Mean pressure across IONAIONA Islands Of North Atlantic. Yes I know there's an island called Iona, but this is so I don't have to use the term 'British Isles' when referring to the whole of Ireland and the UK. in May was around 3 hPahPa A Hectopascal is the SI unit of pressure and identical to the Millibar below the LTALTA Long Term Average. This is usually defined as a 30 year period by the WMO.. In fact lower than average pressure was strung out westward across the central Atlantic all the way to the United States. Mean higher than average pressure persisted across Scandinavia (+6 hPa) resulting in a very slack pressure pattern across the country.