The claim

  1. High outside temperatures and strong sunlight often lead to spontaneous fires.
  2. Broken glass in the wild often causes fires due to the burning glass effect.
  3. Spontaneous combustion caused by solar radiation is a common cause of fires.
  4. Water droplets on plants and skin can cause burns due to the burning glass effect.

Our conclusion

  1. The likelihood of spontaneous fires caused by high outside temperatures and strong sunlight alone is low. High ignition temperatures are required to trigger a fire, which solar radiation generally cannot reach without other factors.
  2. The burning glass effect caused by broken glass is possible, but the likelihood of a fire caused by it is very low. Under optimal conditions, in a test by the German Weather Service, only one out of five pieces of glass reached temperatures above 200 degrees, but was unable to cause a fire.
  3. Spontaneous ignition due to solar radiation is unlikely because the ignition temperatures are usually not reached.
  4. The claim that water droplets on the skin can cause burns due to the magnifying glass effect is not tenable. Water droplets evaporate quickly in the blazing sun, and the assumption that wet skin generally leads to more sunburn because the burning glass effect is responsible is incorrect.

It is a physical process in which light rays are bundled through a lens and directed to a focal point. This can cause significant heat generation and theoretically lead to fires. However, the conditions under which the burning glass effect can actually start a fire in everyday life are specific and rare.

Public perception and media reporting often cite examples such as broken glass in nature, which focuses the sun's light and could trigger forest fires. In this context, experts often cite the burning glass effect as a possible culprit.

But how realistic is this representation of the burning glass effect actually?

Studies and reports from experts suggest that the chance of broken glass actually starting a fire is extremely low. A critical analysis reveals that numerous factors would have to come together to make ignition possible. These include aspects such as the degree of curvature and thickness of the glass, the angle of incidence of the sun's rays, the duration of sunlight and the temperature reached.

In fact, reports of spontaneous combustion caused by strong sunlight or the magnifying glass effect are often misleading. They do not take into account that very high temperatures are required for ignition. For example, the ignition temperature for wood is between 280 and 340 degrees Celsius, for straw between 250 and 300 degrees Celsius (information comes from the Chemie.de ). Such temperatures are generally not reached by focused sunlight, neither from broken glass nor from sunlight without a lens effect.

Water drops, sunburn and burning glass effect

Drops of water on the skin do not cause (severe) sunburn, although the opposite is often said. The theory that water droplets act as magnifying glasses and intensify sunlight is largely a myth. It is based on a misunderstanding of the burning glass effect, in which sun rays are focused through a lens to reach high temperatures. Scientific research and expert opinions show that water droplets on the skin do not meet these conditions. In addition, sun rays, even when focused through water droplets, do not reach the temperatures necessary to burn the skin. The main cause of sunburn is direct UV radiation, not the burning glass effect of water droplets. Therefore, it is important to adequately protect yourself from the sun to avoid skin damage.

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Why it burns in summer...

Instead, the real “culprits” for fires are usually human error and carelessness. Carelessly thrown away cigarettes or improperly extinguished charcoal pose a far greater fire risk than the widely misunderstood burning glass effect. This is because these items can reach temperatures well above the ignition temperatures of common fuel.

Why in summer? Quite simply: it's warm. It's vacation. It's been light outside for a long time. People like to be in nature and behave carelessly. Leftover embers after a barbecue, carelessly disposed of cigarettes. Generally human error. Also a reference to previous fact checks .

Important: Of course, please do not dispose of any rubbish, broken glass or glasses in nature, even if they do not start fires!

Also read: Fact check: The Xylocopa bee – Huge, harmless and essential for our ecosystems

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