Tide Patterns

 Here is a plot of every predicted high tide at Kahului Harbor in 2023: Intuitively, one might imagine that  "new moon tides are more powerful than full moons due to the moon and the sun pulling together from the same side of the Earth."  We will deal with the theory in the next blog post, "Does the Moon Really Lift the Ocean?", but for now let's just look at the data. New Moons vs Full Moons From the figure above we can see that for Maui at least, full moons are just as influential, and in fact there are more full moons where the associated high tide is significantly higher than its new moon neighbor tides.   We do see a peak in tide heights in January, which is probably due to the Earth reaching its closest approach to the Sun on about January 3.   King Tides But the highest tides appear to be in summer, with the peak at full moon on August 1.  What causes this?  One would think that summer higher tides would be associated with the Sun bein...

We tend to think of the Moon's gravity "lifting" up the ocean underneath the moon in order to create the tidal bulge.  This is not how it works.  The earth's gravity is far too strong, and the ocean is too heavy, to be "lifted" by the Moon, 240,000 miles away.  Do you feel lighter when the moon is overhead?  Probably not, since the Moon's vertical force has been calculated to be one ten millionth of the Earth's gravity at points directly underneath the Moon. At points directly under the moon, such as point Z in the figure below, the gravity vector caused by the moon is perpendicular to the Earth's surface and has zero effect on the tides.  However at points not directly underneath the Moon, the gravity vector towards the Moon has a horizontal component which produces a "tractive force."  This force operates horizontally along the surface of the sea, and the sea presents relatively little resistance.  The sea is pulled from both sides to...

 What is the relationship between transits of the moon and the timing of the associated high tides on Maui? Here is a plot for 2023: Shown above are the tide lags for both upper transits (blue) and lower transits (hollow red). Positive lags mean high tide trails the transit.  We might think of this as the "normal" result, since the ocean system shouldn't be expected to respond to the moon's movements instantaneously.  A negative lag means the transit actually occurred after the high tide.   Positive lags predominate, as you can see.  The typical lag range is about 4 hours, with the mean lag for the year being about 2 hours.   Here is a closer look: As you can see, lags tend to decrease around the times of full and new moons.  This is called "priming".  For an simple example of priming see the Professor Tony Phillips example in the Reference blog.  

 Bedford Institute, Halifax, Nova Scotia, Canada: Long Island, NY: San Francisco:

 References The Ocean Isn't Being Lifted, PBS, 2015, Youtube Virginia Institute of Marine Science, Tide Analysis Hicks, "Understanding the Tides", 2006 Parker, "Tidal Analysis and Predictions", 2007 Schureman, "Manual of Harmonic Analysis and Prediction of Tides", 1958 Professor Tony Phillips, Stonybrook, "Priming and Lagging" OCE: Analysis of Oceanographic Data, Dan Kelley and Clark Richards The plots here were done in the R environment.  The harmonic analyses and astronomical calculations were made possible by the OCE package, authored by Dan Kelley and Clark Richards, oceanographers from Halifax, Canada.  Many thanks to Dan and Clark for their illuminating conversations and emails. Contact me at gmail, same subdomain as this blog.