Analysis: Temperatures in excess of 30C, as occurred yesterday are rare, writes Brendan McWilliams.
The line between the unusual and the exceptional is difficult to draw.
Temperatures in excess of 25C are certainly unusual on this island, being experienced on only two or three days in the average summer.
But temperatures in excess of 30C, as occurred in several places yesterday, are very rare indeed. They are clearly exceptional since they occur perhaps only once or twice a decade, and then only in inland areas of the country.
Prior to yesterday, the most recent occasion on which we experienced an officially-recorded temperature in excess of 30C in Ireland was in Kilkenny on August 2nd, 1995.
We have had temperatures in the high 20s during several summers since then, most notably in 2003 and 2005 when 29C was was registered several times.
However it is 11 years since the 30C threshold was last breached.
This is record-breaking territory for Ireland; after all, the highest temperature ever measured here was 33.3C, also at Kilkenny Castle on June 26th, 1887.
The highest reached in the 20th century was 32.5C at Boora, Co Offaly, in the glorious summer of 1976.
The immediate cause of our recent spell of good weather is the same as always - a friendly anticyclone loitering for a time in our vicinity with benign intent.
Such temporary extensions of the semi-permanent anticyclone down near the Azores occur once or twice every summer, and they interrupt for a time the more normal pattern of depressions and fronts moving eastwards across the Atlantic.
The clear skies of an anticyclone allow the sun to do its work, but to produce record temperatures additional factors have to slip quietly into place.
The weather, first of all, needs to have been relatively dry for some time previously so that the ground contains very little moisture.
Little or no heat is, therefore, absorbed in evaporating water from the soil, and the full power of the sun's radiant energy is available to heat the ground.
Also necessary for very high temperatures is that the anticyclone be positioned in such a way that the air wafted in over Ireland has originated over hot, continental Europe rather than over the ocean.
In this respect, the location of this year's Bastille Day anticyclone, which formed over the Mull of Kintyre, was ideal.
It is also noticeable during a fine summer spell like this that the maximum temperature achieved each day often increases as one day follows another.
Each day the soil absorbs more heat during the day than it can effectively surrender during the short night.
The next sizzling day begins from a higher thermal baseline, so to speak, and, other things being equal, a higher daytime maximum temperature is likely to be reached.
This continues, with higher maxima day by day, until some seminal change occurs in the prevailing pressure pattern - as indeed seems to be happening at the time of writing with a westerly drift of wind poised ready to make its inevitable return.
Heatwaves in themselves are not uncommon, and over the decades we have had many fine spells comparable to this one.
But we live in a time when global warming is an ever-present, over-arching fear, and a phenomenon which most meteorologists now believe to be likely to produce potentially catastrophic consequences in the not-too-distant future.
Are we experiencing the early signs?
Weather people will view the current scene in the context of the extraordinary European summer of three years ago, 2003, when temperatures in parts of southern France were a full 5C to 7C above the long-term average.
At that time Switzerland, a country not often given to extremes, was warmer than at any time in the previous 250 years.
That particular heatwave was difficult, if not impossible, to explain without bringing global warming into the equation.
Should there be a repeat this summer it would be difficult to resist the conclusion that there must be a connection between a trend towards hotter European summers and the inexorable rise in average global temperature that has been in evidence in recent decades.