Paradox of high pressure in cold places


If you have looked at a barometer in recent days, you may have noticed that it read exceptionally high. An average pressure value has the needle indicating "12 o'clock" or thereabouts, but with the pressure over Ireland these times hovering in the region of 1040 millibars, or hectopascals as we like to call them nowadays, the pointer has been holding steady near "a quarter past".

Like any fluid, our atmosphere exerts a pressure on anything immersed in it. This is a direct consequence of the not inconsiderable weight of the air itself, and any part of the surface of the earth may be thought of as supporting the air directly overhead.

Atmospheric pressure, however, varies in both space and time. Warm air is less dense, and therefore lighter, than cold air, so changes in temperature at various levels in the atmosphere result in changes in the pressure near the ground. 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 mid-summer.

It is something of a paradox that extremely high values of atmospheric pressure occur, almost invariably, in winter. High pressure values are associated with anticyclones, and at the risk of over-simplification, one could say that these tend to form where the atmosphere, throughout its depth, is relatively cold. When this is so, the column of air above a particular spot will weigh more than a corresponding column elsewhere, and the extra weight causes the pressure at the surface to be higher than the average. For these reasons, summer anticyclones tend to form over relatively cool ocean areas; high pressure in wintertime, on the other hand, is more likely to be found over the cold continents, so the anticyclone close to Ireland at the moment might be thought of as something of an anomaly.

The Siberian winter anticyclone, which forms every year over the vast frozen land areas of eastern Russia, is a dominant feature of the global weather pattern.

Not surprisingly, it was there that the highest pressure ever measured was recorded; the barometer touched 1084 hPa on December 31st, 1968, at a place called Agata, when the temperature was a chilling minus 46 degrees Celsius. In Ireland the record is 1052 hPa, noted at Valentia Observatory, Co Kerry, on January 20th, 1905.