Tidal action results when the waters of oceans and large lakes are affected by the gravitational pull of our sun and moon. The rotation of the earth causes this gravitational effect to vary geographically in a cyclic way, with a period corresponding to a "tidal day," the time of rotation of the earth with respect to the moon, approximately 24.84 hours. The term "tides" is a generic one and refers to the alternating rise and fall of the oceans with respect to the land. Although tidal action causes water to move horizontally as well as vertically, we measure tides as the height of the water surface above a reference point (mean sea level) at a particular geographic point on a shoreline.
Water levels fluctuate in a consistent way in response to tidal movement, but they are modulated by a number of other factors and so are not perfectly regular. In addition to the gravitational pull of the sun and moon, tides also are influenced by coastline configuration, water depth, and seafloor topography, as well as hydrographic and meteorological conditions.
Tides also vary by geographical location and by season. Tides are higher when the Earth is closest to the moon and strongest overall when the moon and sun are aligned, increasing the total gravitational pull. In the northern hemisphere, tides are lowest during the winter when the Earth is farthest from the sun. The tidal range (difference between high and low tide) also varies throughout the year and is largest during the spring.
There are three basic types of tides: semidiurnal (twice-daily), mixed (also twice-daily), and diurnal (daily). A semidiurnal tide has two high waters (high tides) and two low waters (low tides) each tidal day, with the two high waters approximately equal in height, and the two low waters also approximately equal in height. A mixed tide is similar to the semidiurnal except that the two high waters and the two low waters of each tidal day typically have marked differences in their heights. A diurnal tide has one high water and one low water each tidal day.