , and Black or Red Tide s. The term ''tidal wave'' appears to be disappearing from popular usage . SEE ALSO
EXTERNAL LINKS
Tide predictions
REFERENCES AND NOTES |
|
|