2003 European Heatwave
The summer of 2003 in central Europe was the hottest ever recorded. Record high temperatures were recorded across many countries, including the UK, although France was probably the worst hit county. Estimates vary, but over 20,000 people died as a result of the extreme heat. It is likely that the summer of 2003 was the hottest in at least 500 years for Europe. Along with the extreme heat, came drought, which lead to crop shortages and significant impacts on water ecosystems. The heatwave was essentially the result of a very stagnant pattern of weather throughout the summer - an area of high pressure positioned itself over much of Europe and wouldn't budge, preventing areas of low pressure from migrating into Europe from the west and cooling things down. 
Difference in average temperature from 2003 for the date range 20th July - 20th August.
Credit: University of Reading.
Surface pressure anlysis for the 19th July 2003. This is just a random day during the summer, but by this stage, high pressure had been stagnant over central Europe for several weeks, allowing the heat to build. 
Credit: Met Office. 
For much of the summer, central Europe was baking, with temperatures soaring to record highs. However, the high pressure was always just out of reach of the UK throughout the beginning of the summer. Instead, cool Atlantic flow was the dominant air source for the UK, keeping temperatures around average. These areas of low pressure were being diverted around the "blocking" high, allowing the skies to be kept mostly cloud free, and for temperatures to soar over Europe. 
Surface pressure analysis for the 10th August 2003. This chart depicts the climax of the heatwave event across much of central Europe and the UK. Temperatures climbed into the low 40s celcius across much of mainland Europe. This hot air was fed gently towards the UK from the south-east, allowing temperatures to climb here too. 
Credit: Met Office. 
The heatwave in the UK began in the last week of July and ended in mid-August. Temperatures gradually climbed higher day after day and the 10th of August 2003 was the hottest day ever recorded in the UK, with a high of 38.5°C recorded at Faversham in Kent. This temperature is incredibly high for the UK, considering that in an average summer, one might only expect a few days with a temperature of over 30°C recorded somewhere. The heat really began to build from the 5th of August, with the Channel Islands experiencing the heat first - a maximum of 34.8°C was recorded at Jersey Airport. However, the overnight minimum of 23.7°C, recorded here for 4/5 August is perhaps the most astonishing statistic. On the 6th of August, temperatures ramped up further, reaching a maximum of 36.4°C at Gravesend, Kent. Temperatures decreased slighty for the 7th and 8th of August, before the temperature hit over 36°C in several parts of London on the 9th. The highest maximum ever recorded in Scotland was beaten on the 9th, with 32.9°C recorded at Greycrook in the Scottish borders. On the 10th of August, no fewer than 13 weather stations recorded higher maximum temperatures than the UK's previous record high of 37.1°C at Cheltenham on the 3rd of August 1990. Fortunatly for the UK, the wind soon began to blow off the Atlantic and cooled everything down! 
Maximum temperature contours for the 6th, 9th and 10th August 2003. 
Credit: Met Office
Table displaying maximum temperatures recorded on the 10th August 2003 at various weather stations in the UK. 
Credit: Met Office. 
What was unusual about this weather event was that the high pressure remained stationary for so long; normally, relatively short bursts of heat might be expected from an area of high pressure, before a front moves in and cools everything down. However, the extreme heat smashing temperature records was only part of the story. With crops across Europe already feeling the effects of a harsh winter and a late spring frost, the heatwave finished off much of the surviving crop. The low rainfall and high temperatures meant that crops were trying to grow in soil that was rapidly drying out, which reduced crop yields. The hot weather was also devistating for alpine glaciers, whose mass was reported to have decreased by up to 10% in some cases. The rate of water loss was over 5 times as great as during an average summer.
The heatwave also had major impacts on energy, especially in France. Generally, during a period of hot weather, energy demands increase as people turn up their air conditioning and refrigerators. However, in France, most of the energy is supplied by nuclear power plants, which need to be cooled down by river water. In some places, the river levels dropped so low that the nuclear reactors had to be shut down as they could not be cooled effectively. A devistating impact of the heat and drought was on vegetation: there were huge forest fires across Europe that summer, with an estimated 650,000 hectares of forest burnt across many countries. In fact, Portugal had 5.6% of its total forest burnt. 
Closer to home in the UK, hospipe bans were introduced as water levels dropped to record low levels in many reservoirs and some drinking water supplies were also affected. The London underground, famous for not having air conditioning, became unbearably hot, while railway tracks buckled and roads began to melt. On a positive note, the hot weather boosted tourism as many people decided to holiday in the UK while the weather was good. 
Visible satellite image of Europe underneath the blocking high pressure - clear skies allowed temperatures to soar.
From: http://www.eurofora.net/newsflashes/news/weathersciencerussianheatwavemassue.html?mylang=italian
Very low water levels at Haweswater Reservoir, September 2003. 
From: https://en.wikipedia.org/wiki/2003_European_heat_wave