A QUICK REMINDER ON TIDES
At their simplest, tides come in and out twice a day, regressing about 50 minutes each day. In other words, if high tide is at noon one day, the next day it will be high tide around 1250. If you’re good at maths, you’ll see that in 14 days’ time, you will be more or less back to where you started from. So, if it’s high tide at noon on a Monday, two weeks later it will be high tide close to noon again on a Monday.
In the Mediterranean the volume of water is relatively small, so the tidal range is only a few centimetres in most parts. In the Forth estuary at Port Edgar, the range can be 5m at spring tides (ie the high tide is very high, and the low tide on the same day is very low). Neap tides are the opposite: high tide is not very high, and low tide is not very low.
The tides are caused by the gravitational pull of the moon, with a little help from the sun. When sun and moon are in alignment, they help one another and you get big spring tides. When they are aligned at right angles to one another, the gravitational effect of the moon is reduced by the pull of the sun, and you get neap tides. The
moon and sun align at full moon and new moon, and spring tides occur a day or two after full moon and new moon. Spring tides and neap tides follow approximately a two-week cycle. So, if there are spring tides around the 10th of the month, they’ll occur again around the 24th of the same month. Keep in mind that spring tides tend to be particularly strong near the equinoxes: March/April and September/October.
The tides are also affected by local topography, by air pressure and by the wind. Low air pressure on the sea allows it to rise a little. For the technically minded, you could expect 1cm of a rise in sea level for a drop in pressure of 1hPa. There are many tidal anomalies too. For example, round Islay and Rathlin Island in the North Channel, the tides are a bit variable. On the River Forth near Stirling, the tide often surges up twice in every cycle, giving a double high tide every 12+ hours. Sometimes you even get a tiny tidal bore flowing round the bends towards Stirling, which is always fun to see.