A quick cuppa as Caithness sees exceptionally low atmospheric pressure
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THE phrase “a quick cuppa” took on a new meaning for Caithness householders on Sunday as exceptionally low atmospheric pressure brought a significant lowering in the normal boiling point of water.
The phenomenon was highlighted by the John O'Groat Journal's monthly Weather Watch columnist Keith Banks, from Wick.
Keith explained: “Most people are aware that, under normal circumstances, water boils at 100C (212F). In brief, when those 'normal' conditions exist, at 100C, water has a specific pressure called its saturation vapour pressure, that is equal to the normal atmospheric pressure, and at this temperature boiling occurs.
“Boiling is a special type of evaporation where what is called the phase change from liquid to vapour takes place within the liquid, and not at its surface, as occurs in evaporation.
"Bubbles of vapour can form inside the liquid ONLY when their pressure just exceeds the pressure exerted by the atmosphere. The liquid is now said to be 'boiling'.
“When the atmospheric pressure is less than normal, boiling will therefore, occur at a lower temperature.
“Caithness was fortunate to escape the wrath of Storm Ciara, experienced just about everywhere elsewhere across the UK. Nevertheless, Storm Ciara did 'impact' on the residents of the far north – albeit in a very discreet, subtle and benign fashion – as its centre drifted right over Caithness during Sunday morning and afternoon.
“In Wick at 3pm the air pressure had fallen to a near record-breaking low of 948.4mb, or just 711.4mm of mercury. The lowest atmospheric pressure reading on record for Wick is currently 942.6mb (707.0mm), recorded on December 20, 1982. The average atmospheric pressure at sea level is about 1013.3mb or 760mm of mercury.
“This event and resultant phenomenon, in terms of such an exceptionally low atmospheric pressure occurring over the far north that significantly lowered the normal boiling point of water, certainly benefited those partaking in a cuppa in Caithness on Sunday. It was literally a case of a 'quick brew', with the kettles boiling at just 98.16C (208.7F) at 3pm!”
