September is the first of a quartet of months that take their names from Latin numerals. But the names September, October, November and December make sense only when you recall that the ancient Romans began their year in March, not January.
As it happens, September's equinoctial sun is of equivalent strength to that of March, but the early autumn is made softer and more mellow by the lingering warmth of recent summer, and by the still-luxuriant foliage upon the trees.
For many people, September is the most pleasant month in all the year. It can be a windy, blustery month. September is often characterised by a regular procession of depressions moving eastwards across the Atlantic, and passing close to Donegal and Mayo.
Occasionally, one of these may harbour the atmospheric remnants of some almost forgotten transatlantic hurricane, as, for example, did the storm we recall as Hurricane Debbie that reached our shores September 16th, 1961. Many of the records for extreme wind-speeds established on that day still stand.
But it often happens, too, that the march of the Atlantic lows is halted for a time by a strategic anticyclone, and one or more spells of fine, dry, quiet weather often come along. This, combined with the lengthening nights, makes it an ideal month for the dews and mists that we associate with early autumn.
The quiet air, cooled by the ground at night, slides down into the valleys and leads to "ponding" of the cold air in undulating countryside - a phenomenon often detectable by the home-bound cyclist on a fine September evening.
September is a month of slow transition. On a typical August day, for example, the temperature climbs to about 20 Celsius; in October, on the other hand, the average maximum is only 14. September takes the middle ground, with an average daily maximum temperature of around 16 or 17.
September nights are cool. Ground frost becomes a common occurrence as the month matures, with the ground temperature slipping below freezing point at least three or four times in the second half of the month. The air temperature on the other hand - the temperature four feet above the ground - drops below zero only very rarely.
September is neither particularly wet, nor very dry; most places experience between 100 and 150 millimetres of rain, with an extra 50 to 100 millimetres in places some height above sea level. But with the evenings drawing in, we can expect rather less sunshine: four or five hours per day is the norm for September, about an hour less than the average for August.