What the eight do not know
A harmonic prediction is astronomy. The sea also has weather in it, and the weather is what will put you aground.
Three things that move the water and are not in the sum
- Barometric pressure. Roughly one centimetre of level for every millibar away from 1013. A deep low sitting over the coast lifts the whole curve — every high and every low — by a third of a metre for as long as it stays.
- Wind. A hard onshore blow holds the water in: high water comes in higher and stays longer, and low water may not arrive at all. Offshore does the reverse, and the reverse is the one that grounds people.
- Fresh water. In an estuary, river flow after rain sits on top of the tide at low water and hardly shows at high water. It is why Nethermill Reach has the worst residual on this list.
Published residuals
Observed minus predicted, over the last complete year at each gauge. These are ours to be judged on and they are on the site rather than in a support article.
| Station | RMS | Worst | Neap range | Worst as % of neap |
|---|---|---|---|---|
| Halberry Roads | 0.11 | 0.62 | 3.70 | 17% |
| Skellow Bar | 0.19 | 0.94 | 2.54 | 37% |
| Nethermill Reach | 0.23 | 1.35 | 3.12 | 43% |
| Cargreave Sound | 0.08 | 0.35 | 0.82 | 43% |
| Ossett Head | 0.09 | 0.48 | 2.14 | 22% |
| Feltwater Ness | 0.14 | 0.71 | 0.38 | 187% |
- Halberry Roads
- Open coast. Surge is the whole of the error.
- Skellow Bar
- Eight constituents under-carry a bar. The second high water is right to within twenty minutes and the stand between them is not.
- Nethermill Reach
- River flow after heavy rain lifts low water and the prediction knows nothing about rain.
- Cargreave Sound
- Small range, deep water, little to go wrong.
- Ossett Head
- The best-behaved station on the list.
- Feltwater Ness
- The residual is larger than the neap range. On a neap here the weather is the tide.
The number worth reading twice
At Feltwater Ness the worst residual is 0.71 m against a neap range of 0.38 m. On a neap there, the weather is larger than the tide. A prediction that did not say so would be worse than no prediction, because it would be believed.
And the honest bit about the eight
A full port analysis carries sixty constituents or more. We carry eight, because eight fits offline on a phone and covers open coast well. On Skellow Bar it does not fully cover the stand between the two high waters, and the station page says so instead of the marketing page not mentioning it.
Something not matching what you saw? The duty watch wants it — that is how these residual figures get better. 0191 498 0272 or send it in writing.
Wrackline is an aid to planning and is not a substitute for an official tide table. Do not navigate on it.