Solent tide timing
Why Solent tidal-stream atlas stages need clock-time correction
Tidal-stream atlases describe the stream at stages such as HW−6, HW−3, HW and HW+4 relative to Portsmouth high water. Those pages are arranged as a regular sequence, but the observed tide does not progress through every part of the cycle at a perfectly uniform rate. The result is that an atlas stage can occur earlier or later than a simple one-hour count from high water suggests.
Why the length of the tide cycle changes
The familiar figure of roughly 12 hours 25 minutes describes the dominant lunar semi-diurnal rhythm. It is not a guarantee that every local interval from one high water to the next, or from high water to low water, will be identical. The observed tide is the combined result of several astronomical constituents, each with its own period and phase.
As those constituents reinforce or oppose one another through the spring–neap cycle and over longer astronomical cycles, the shape of the local tide curve changes. One part of the curve may become steeper while another becomes flatter. High water may arrive a little earlier, low water a little later, or the interval between them may be redistributed rather than divided into six equal hours.
In the Solent and surrounding Channel, shallow-water effects add further distortion. Friction, changing depth, coastline geometry and higher harmonics can stretch a stand, compress a falling or rising section, and contribute to features such as prolonged or double high water. The stream pattern represented by an atlas page remains useful, but the clock time assigned to that page needs to follow the real shape of that day's tide.
What that means for HW± atlas pages
A printed atlas normally treats HW−6, HW−5, HW−4 and the following stages as evenly spaced reference pages. In reality, the time occupied by each segment of the tide cycle can vary. Tidal Deltas measures the position of the selected time within the actual high-water and low-water sequence, then assigns it to the corresponding corrected stage rather than assuming every stage is exactly sixty minutes long.
A practical example
On a strongly distorted day, a standard stage calculated for 21:31 may correspond more closely to 20:10 after the timing correction. The atlas page has not changed. What has changed is the clock time at which the real tide reaches the part of the cycle represented by that page.
What Tidal Deltas currently provides
- • Selected-day HW/LW stage timing.
- • Signed corrections in minutes.
- • Corrected clock times.
- • Automatic matching to the relevant tide-cycle stage.
The current product corrects timing only. It does not adjust tidal-stream rate, direction, wind setup, atmospheric pressure or live surge residuals.
How to use the result
Use the corrected time to identify when the corresponding tidal-stream atlas page is most relevant. Continue to use official tide tables, charts, local notices and appropriate navigational judgement.