Air-pressure related variables

Average air-pressure at sea level

 
 

Figure 41 - Average air-pressure at sea level each year (in hectopascals, hPa) for Scottish regions, from 1961 to 2004, with smoothed curves showing a running average

Table 21 - Changes in average sea level air-pressure (hPa), from 1961 to 2004. One hectopascal is equivalent to one millibar.

 

North Scotland East Scotland West Scotland Scotland
Spring 0.2 0.5 0.5 0.3
Summer -0.4 -0.4 -0.6 -0.4
Autumn -0.5 -0.6 -0.9 -0.6
Winter -2.6 -1.7 -1.3 -1.9
Annual -0.8 -0.5 -0.5 -0.7

 

Regional trends

 

  • Given that the average sea level pressure for Scotland is about 1012 hPa, it is clear that any changes are low - much less than 1%.
  • Changes from year to year are high but the link between the three regions is also high. This is consistent with the large-scale nature of atmospheric pressure patterns.

Spatial trends

 

  • The summer map shows that there is no consistent pattern of change, but in winter months a pattern is clear.
  • Average winter pressures have been falling in northern Scotland, particularly over the Outer Hebrides, Orkney and Shetland Islands, from 1961 to 2004.
  • At the same time, there has been little change to average sea-level pressure in winter in southern Scotland.
  • Given that the area north of Scotland is mainly one of low pressure (in other words, the so-called Icelandic low), this suggests that low pressures have become lower. It also suggests that the average winter pressure gradient across Scotland has increased since 1961. However, we know that the change is small and may not be part of a measurable trend.

Future trends

 

  • Maps of change in average sea-level pressure were also not given in the UKCIP02 scenarios report. However, they do comment on it in the text.
  • Changes in spring and autumn may be small. In winter the north-south pressure gradient may increase resulting in stronger winds in southern and central Britain but little change in Scotland.
  • Average sea-level pressure can be used to investigate possible changes in North Atlantic storm tracks and the North Atlantic Oscillation (NAO). The UKCIP02 report suggests a possible shift southwards of the storm tracks current position, which may result in stronger winter winds across southern England.
  • It is expected that the NAO index may become mainly positive resulting in an increased north-south winter pressure gradient across Britain. This is consistent with more winters where the prevailing winds are more ‘westerly', in other words, milder, wetter and windier. The change in average sea-level pressure is broadly consistent with these expected future changes.

 

Figure 42 - Patterns of change in the average sea level air-pressure (in hPa) each year, between 1961and 2004, for the summer and winter quarters.