High & Low Tide Times, and Monthly Tide Charts
What are Tides?
Tides are the ‘heartbeat’ of our ocean. Much like the sun rises each morning, tides occur predictably around the world. Beaches and treacherous reefs are often visible for enjoyment or avoidance (respectively!), but other times are obscured under many feet of ocean water. This regular rise and fall of water seems mysterious, but is a perfectly well-understood and explained phenomenon: a phenomenon caused by conditions far outside the bounds of our planet earth.
All boaters, fishermen, and people who spend time on the coast should have an understanding of how tides work. Knowing what to expect can help you stay safe on the water, help you find more fish, and even help lessen your fuel consumption!
What Causes Tides?
Tides are extremely long waves created by the gravitational pull of the moon and sun on the water. The water level rises near the coast as the crest of the tide (wave) approaches, and falls as the crest moves out into the ocean. These waves move very slowly compared to regular shoreline waves and for this reason are called ‘long-period’ waves.
The moon plays the dominant role in generating our tides. While the sun is larger (generating more gravity), its much further from Earth and therefore has about half of the moon’s gravitational pull on our ocean.
Because tides are primarily caused by the moon, most coastal areas experience two high tides and two low tides each lunar day. However some places will only experience one tidal cycle a day! A lunar day–a bit longer than a regular or solar day due to the moon revolving in the same direction as the earth spins–is 24 hours and 50 minutes. Generally each high tide occurs 12 hours and 25 minutes apart, with low tide occurring between.
Understanding High Tide and Low Tide
About “High Tide”
High tide is when the water level is at its highest point in any location. This occurs as the crest of the wave reaches this specific location. The tide is “rising” as it approaches this point, and begins “falling” after it has reached high tide. Often, the tide will leave a “high-tide line” composed of sea plants, foam, and other detritus deposited by the high water.
About “Low Tide”
Low tide is the opposite of high tide: the water has reached its lowest point. This occurs in the trough–or the lowest part–of the wave.
What Are “Spring Tides” and “Neap Tides”?
While the height of high tide and low tide varies in a predictable way, sometimes the gravitational pulls of the sun and moon combine to create tides with extreme highs and lows called “spring tides”. When the reverse happens, and theses forces go against each other, milder tides known as “neap tides” result. Generally there are two sets each of spring tides and neap tides each lunar month, alternating each week.
Tidal height will also vary due to the moon’s elliptical orbit around Earth, and Earth’s elliptical orbit around the sun. When the moon is closest to Earth–at perigee–it’s gravitational pull on Earth is greatest and tides are larger. They are smaller when the moon is further away, at apogee. In North America, the sun is closest to Earth each year on January 2nd, and resulting in larger-than-average tide ranges at that time of the year. It is furthest away on July 2nd, generally generating smaller ranges in tidal height.
While the timing of high and low tide is important to know, the range is also a critical piece of information. US Harbors’ tide charts provide this tidal data specifically calculated for your coastal location.
How Do Our Tide Charts Work?
US Harbors has been providing the best tide charts for over 10 years: they’re easy-to-read, printable and nice-looking monthly charts calculated specifically for the tides in your area. Here are some of the common questions our users ask.
Q: What are our tide high/low measurements based on?
A: The baseline for giving how high or low the tides are is called the MLLW, or Mean Lower Low Water. This is the average low water height for a complete 19-year Metonic Cycle (also known as the National Tidal Datum Epoch) – basically the time it takes for a high-tide to occur at exactly the same time and date. This number is what is used on nautical charts. So if the low tide has a negative number then you know the tide is going to be further out than normal.
Q: Why are there blank cells in the tide chart?
A: Sometimes you might notice in our tide charts that the high/low for a given date is not there. The reason for this is the fact the tides don’t quite fit into a daily cycle, and actually average 24 hours and 54 minutes between complete cycles. So those ‘missing’ cells are where the tide time has jumped into the next day.
Q: Why do some cells show AM or PM, but not others?
A: Similar to the answer above, there are instances where 2 high tides occur in one 12-hour period. If that is the case, the second time will be in the second time column (PM) but will say AM so you know it is in same period as the first time of the day. These are rare, depending on the layout of the harbor and surrounding geographically features.
Q: How do we calculate “King Tides”?
A: The truth is we don’t predict King Tides in our tide charts. ‘King Tides’ refer to the highest tide of the year. There are many factors that can increase the height of the tide the are not caused by the predictable phases of the moon, which our tide charts are based on. Things like flooding and river surges can change the height of the tide on any given day. This is often true when snow melt is running off the mountains and hence they are often also known as ‘Spring Tides’. Each lunar cycle there are perigean high tides, when the moon is at its closest. The tide will only increase by a few inches, but when it coincides with the conditions mentioned above it can cause King Tides.
Q. How do the currents affect the tides?
A: Many mariners use a general rule that says that slack water (no movement of water in any direction) occurs at high tide and low tide. However there are exceptions depending on the location and its currents.
There are other factors that affect the appearance of slack water:
- Wind driven currents can alter the timing of slack water.
- Changes in river flow due to rain and runoff (or lack of it). – Changes in the freshwater river flow of only 1/4 of a knot can shift times of slack water by as much 30-45 minutes.
There is no consistent relationship between tides and currents; and even at the same location, that relationship can be significantly different on two different days, or even two different parts of the same day.
A common rule regarding slack water and high tide is a reasonable approximation of when high or low tides may occur; with an error of up to +/- 90minute, and possibly up to 2 hours, in some cases. More on tides and tidal currents can be found here.
Q: What are the best resources for learning more about tides?
A: There are several really great organizations with scientifically accurate information on tides:
- NOAA’s Educational Resources on Tides
- NOAA’s Tides & Currents Site
- The National Oceanography Centre (UK)
We also have several interesting and helpful articles on Tides: