Slipping jet streams
| 15:18 UK time, Monday, 27 July 2009
So what's happened to our summer BBQ weather? June was drier, warmer and sunnier than average but we've had our fair share of rain in July.
Ray Garner from Llanfynydd, Carmarthenshire has recorded over 290 mm of rain so far this month at his weather station.
That's three times the monthly average of 95.5mm. This July is actually wetter than July 2007.
So what's causing it? The reason for all this the rain is down to the position of the jet stream.
The jet stream is a like a river of fast moving air high in the atmosphere. It normally flows from the west to east across the Atlantic, but sometimes from north to south.
It's the dividing line between cold Arctic air and warm air from the tropics. This dividing line is most pronounced in winter and that's when the jet stream is at its strongest and winds in the centre can reach 275mph.
Today's jet stream can be see here in yellow:
Jet streams meander around the globe like a snake, dipping and rising, splitting at times and even disappearing altogether to appear somewhere else.
It's currently much further south than it should be and bringing us rain and showers. Why this happens is unclear but its not unusual.
When the jet stream points towards Britain, it means wet and windy weather. Flooding of course is nothing new but with climate change, winters are predicted to get wetter with heavier rain and an increased risk of flooding.
In summer, the jet stream tends to move north between Iceland and Scotland allowing high pressure over the Azores to move towards Britain.
This brings warmer and drier conditions. However in Summer 2007 and 2008 this didn't happen and it was the wettest summer since 1914.
Hopefully it will shift further north in August and we'll get some high pressure and proper
summer weather but in the meantime, we'll just have to wait and see.
E-mail unusual cloud photos to me at wales nature and I'll feature some of the best ones in the blog.
Derek
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