The immediate cause of the current fine spell is the same as always: an anti-cyclone hovering in the vicinity of Ireland to deflect from our shores the normal eastward procession of troublesome weather fronts and deep depressions.
This particular zone of high pressure has been with us for most of the current month, extending southwestwards over Ireland from Scotland and the North Sea. The result, as always in such a situation, is a spell of dry and sunny weather.
Dry weather in itself is not unusual about this time of year. April is the driest month of the 12 in most parts of Ireland, although it normally brings a rainfall total of some 50 to 60 millimetres; so far this month most of Ireland has had no rain at all, and those places not completely dry have experienced the merest shadow of a proverbial April shower.
There are signs in agricultural circles, indeed, that this dearth of moisture is having an effect in retarding growth to some extent, but we have been cushioned from the worst effects of a deficit in soil moisture by the relatively plentiful rainfall accumulated during the winter months.
Temperatures, too, are well up on average, running about 1.5 degrees above the seasonal norm and continuing the now almost monotonous trend for successive months to be warmer than historically they ought to be. Afternoon temperatures have been in the high teens almost since the month began, and the forecasters tell us that the thermometer will probably touch 20 degrees in some places over the weekend, a temperature that is high for April, but not exceptional; statistically the temperature creeps above this value on some day during this month in one out of every four of five years.
The gradual increase in the daily maximum temperature as a dry sunny spell progresses is explained, other things being equal, by the fact that when there is little moisture left in the surface layers of soil, no heat is absorbed in evaporation, and virtually all the sun's radiant energy is available to heat the ground and raise afternoon air temperature to ever higher values.
Consistent with the increase in average temperatures in recent years has been a continuation of the trend for earlier spring budding and related vernal developments. It is clear from satellite observations and other evidence that the onset of spring in the northern hemisphere has been getting earlier each year, occurring on average about 10 days sooner now than it did in the middle of the 1960s.
But going back even further and looking at phenological records for the past 300 years, the opposite appears to have been the case for the greater portion of the period. Spring in the northern hemisphere occurred later and later each year until around 1940, the cumulative delay amounting to around a day per century, which, it seems, was to be expected because of the idiosyncrasies in the orbital characteristics of the Earth. But then there was a dramatic shift, and spring began to arrive earlier, rather than later, and at an accelerating rate. And this, of course, is entirely consistent with current greenhouse theories about global warming.
Although the precise environmental factors sparking the start of growth are subtle and complex, it is estimated that a rise of one degree accelerates the onset of spring by about a week.