Whatever’s This Weather? Posted on 10th July 2012 by Laurence Cloud Front over Lyme Bay What is it with this weather? Am I the only person noticing significant changes in the British weather pattern? I’ve done some research on monthly rainfall figures; but there’s no real trend to show that it’s getting much wetter, and, as shown on the chart below, the trendline for annual rainfall over the last 100 years shows only about a 2.5% increase over the period, which isn’t significant, and the recordings don’t go back far enough historically to identify whether there is actually any trend at all. The other issue is that these recordings can only be either averages for a very large geographical area, or recordings for one specific place. The reality is that conditions vary dramatically, even over short distances. The Met Office has reported that some parts of the UK has already had 250% of its average July rainfall in the first 10 days of the month, but this in no way says either way whether it will stop raining now for the rest of the month or whether it will continue with more extremes. In fact, a trend line plotted over the July rainfall totals since 1948 shows that July rainfall has actually gone down by 25%. Additionally, data since 1948 shows that the rainfall trend for May, June, July, August, September and November has actually gone down since 1948. It feels as though the country doesn’t get a summer any more. Perhaps it doesn’t, or not in the traditional sense that we perceive Summer as between Spring and Autumn, and coinciding with schools’ Summer Holidays. The weather conditions we get depend on the Jet Stream, a stream of air that moves above the Atlantic, west to east, at between 11 and 17 kilometres above sea level, travelling at around 160kph. The direction of the Jet Stream shifts, sometimes it flows north of the UK, sometimes south of it, and sometimes over the top of it. At the moment, it is passing to the south of us and is allowing the unsettled conditions to its north to spread over our country. It’s not really understood why the shift happens, but it really a significant impact on our country’s weather. The extreme weather we are getting, then, isn’t so much the large volumes of rain overall, but the intensity of the rainstorms that we appear to be getting. I think the Met Office has had warnings for heavy rain in place over some part of the UK nearly every day for several weeks, and the sight of locally torrential downpours appears, at least to me, to be something that we will see more of in the future. Your comments would be appreciated. Annual UK Rainfall Totals since 1910 [For more information on the Jet Stream, this BBC article provides an excellent explanation: http://www.bbc.co.uk/news/science-environment-18783422].