Rain, rain, go away and push the jet stream back the right way

Tuesday, 17 July, 2012

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Looking for something to blame for the apocalyptic weather of the summer of 2012? Blame the jet stream.

Far above our heads, far above our umbrellas and even far above the rains clouds (no they don’t go on forever) is a huge river of wind, which flows above the upper atmosphere. This is the jet stream, and its current path is the cause of what is turning out to be one of the most miserable summers on record.

First identified in the 1920s by the Japanese, and first experienced first hand by World War II aviators, the jet stream is a massive and mysterious driver of our weather. It usually passes from west to east across the Atlantic, following a fairly steady path. However, its location is further south than usual. The result? Wet and windy weather, weather warnings, flooding and, misery, particularly for southern counties.

Normally, we would expect the pattern of the jet stream to keep shifting, for its shape to switch every few days and for our weather to change as a result. Instead, for week after week, and possibly for weeks ahead too, the meanders of the stream are sticking to the same shape so repeated rainstorms have become the norm.

The big mystery is why this current pattern is so static. The high-altitude winds that make up the stream are themselves still racing along but their path remains stuck in place so our battering continues. This is one of the major puzzles for weather specialists and the science behind this is fairly young.

One attempt at an explanation involves so-called Rossby Waves, named after the Swedish meteorologist Carl-Gustav Rossby whose research was published back in 1939. The science is complex but it seems that if it gets locked into the wrong position, with a pattern of large waves, heavy rain is the result.

On top of this, there is the related question of climate change. Most researchers are extremely reluctant to blame any single weather event on global warming. However, Dr Peter Stott of the UK Met Office says that since the 1970s the amount of moisture in the atmosphere over the oceans has risen by 4%, a potentially important factor. This could make the difference between a place getting flooded or not getting flooded.

So there are no exact answers, and plenty more questions yet remaining. In the meantime, spare a thought for birds, bees and wildflowers. Populations of some species have been decimated, with knock on effects likely to last into next year and beyond.

On the other hand, mosses and grasses, nettles and brambles are amongst the winners, thriving in the wet, as we look up at the skies in wonderment.