What do tide numbers mean

These also occur every two weeks, on the first-quarter and last-quarter moons. The question of time deserves some consideration as well. While clocks and calendars tell us a day is 24 hours long, a week seven days, and a month 28, 30, or 31 days long, a tidal day is 24 hours and 50 minutes long and the period during which the moon goes through all of its phases is Anglers quickly learn to "round off" these changes by assuming that each day the tide is "about an hour later" actually 50 minutes.

Another "round off" is to assume that quarter-moon tides come a week after spring tides. It's actually 7. Though tides are primarily affected by the location of the moon and sun, the shape of the basin in which the tide takes place also affects them. It's one thing when the basin is the Atlantic Ocean and quite another when the basin is the relatively small, shallow Gulf of Mexico.

Semidiurnal simply means that there are two high and two low tides each 24 hours and 50 minutes called a tidal day , with both highs about the same height and both lows about the same height.

Finally, diurnal tides have one high and one low during a tidal day. Everything discussed thus far relates to the astronomical tides, that is, the tides that are predicted to occur due to the gravitational attraction of the earth's waters by the moon and the sun. The actual tides, however, may be somewhat different from the predicted tides, due to meteorological conditions.

Barometric pressure can cause the tides to fluctuate up to 6 inches. The tides are higher when the barometric pressure is low, and vice versa. A more important factor affecting the tides is the wind. As the wind blows it "pushes" the water in the direction the wind is blowing. The wind affects the rise and fall of the tide by retarding, or increasing, the tidal currents. Wind direction, strength and the amount of time it blows are the main variables.

A brisk wind of knots may not seem to have any effect on the tide for a couple of hours, but let it blow for a half day or more and the tide times will be quite different from those predicted. If you are fishing along the lower Gulf Coast and the wind is blowing from the east, the effect is to blow water out of the backcountry and bays, into the Gulf.

This causes extremely low tides and may hold back the incoming high tide, making it smaller and later than predicted. This is calculated by measuring how long it takes waves to hit a specific marker, usually a buoy. Swell direction lets you know what angle waves will approach from. Swell direction is marked on a tide table with a letter. Tides can also differ greatly among sites that are very near one another, so ensure that your tide table is as area-specific as possible.

Finally, tide predictions are just that… predictions! Aside from the moon, sun, and gravity, other factors affect tides, such as local weather and wind patterns. Be aware that the tides on the beach may not follow the tide table perfectly, and always exercise caution in or by the water. Photo by scot Never again will your sandy creation be thwarted by unanticipated tides!

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Join the Swim Guide community today. Remember that high and low tide times will be different every day, so do not make plans for the future based on a chart for the present. Understand Chart Datum. This term refers to a plane of reference against which tidal heights are measured. It is an average of low tide heights. This number is set low so that most low tides will not fall below it.

It is also the reference point that nautical charts use to measure depth. Find high tide information. This number indicates how high above Chart Datum the tide will be at its greatest swell.

A high tide indicated as an 8 tells you that at its highest, the water will be 8 feet 2. Remember that outside the United States the metric system prevails and measurements will be in meters, not feet. Find low tide information. Low tide will be expressed as a small number, or possibly a negative number.

This number indicates the relationship between the water depth at its lowest ebb and Chart Datum. Because Chart Datum is an average, not the absolute lowest the tide could ever be, sometimes low tide will fall below the average low water mark. When low tide is expressed as a negative - , it means the tide will fall below Chart Datum.

For example, -1 means the low tide will be 1 foot 0. The low tide may also be a positive number. The number 1.

Read the time. Depending on location, high and low tides may occur twice in a day or only once. The time may be listed using a 24 hour clock, also known as military time. Be certain that you have distinguished properly between morning and evening tide times.

To read a military clock, remember that the numbers continue onward from noon. So could also be written as because it is noon 12 plus 1 hour If the number is large and you don't want to count up, simply subtract 12 to get the number in the a. For instance, will be 11 pm. Understand swell direction. This information, often indicated by a letter, tells you from what direction the waves will be coming.

For instance, the letter W tells you that the waves will be coming from due west. This information is particularly important for surfers. If the beach faces north, but the waves are coming from the northwest, the waves will break differently as they hit the beach or point break. Read height and interval. Some tide tables include predictions about wave height, also referred to as the swell, and wave interval.

Wave height is the distance between the crest of the the wave and the trough, or the lowest point in a wave. The interval indicates the average number of seconds that elapse between waves as they hit an offshore buoy. Shorter intervals make for gentler waves, which are safer for swimming. Large swells and long intervals make for massive surf. Part 2. Learn tide patterns. Understanding the local pattern can help you predict whether you will have 2 daily opportunities to navigate or beachcomb, or only one.

In general, it takes around 6 hours for a tide to fully recede and hit the lowest point, then another 6 hours for it to come back in to the highest point. In many coastal regions, tides rise and fall twice every day. These tides are called semidiurnal tides and are common on the east coast of the United States.

In some places one of the high tides is dramatically higher than the other, a pattern called mixed tides. This occurs in many places on the west coast of the United States. In some places there is only 1 tidal cycle per lunar day, with 1 high and 1 low tide in a hour period.

This is common in many places along the Gulf of Mexico. Take advantage of high tide. When the water is at its highest, boats can safely pass over obstacles that might otherwise cause problems, such as reefs or sand bars.

This is often the best time to go in or out of harbor. Notify of. Notify of all new follow-up comments Notify of new replies to all my comments. Inline Feedbacks. Luke Simonds. Reply to A. Vincent Imbro. Reply to Vincent Imbro. Reply to Tripp. Buddy Harrison. Excellent refresher Luke! Reply to Buddy Harrison. James Biggart. Last edited 1 year ago by James Biggart. Reply to James Biggart. The depth numbers are based on the chart datum, which is the mean low water mark.

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