Tides refer to the rise and fall of sea levels caused by the gravitational pull of the sun and moon coupled with the Earth's rotation. Because the moon is closer to the Earth than the sun, it has a stronger affect on tides. The Office of Naval Research's Science & Technology Focus says “gravitational pull depends on distance and mass.” The moon's gravitational pull also depends on the part of the globe the tide occurs, meaning some places have weaker tides while others experience stronger tides. Different kinds of tides display unique characteristics, which may help people visiting the coast.
Spring and Neap Tides
Spring tides are strong tides that occur during full moons and new moons. “Types of Tides and Tidal Currents” by Kenn Oberrecht, published by Oregon.gov, says these tides are strong because the moon and sun align, causing extremely high or extremely low tides. Though they are called spring tides, they occur twice a month at any time of the year. A proxigean spring tide, a rare high tide, occurs during the new moon phase when it is close to the Earth. HiWAAY Information Services says proxigean spring tides may occur every year and a half. Neap tides occur when the sun and moon form a right angle, according to Science & Technology Focus. The moon is in its first and third quarter phase, which counterbalances the gravitational pull between it and the suns. Neap tides are characterized by minimal high and low tides.
Diurnal and Semi-diurnal Tides
Diurnal tides happen within in a 24-hour period, characterized by one high water and one low water tide a day. The Florida Department of Environmental Protection's Land Boundary Information System (LABINS) says areas along the Gulf of Mexico located west of Apalachicola, Florida, experience diurnal tides. Semi-diurnal tides consist of two high tides and two low tides in a 24-hour period. They usually occur in waters along the Atlantic Ocean.
LABINS defines mixed tides as tides with conspicuous “diurnal inequality in the higher high and lower high waters and/or higher low and lower low waters.” In general, they alternate between diurnal and semi-diurnal tides. Mixed tides generally occur on the West Coast. Occasionally, they occur on the Gulf Coast.
Riptides are underwater currents that go away from the shore, unlike tides affected by the sun and moon. According to the Department of Atmospheric Science at the University of Illinois, riptides occur when hurricanes and tropical storms push waters toward the shore while currents try moving away from the shore. This push and pull cause underwater gaps. Swimmers trying to reach the shore during riptides often have to fight against currents or get swept into the sea.
Each kind of tide exhibits different characteristics, depending on position and distance from the sun and moon. They are also unique to their global proximity and whether or not they take place on the water's surface or underneath. All of these factors work together to cause tides to take place at certain times a day and how frequently they come. Beach goers may use this information to tell them when to go near the water.