By Philip Eden

The so-called June monsoon has been fingered in the news media for the appallingly cold, wet and dull start to summer. But this reveals a fundamental misunderstanding of what the June monsoon actually is.

The term "monsoon" is supposed to carry its original Arabic meaning - a seasonal wind - rather than the torrential downpours that most people think of when the word is mentioned. In India the southwesterly monsoon of June to September delivers much heavy rain, but the northeasterly monsoon of November to April brings dry and sunny weather.

Thus, over the Atlantic and Europe, climate-experts believe that the typical slow-moving weather systems of May and early-June - either high pressure, or, as this year, low pressure - give way to the westerly winds of high summer. These westerly winds bring frequent cloud and rain to northern and western parts of the UK, but southern, central and eastern regions sometimes see a marked improvement in the weather.

Professor Hubert Lamb, Britain's leading 20th century climatologist, calculated that the midsummer arrival of the westerlies occurs in a recognisable form in approximately 70 per cent of years. He, by the way, preferred the more prosaic term "June return of the westerlies" which is, of course, much less likely to be misunderstood.

He pointed to several roughly coincident changes in the atmospheric circulation during late-June or early-July which might encourage this particular "singularity" in the climate of northwest Europe. He cites, for example, the disappearance of snow and ice from northern Canada, the earlier warming of the north Pacific Ocean compared with the north Atlantic Ocean, and the shift of the jet stream over Asia from its normal winter position over northern India to its usual summer location north of the Himalayan massif. Jet streams are conveyor-belts of very strong winds high in the atmosphere which control the behaviour of the depressions on our weather charts.

The date of the switch in weather pattern can be very variable, ranging from the first week of June in some years (as in 1944 when the D-day landings were postponed by a day) to the second week in July as happened in 2004. This year the return of the westerlies hasn't happened yet, and it remains to be seen whether 2012 is one of the 30 per cent of years when it fails completely.

By Philip Eden