Archive | May, 2014

Instability and Stability In the Atmosphere

9 May

Today has been a great example of how the atmosphere can quickly progress from being unstable to stable due to larger scale processes and the differences at the surface can be quite marked. To start with the following 2 atmospheric soundings for Leeds have been used to highlight how the instability changed very quickly from late morning compared towards the afternoon;

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I’ve highlighted on both soundings where the atmosphere was unstable up to. Clearly you can see on the left image that around late morning the atmosphere was unstable to near 24,000ft which resulted in deep, towering clouds and the resultant conditions at the surface was frequent and often heavy downpours and with longer spells of rain. In contrast note how by 1700BST and the late afternoon the atmosphere is far more stable with clouds only reaching heights of around 7,000ft as subsidence (sinking/warming air) takes place above this level with a temperature inversion.

The primary reason for this change in instability from a particularly unstable atmosphere to a far more stable atmosphere is due to broader synoptic scale changes that have taken place and these are highlighted on the following images;

modelanalysis1

On the left image which is 0600Z you’ll note that low pressure at the surface is crossing Scotland whilst within the upper atmosphere a marked trough is also moving across the UK in direction association with the surface low pressure. This upper trough is placing particularly cold air over N England and the Leeds region. This helps to increase instability and is directly related to the first atmospheric sounding from late morning which is clearly highlighting the unstable atmosphere. Now in contrast note how by 1800Z the surface low pressure and upper trough have cleared away to the east of the UK into the North Sea and in it’s place is a transient ridge of high pressure both at the surface and within the upper atmosphere. This is also highlighted by the change in colours across the UK  on both images (trough to a ridge pattern). This rise in pressure both at the surface and within the upper atmosphere coincides with a rise in temperatures too and this prohibits or at least limits convection from taking place.  Once again this can be directly related back to the second atmospheric sounding for 1700BST.

Somewhat ironically the reason for the lack of instability and general reduction in showers is the development of an area of low pressure to the west of the UK in which the ridge of high pressure builds ahead of for a short time. What happens after this is the upper ridge quickly moves across the UK and allows low pressure and another significant upper trough in the atmosphere to approach from the west.

So, in summary. To aid instability in the atmosphere you need lower pressure and colder temperatures within the middle and upper atmosphere in particular. This combines with warmer temperatures at the surface to create an unstable atmosphere, as per the atmospheric sounding from 1100BST. To reduce instability and create a more stable atmosphere (better weather/conditions) you need higher pressure and generally warmer conditions within the middle and upper atmosphere and this, as a result, limits convection and improves conditions at the surface.

Finally to highlight this are two satellite images one from 1045BST and note the extensive cloud evident over N England and the second image is for the early afternoon and already there is less cloud and more in the way of bright or sunny spells developing as the instability is replaced by stability within the atmosphere, particularly so across NW England.

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