|Summer solstice celebrations at Stonehenge, UK [AFP]
In many parts of Europe the summer solstice marks the start of high summer. With the sun at its highest point in the sky, who could blame people for thinking that fine weather was assured. Daylight in the northern hemisphere may be at its greatest but unfortunately the sun’s position coincides with a spell of very unsettled weather, known as the European Summer Monsoon. But is it really a monsoon, in the true sense of the word and how can we identify it?
That European early summer weather is changeable, to say the least, is not in doubt. The start of the European rock and pop season is marked by the Glastonbury Festival. Despite the appearance of many of the top rock acts from around the world, footage of the event often seems to focus on festival-goers dancing in knee-deep mud.
For much of western Europe, which lies under the influence of mild, but moist Atlantic airflow, fine summer weather rarely lasts for more than a few days and is likely to be punctuated by showers or longer spells of rain. However, looking more closely at the weather statistics, a significant pattern begins to emerge.
Building pressure, which leads to more settled weather, tends to establish itself during May, but come early June those damp westerly winds return across the continent, bringing unsettled weather. The climatologist H.H. Lamb identified the periods June 9 to 18, 21 to 31 July and 1 to 10 August when the weather was likely to be dominated by low pressure and changeable conditions. He found that over a period of 70 years these unsettled periods returned with a regularity of between 84 and 89 per cent.
Although the first of these periods ends just before the summer solstice, it does coincide with the start of the Wimbledon Open Tennis Championships. The regularity of delays and interruptions to the tournament eventually forced the All England Tennis Club to invest $US 130 million in 2009 to keep the summer rains at bay.
A definitive explanation for these wetter phases during the European summer is hard to come by. Although there is no direct link and little comparison with the intensity of the Asian and African monsoons, it has been suggested that it may be linked to a shift in high level, jet stream winds over the Himalayas which directly affect the monsoon in these other regions. But some climatologists also point to the influence of melting snow and ice over northern Canada and warming in the north Pacific Ocean.
Whatever the cause, the effect is to dampen people’s hopes of a fine summer and climatologists think the European Summer Monsoon could become a more pronounced feature as global warming continues.
Source: Al Jazeera