<P> Atmospheric tides are negligible at ground level and aviation altitudes, masked by weather's much more important effects . Atmospheric tides are both gravitational and thermal in origin and are the dominant dynamics from about 80 to 120 kilometres (50 to 75 mi), above which the molecular density becomes too low to support fluid behavior . </P> <P> Earth tides or terrestrial tides affect the entire Earth's mass, which acts similarly to a liquid gyroscope with a very thin crust . The Earth's crust shifts (in / out, east / west, north / south) in response to lunar and solar gravitation, ocean tides, and atmospheric loading . While negligible for most human activities, terrestrial tides' semi-diurnal amplitude can reach about 55 centimetres (22 in) at the equator--15 centimetres (5.9 in) due to the sun--which is important in GPS calibration and VLBI measurements . Precise astronomical angular measurements require knowledge of the Earth's rotation rate and polar motion, both of which are influenced by Earth tides . The semi-diurnal M Earth tides are nearly in phase with the moon with a lag of about two hours . </P> <P> Galactic tides are the tidal forces exerted by galaxies on stars within them and satellite galaxies orbiting them . The galactic tide's effects on the Solar System's Oort cloud are believed to cause 90 percent of long - period comets . </P> <P> Tsunamis, the large waves that occur after earthquakes, are sometimes called tidal waves, but this name is given by their resemblance to the tide, rather than any actual link to the tide . Other phenomena unrelated to tides but using the word tide are rip tide, storm tide, hurricane tide, and black or red tides . Many of these usages are historic and refer to the earlier meaning of tide as "a portion of time, a season". </P>

When is it going to be high tide