2.4 Tidal range

It is often useful to know whether the tide is a spring tide, a neap tide or somewhere in between.

Simply inspecting the tide table can give some information - spring tides will be associated with higher high waters and lower low waters. Some tide tables will indicate the days on which spring or neap tides occur.

I find it useful to calculate a tidal factor – the ratio of today’s tidal range to the typical tidal range for a spring tide. This will be 1 (or 100%) for a typical spring tide and about 0.5 (or 50%) for a typical neap tide. Calculating this is straightforward, but a calculator can be handy:

Factor = tidal range today / tidal range at mean springs

For example, if the tidal range today is 6 meters and the tidal range at mean springs is 8.3 meters, then the factor is 6/8.3 = 0.72 = 72%

Data on the tidal ranges at mean springs can be found:

  • In the back of Pesda Press’s sea kayak guidebooks.
  • At https://www.ntslf.org (under ’highest & lowest predicted tides) - you’ll need to subtract mean low water springs from mean high water springs to get the mean spring range
  • In any nautical almanac (e.g. The Reeds Small Craft Almanac)

For example, the mean tidal ranges at springs and neaps are given at the top of the tide table below:

First row of tide table for Plymouth

What is the tidal factor on June 1st 2020?

On June 1st, we have the following tidal data for Plymouth (adding an hour for BST):

08:28 BST Low water 1.8 metres
14:43 BST High water 4.9 metres

The tidal range on July 4th is 4.9-1.8 = 3.1 metres.

The mean range at springs for Plymouth is 4.73 metres. So the tidal factor is 3.1/4.73 = 0.66 = 66%.

The tidal factor is 66%, which is part way between neaps (50%) and spring (100%), but closer to neaps than springs.