We need to pay attention to the weather forecast over coming days, writes Brendan McWilliams
Our worst nightmares about global warming include an inexorable rise in sea level accompanying the melting of the polar ice caps. But without any help from climate change, the level of the sea can rise at times to a level sufficient to cause serious concern. Such an event is predicted for next weekend; exceptionally high tides are expected to occur, but we have no particular cause for worry unless the weather decides it must conspire against us.
The tides, as we know, rise and fall because our oceans are subject to the gravitational attraction of other bodies in the solar system. By far the most important are the sun and the moon. Lunar gravitation produces two "bulges" on our oceans - one almost directly under the moon and the other on the opposite side of the world. Half-way between these two mounds of water, on either side of the earth, lie two regions where the sea is a good deal shallower than otherwise it ought to be. As our planet revolves every 24 hours, the zones of high and low water retain their position relative to the moon, so that as a general rule two high tides and two low tides occur at each point on the earth's oceans every day.
But the sun also causes tides, which on their own would be slightly less than half the magnitude of the lunar tide. Sometimes the two tidal effects work against each other, but at other times they co-operate, notably at the new and full moons when the earth, moon and sun are all arranged more or less in a straight line. On these latter occasions, with the two external bodies pulling on the world's oceans in tandem, as it were, high tides are higher than usual; they are called "spring" tides - no relation of the season. A spring tide typically occurs about two days after the new or full moon. On such occasions the low tides, too, are much lower than usual.
Other astronomical coincidences can make spring tides even higher. The moon's orbit around the earth is not circular, but an ellipse, the distance between the two bodies varying during the monthly cycle by around 48,000km (30,000 miles). At a certain time in its every orbit, the moon is at perigee, at its nearest point to earth, and its gravitational attraction is enhanced. A "perigee spring tide", therefore - a spring tide that coincides with perigee - is very high indeed.
Very high tides are also encouraged when the three bodies, the sun, moon and earth, align themselves closer to an exact straight line than usual. In the case of the sun, the optimal situation in this regard occurs near the equinoxes, when the earth's equatorial plane is aligned with the orbital plane of the earth around the sun; this produces the so called "equinox spring tides". And finally, lunar tidal forces are stronger than usual when the declination of the moon's orbit is at a minimum - when, every 18 years or so, the plane of its orbit coincides as closely as it can with the plane of our equator.
For all these events to take place simultaneously is akin to the occurrence of a full line of cherries on a fruit machine. But this is what is about to happen. A full moon will occur next Thursday, September 7th, and its orbit is such that full moon coincides with lunar perigee; the autumn equinox, moreover, happens only two weeks later. So two or three days after the coming full moon we will experience equinoctial perigee spring tides, enhanced by a low lunar declination.
Now, tides like these in themselves are of no particular significance since the water rises only perhaps 10-20cms above what we might call the "normal" level of a high tide. Most coastal systems can cope with minor anomalies like this. Only when the weather co-operates by raising the level of the water even further are flooding problems likely.
Strong onshore winds, for example - particularly if the pressure pattern is such that they have acted on a stretch of water over a long distance - "pile up" water against a coastline, and can cause a further increase in sea level of perhaps a metre. And if there is a deep depression in the vicinity, the low pressure allows the sea to rise like a barometer by about 30cm for every 30 hectopascals that the atmospheric pressure drops. When we recall that the approach of a depression may cause the pressure to fall from the normal of about 1013 hPa down to 980 hPa or less, the potential rise in water level, even due to the pressure drop alone, is quite significant.
The current danger period is not confined to the September full moon, although this is the time of maximum astronomical assistance to high water levels. The August full moon also caused high spring tides, and the October full moon will produce similar effects around October 8th; indeed local topographical peculiarities result in this latter date being of more significance along our western coasts.
It is too early for confident predictions about next weekend's weather. The indications are that winds are unlikely to be exceptionally strong and no deep depression is expected. But more reliable predictions will only be possible in a day or two.