European monsoon

Return of the westerlies

Issued 18/05/2015

As we head through June you may hear the terms "European Monsoon" or the "return of the westerlies" being used if an unsettled and wet spell of weather develops of if a festival turns into a mud bath. What do these terms refer to?

The terms are loosely defined but have commonly become used to describe periods in June and July when westerly winds may become more frequent in the UK and north west Europe. In the UK a mobile westerly flow bringing cloud and rain in from the Atlantic is of course possible all year round, usually reaching its peak in the winter months. However, there is a tendency for it to become weaker during May before sometimes strengthening again in the first half of the summer.  

By Domharrison (Own work) [Public domain], via Wikimedia Commons

A monsoon is defined as "a seasonal reversing wind accompanied by changes in precipitation". The Indian monsoon is probably the classic example. What we experience in the UK and north west Europe doesn't fit the definition of a monsoon. Despite this the term has become popular to describe periods of unsettled weather which can develop in the first half of the summer.

Causes of the European Monsoon

The causes aren't well understood. One possible explanation is the warming over continental Europe. There are also suggestions that it results from changes to the circulation in other parts of the world.

What about this year?

It is tool early say. Medium range computer models are showing an increasing chance of high pressure becoming established near the UK as we head into June. This means drier and warmer periods of weather become more probable at this time of the year.

Summary

Should an unsettled period develop in June or July expect to hear the terms "Return of the westerlies" and "European Monsoon". These refer to westerly winds bringing cloud and rain back in from the Atlantic. The pattern in the UK and north west Europe does not fit the definition of a monsoon, nonetheless, if you're at Glastonbury and it turns muddy that probably won't be high on your list of concerns!

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