For all the damage that they caused, the recent storms did not come as a surprise. They were spotted, each in its turn, several days beforehand as relatively minor features of the weather map on the far side of the Atlantic. All the signs indicated they would move steadily north-eastwards in the direction of Ireland, becoming more and more vigorous as the pressure at the centre of the vortex dropped.
The lower the central pressure of a "low", the more isobars required to delineate the sharp gradient between its central pressure and the "normality" beyond its sphere of influence, and therefore the more vigorous and stormy the expected weather.
The strength of the wind is known to be "inversely proportional" to the distance between adjacent isobars: the closer they are together, the stronger the expected wind will be.
It was encouraging on this occasion to find that the computer forecast models captured the development of the depressions very well indeed, and predicted the essential characteristics of the pressure pattern with considerable accuracy. Provided by the computer with successive positions of the low, forecasters were able to estimate expected wind speeds hour by hour, and the computer output also provided estimates of likely rainfall.
Depressions as intense as these are not unusual. Many appear in the north Atlantic every year, but they only become noteworthy as far as we in these islands are concerned when they pass close enough for their strong winds and rain to interfere with us. Their typical path varies with the seasons. During the summer the lows tend to follow a track that takes them well to the north of Ireland, which partly accounts for the fact that storms are less frequent in summertime; but the preferred path of wintertime depressions lies very close to Ireland, so December storms are not a rarity. In fact, as most of us remember very clearly, we had two of almost equal severity at Christmastime last year.
But winter storms in general are more intense than their summer counterparts. The atmosphere in the northern hemisphere is at its most energetic at this time, because the temperature contrast between the equator and the poles is greatest; the average temperature of the equatorial zones varies little with the seasons, but the northern latitudes become progressively colder as we advance deeper into winter.
The interaction between the very cold air to the north and the warmer air further south provides the stimulus for the development of deep depressions which sweep across the Atlantic, and now and then, as we have seen, wreak havoc on our shores.