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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Big Full Moon and Perigee Spring Tides
From: John Huth
Date: 2011 Mar 17, 12:18 -0400
HAT - Highest astronomical tide
LAT - Lowest astronomical tide
Highest astronomical tide (HAT) is the highest level, and Lowest astronomical tide (LAT) the lowest level that can be expected to occur under average meteorological conditions and under any combination of astronomical conditions. HAT and LAT are not extreme levels, as certai meteorological conditions can cause a higher or lower level, respectively. The level under these circumstances is known as a 'storm surge' ('negative surge' in the case of level lower than LAT). HAT and LAT are determined by inspecting predicted sea levels over a number of years.
Note
The value of HAT and LAT may not have the same value as other reference sources. The value given in any source is dependent on the years of inspection, the period which the inspection covered and the exact location and calibration of the tide gauge used. The values listed here have been produced from predictions over a 19 year period from 1996 to 2015 with the gauges being maintained and calibrated to a uniform standard by the Tide Gauge Inspectorate at Proudman Oceanographic Laboratory
From: John Huth
Date: 2011 Mar 17, 12:18 -0400
I tried to hunt around a bit for the definition of high and low astronomical tide. I couldn't get a really good answer, except an 'operational' definition, listed below. US charts use MLLW for the bathymetric datum - "mean lowest low water" - i.e. over the course of a lunar month.
LAT, "lowest astronomical tide" seems to be the lowest level based on measurements over the course of some length of time. So, perhaps Frank's perigean moon near the equinox qualifies?
Still, I can imagine far more rare circumstances that would contribute to a super-high or super low tide, but these might happen once every 19 years or so.
LAT - Lowest astronomical tide
Highest astronomical tide (HAT) is the highest level, and Lowest astronomical tide (LAT) the lowest level that can be expected to occur under average meteorological conditions and under any combination of astronomical conditions. HAT and LAT are not extreme levels, as certai meteorological conditions can cause a higher or lower level, respectively. The level under these circumstances is known as a 'storm surge' ('negative surge' in the case of level lower than LAT). HAT and LAT are determined by inspecting predicted sea levels over a number of years.
Note
The value of HAT and LAT may not have the same value as other reference sources. The value given in any source is dependent on the years of inspection, the period which the inspection covered and the exact location and calibration of the tide gauge used. The values listed here have been produced from predictions over a 19 year period from 1996 to 2015 with the gauges being maintained and calibrated to a uniform standard by the Tide Gauge Inspectorate at Proudman Oceanographic Laboratory
