## 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:

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.