THERE was a time when it was believed that two full moons in the same month brought trouble in the form of floods.
We nearly had the two this month, but not quite one has just gone, and the next is due in early February. The floods arrived, however, and the moon may not have been entirely innocent.
Rural and coastal flooding often arise from quite different causes in the case of the former the reasons are relatively simple. The low gradient of the flat midlands of Ireland makes for slow and ponderous rivers with a poor carrying capacity.
When there is an excess of rain fall they sometimes cannot cope and overflow their banks. The heavier and more prolonged the rain, the greater the degree of flooding.
Coastal flooding of the kind to which places like Cork and Waterford are prone, on the other hand, may often arise for a combination of quite different reasons.
Although excessive rainfall may well play a part, the primary cause is often an anomalous rise in sea level, brought about by a conjunction of unfavourable tides, strong winds, and even the barometric pressure pattern on the weather map.
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 in this respect are the sun and the moon, and twice each month they co operate fully with one another.
At new moon and at full moon the sun, the earth and the moon are virtually in a straight line with the two bodies pulling on the oceans together, as it were, the high tides are much higher than usual. We call this a spring tide, although it has nothing to do with the season of spring.
Other astronomical coincidences can make matters worse. At certain times in every revolution, the moon is at its perigee, the nearest point to earth, and its gravitational attraction is correspondingly enhanced. So a perigee spring tide is higher still.
Moreover, the distance between the sun and the earth also varies the sun is at its closest to us in early January (paradoxically, since it is the middle of our northern winter) and this means that a perigee spring tide around this time of year may be very, very high indeed.
Meteorological factors also play their part if the pressure pattern is such that strong winds can act on a stretch of water over a long distance (if they have what we call a long fetch) they "pile up" water against an adjacent coastline, causing an increase in sea level of several feet.
And a deep depression in the vicinity causes even further problems. The low pressure at the centre of a deep depression allows the sea level to rise like a barometer by about one foot for every 30 hectopascals (hPa) that atmospheric pressure is below average.
Considering that the approach of a depression may cause the pressure to fall from the normal of about 1013 hPa down to 950 hPa or less, the potential rise in water level due to the pressure drop is quite significant.
Not all these factors have been in operation over the past week, which explains why our southern cities, despite the heavy rain, were not the worst affected.
But many of the danger elements were there the moon was full last Saturday, and a number of very deep depressions passing close to Ireland in the Atlantic provided very strong on shore winds for much of the time.
Had any of these "lows" followed a track a little closer to us, and passed at high tide, the barometric pressure would have been much lower and the potential for flooding along the southern coastline very much worse than was in fact the case.