Air-pressure related variables



This section presents the analysis for:



Large-scale pressure patterns are responsible for many aspects of Scottish weather. Lowpressure systems pass across the country bringing weather that is mainly wet and windy, while high pressure is associated with less changeable and often drier conditions. Figure 40 shows the network of air pressure and sunshine recording stations that have provided the information we have used in this analysis. The positioning of centres of high and low pressure also affects the flow of air across Scotland and hence the wind speed and direction.


The information on wind speed and direction greatly depends on the site of the weather station. For instance, there will be a marked difference between an observation in a mountain valley compared to one taken, at the same moment in time, on nearby flat arable land. Also, if the measuring instrument is moved on the site, there are new buildings close by or if a site is moved there will be an effect on the characteristics of wind measurements. Although we have information about average wind speed it is difficult to use. Analysis has identified downward trends in average wind speeds, but we believe that these trends are likely to be a result of irregularities in the information from some of the stations (in other words trends within the information may relate to changes in the way it has been measured rather than changes in the wind speed or direction itself). For this reason we have not mapped patterns of change in wind speed. Instead, we have taken records from three observing sites, where we know the records are reliable.


Calculated variables

These derived quantities relate to both pressure at sea level and wind speed and show how often strong winds or gales take place. One of the quantities that we can calculate from the information we have on wind is a measure of the number of days in a year that we can consider as a day of strong wind. We have recorded changes in this variable in this section. The measure we use is that of a “gale day”. We define this as a day with an average wind speed of 34 knots or more over any 10-minute period.


Figure 40 - Where the air-pressure and sunshine-hour sites are based (January 2001).