14 Jan 2015

Neap tides.

Not all tides are equal; the high and low tide heights vary throughout the lunar month depending on whether the sun and moon's gravitational pull are acting together or against one another. The moon has the greatest influence on the tides, the distant sun's is a kind of gravitational top-up.

When the moon and sun are acting in opposition, the tides are known as neap tides; the tidal range is lower. So today's high tide (1001 at Whitby) is 4.4 metres above chart datum (chart datum is a virtual line around the coast representing the lowest ever tide) and the low tide will be 2.5 metres above chart datum.

Here's what todays neap high tide looks like at Runswick looking towards Kettleness:
And neap low tide...
That was a vertical tidal range of 1.9 metres, which exposed 58 metres of beach from the high tide strand to getting my boots wet at the low water mark.

High tide, looking towards Port Mulgrave:
This isn't solely an excuse to publish another pretty picture of the thatched former coastguard cottage; it also illustrates the stress that beach-living seaweeds have evolved to cope with. The seaweed on the exposed rocks below the sea wall (mostly Ascophyllum nodosum, or knotted wrack) aren't covered even at the top of the tide. Some of these may not get a good dunking at high tide for another two or three days. It's very calm today and they weren't getting splashed by breaking waves. In the meantime they are dealing with temperatures of 2 celsius, and in summer during the three or four days of neap tides they will probably be exposed and dehydrated under a hot sun, and will survive. Living in marginal conditions is an evolutionary strategy that is fine if the marginal conditions don't change much. And neap low tide....

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