UPDATE: We’ve written a further post explaining a little more behind the continuing dissapointing weather that we have seen through the summer so far. You can read this at ‘The UK’s wet summer, the jet stream and climate change*‘
After an unusually dry, sunny and warm March, April has seen some very wet and unsettled weather with below average temperatures. So what has caused this about-turn in the UK’s weather? There are many factors which can impact the notoriously changeable weather in the UK, so no single one on its own can be said to be fully responsible. However, it is possible to isolate contributing factors and, in this case, one of those is the northern hemisphere jet stream. This is a narrow band of fast flowing westerly winds (ie blowing from west to east) in the high atmosphere. This band moves around and also changes its track, from a fairly straight line to something more closely resembling a meandering river. Its position can, and does impact weather in the UK and other parts of the northern hemisphere. In both March and April we have seen what we term a ‘blocking pattern’ in the jet stream, where it meanders north and south instead of making its more usual eastward progress. Despite this, March was the 3rd warmest and 5th driest March in the all-UK record going back to 1910, while April has so far been relatively cool with rainfall already 30% above the average for the whole month across England and Wales. So what is causing the difference? It comes down to the position of the blocking feature. In March, the meandering of the jet stream caused it to pass to the north of the UK – anchoring high surface pressure over the UK. This suppressed cloud, increased sunshine and temperatures, and prevented the usual rain-bearing Atlantic weather systems coming in from the west from reaching us. Soon after the start of April, however, the whole pattern moved westwards, so the peak of the northerly meander moved over the North Atlantic Ocean. The UK, in contrast, found itself under the adjacent southerly meander, with the jet stream passing to the south of the UK over France and Spain. This atmospheric set-up brings low surface pressure, cloud and rain. Because the pattern is still blocked, without a west-to-east jet stream to blow the weather system through, the low gets stuck over the UK, resulting in high rainfall totals overall. Like the weather, we can predict the path of the jet stream with a good deal of accuracy up to about five days ahead but it is more difficult to give detail on longer timescales. Therefore it’s not possible to say exactly what the jet stream will be doing in a month’s time, for example, or exactly how it will impact our weather. You can find out more about the jet stream in our YouTube video. http://www.youtube.com/watch?v=GpsRQtk6IfM
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