In 2003, Europe endured the hottest summer on record since at least the Middle Ages, if not longer. For three months, the temperatures soared regularly to levels Europe has simply never experienced. In some regions, even nighttime temperatures did not fall below the 90s. European homes are not built for this kind of thing. More than 46,000 Europeans died.
With the months of unrelenting summer heat came a punishing drought, which helped to fuel a strong Saharan wind. Rain never came, wells ran dry, and the water previously taken for granted ran out. Crops withered, trees dried out, animals died.
Diseased and dead vegetation catch fire easily. Even normal activities can create the spark which sets everything ablaze. A hot automobile exhaust pipe can set fire to nearby grass. Fast-moving trains shed sparks from their wheels. At one point, one third of Portugal was ablaze.
A heat wave which has killed off the local vegetation also kills off its root system. Nothing remains to hold the topsoil in place. If the drought still does not break, that valuable topsoil blows away in the wind. Where the air is dry enough and the wind is strong enough, it can create a dust storm.
Even milder heat waves can kill. The elderly, young, ill, and overweight are especially at risk, as are those with chronic health problems and those who are too poor to be able to afford air conditioning.
The human body tries to cool itself by sweating; so both the evaporated water and lost salts have to be constantly replaced. A thirsty body is already dehydrated. The ability to think clearly is lost early on. By the time more serious symptoms of dehydration hit, it is already too late.
Summer heat waves are often exacerbated by atmospheric inversions which trap pollutants. In a typical city, smog builds up in no time.
It does not even take a particularly hot day for the temperature inside a locked car to reach the broiling point. A child or a dog left in a car will develop heat stroke. If left alone too long, they will die.
A mild heat wave also influences human behaviour: but not entirely in ways one might expect. Individually, people lose energy and aggression as the temperature goes up. As a mob, however: heat makes riots and crime rates worse - but only to a certain point. The precise temperature varies from place to place, depending on what the local people are used to.
Once the temperature exceeds that point, even riots fall apart. There simply comes a point when it is too hot to do much of anything, even random violence.