The seasonal variation is in the magnitude of the tides, with the Sun being more close to overhead in the summer. However, the greatest tides are closer to the equinoxes. Presumably this is to do with the relationship to the Moon's position. Check out the large bores in the Severn prediction page. These vary fom year to year, as the position of the Moon varies. The sun is much more regular.

Severn Bore 2011

I see we've just missed the really big ones for this year. The three star ones next month should be good, too.

You can compare with other years at the foot of the page.

Here's the other one. There's a considerable difference between the morning and evening waves with this one.

Trent Aegir

Our friend up above who was involved with tide predictions obviously wasn't using the machine method, which involves programming with all the possible oscillations due to orbits, perihelion, aphelion, perigee, apogee, nodes, and anything else I may have left out. Lots of interacting sine waves. I saw a programmme about the machine which was more like an offshoot of Babbage's contraption or something by Heath Robinson than a modern computer.

I used to be considerably confused by the two tides. Why, if the water is pulled by the Moon, isn't there only one? But the water the other side is being NOT pulled by the Moon, so is left sticking out.

Penny