The average value of atmospheric pressure over Ireland is about 1013 hectopascals. It is generally higher in summer than in winter, with monthly averages varying from a low of 1011 hPa. in December and January, to a high of about 1016 in midsummer.
It is something of a paradox, however, that extremely high pressure values occur, almost invariably, in winter time.
High values of atmospheric pressure are associated with anti cyclones. At the risk of over-simplification, one could say that these tend to form where the atmosphere is relatively cold.
Cold air is dense, and therefore heavy, and the column of air above a particular spot will weigh more than a corresponding column elsewhere if it contains more than its fair share of relatively cool air; the extra weight causes the pressure at the surface to be higher than the average.
For these reasons, summer anticyclones tend to form over the relatively cool ocean areas of the world, like the Azores high that sometimes extends up over Ireland to produce, for a week or two perhaps, a summer.
In winter time, on the other hand, high pressure is more likely to be found over the cold continents - especially over the vast land areas of eastern Russia where the Siberian winter anticyclone is a dominant feature of the global weather pattern.
Not surprisingly, therefore, it was there in Siberia that the highest pressure ever measured was recorded - 1084 hPa on December 31st, 1968, at a place called Agata, when the temperature was a chilling minus 46 Celsius.
Five other weather stations in the vicinity recorded similarly high pressure values, thus almost eliminating any possibility of error.
Low pressure values, on the other hand, almost by definition, are associated with the great depressions that skim across the oceans of the world. It was a depression passing near the is land of Guam in the North Pacific that provided the lowest barometric pressure ever recorded - the 870 hPa measured there 20 years ago today, on October 12th, 1979.
But, of course, neither of these figures is necessarily the "highest" or the "lowest" in the real sense. The atmospheric pressure is measured with barometers at thousands of individual places around the world, and we know the extreme values recorded at these precise points - but even greater departures from the norm may have occurred from time to time elsewhere, quite undocumented and unknown.
Time is an important factor, too: organised instrumental records of pressure and temperature go back only 150 years or so, and we therefore have no way of recapturing the extreme values of these elements which may have occurred before that time.