Weather Watch: The importance of Polar jet streams

Latest weather news with MetDesk in Wendover
Latest weather news with MetDesk in Wendover

The last few weeks have witnessed a welcome respite in the rain and flooding that dominated the winter across southern Britain with temperatures close to record high levels during the first half of March.

That has allowed river levels to drop sharply in most places and the ground to start drying out nicely.

Having said that, some of the chalk streams in the Chilterns are still rising because the chalk aquifers lag behind the weather by weeks and sometimes months as the winter rainfall only very slowly seeps into the chalk, which acts like a giant sponge.

Meanwhile, northernmost areas, particularly western Scotland, have been much wetter with frequent spells of rain.

This turnaround in fortunes between north and south can be attributed to the position of the Polar jet stream across the North Atlantic. 

Having spent much of the winter crossing the UK at a latitude several hundred km south of its climatological average position, it has now reverted northwards, allowing pressure to rise across southern Britain.

High pressure is associated with descending air and usually equates to dry weather, whilst low pressure corresponds with areas of rising air. As the air ascends, it cools down and can no longer hold all the moisture contained within it. The excess moisture condenses out into water droplets, i.e. cloud, with the result often being rain and wet weather.

For this reason, the northward migration of the jet stream and the low pressure areas that track underneath it, means that the climatological norm of north-west England and western Scotland being wettest and south-eastern England driest has been restored.

As for the forecast here in Aylesbury Vale, there’ll be some more warm spring weather today with hazy sunshine, but that looks like being the end of the unseasonable spring warmth for a while.

Tomorrow will be cloudier with rain overnight.

Friday and the weekend then looks like being cool and showery with temperatures almost 10 degrees down on the last couple of weekends, although there will be some sunshine between the showers.



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