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The Conversation

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imageKeeping cool as Paris sees its hottest temperatures in six decades.Etienne Laurent / EPA

For those that haven’t noticed, it’s been rather warm in Europe. In fact, last Wednesday was the UK’s hottest July day since records began. The temperature at Heathrow soared to 36.7°C, approximately 15°C higher than the average maximum daily temperature for the month. In mainland Europe it has been even hotter, with much of Spain well into the 40s and Paris seeing its highest temperature since 1947.

However, the extreme conditions haven’t been limited to high temperatures – thunderstorms were generated that produced fantastical lightning shows, heavy downpours and hail stones the sizes of ping pong balls.

What causes a European heatwave?

The World Meteorological Organisation defines a heatwave as five or more consecutive days when the maximum daily temperatures are at least 5°C above average for that time of year.

Persistent hot weather of this type is linked to high pressure systems, where air descends, heats up and drys out. This suppresses the formation of clouds and allows for clear conditions for days or even weeks.

Western Europe’s weather is largely governed by the jet stream. This high-altitude, high-velocity river of air meanders around the globe and is constantly changing position.

When an omega-shaped wave is present on the jet stream which arcs over Europe, warm dry air from southern Europe and Africa can be pulled north, pushing temperatures higher than normal. If this upper level feature coincides with high pressure at the surface with relatively low pressure to the east and west an omega block is formed.

This pattern of flow can be very persistent and lead to long periods of fine weather in summer. Decreased cloud cover and transport of warm air over a number of days can produce very high temperatures. Such a situation occurred last week, replacing the usually cool westerly air stream with warm air from over continental Europe.

imageEurope’s omega block on June 30. The omega-shaped jet stream is highlighted in the insert.Severe Weather Europe / EUMETSAT

An important consequence of this situation has been the formation of a Spanish plume, which transports warm dry air from over Spain to the UK (sometimes North Africa, which explains the red dust left on your car after it rains).

This air rises as it travels and acts as a lid under which energy is trapped and builds. This energy is then suddenly and dramatically released through thunderstorms, as the warm air bumps into the colder from the jet steam and further north. The strongest thunderstorms are generated at the edge of the high pressure region – hence the storms over the UK last Wednesday.

Heatwave history

Europe has been affected by two notable heatwaves in recent memory, during the summers of 2003 and 2006. Both were caused by very similar meteorological conditions to those occurring now.

While hot, dry weather might seem like perfect summer conditions, such severe heatwaves can be deadly. The “lid” in the atmosphere causes pollutants to build up near the surface. Exposure to these, together with dehydration and heat stress pose a very real risk. In the heatwave of 2003 almost 15,000 deaths were attributed to the weather conditions in France alone. Heatwaves can also be responsible for destruction of crops, widespread drought, power cuts due to lightning strikes, accelerated glacier melt and destructive forest fires – governments across Europe have plans specifically designed for reducing the impact.

What impact will climate change have on this? It will no doubt lead to higher temperatures in Europe, and with hotter conditions the air is able to hold more water. This means there is more energy that can be released by thunderstorms, which is expected to lead to heavier downpours and other more severe weather.

However, it is impossible to attribute a particular event to climate change. Just like rolling a loaded die, you cannot say whether any particular six you roll was because the die is loaded, only that the chance of a six was higher.

The key components of a heatwave are the flow patterns on a continental-scale, and whether the frequency of these patterns will be significantly changed in an altered climate is still uncertain. As such, it is an important and interesting topic of debate among scientists.

Alexander Roberts receives funding from Natural Environment Research Council (NERC). He is affiliated with Labour Party.

John Marsham is a water@leeds Research Fellow and part funded by the National Centre for Atmospheric Science (NCAS).

Authors: The Conversation

Read more http://theconversation.com/explainer-the-omega-shaped-jet-stream-responsible-for-europes-heatwave-44268

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