During a thunderstorm, when lightning strikes a tree, it does not always cause an immediate fire. There are times when the tree does not start to burn for several hours or even days. This phenomenon is called “latent lightning”.

The lightning strikes the tree and does not remain in the form of an electric current, but enters the interior of the tree until it reaches the roots. This can occur due to relatively high humidity in the environment.

Upon impact, a very slow internal combustion process begins. This combustion is detectable only by thermal cameras. The process can continue for hours or even days until, without warning, the tree begins to burn. This occurs when weather conditions, such as humidity or wind, change. The fire is triggered because oxygen enters through any gaps and feeds the embers generated during that time. If, in addition to this, there is a strong wind, catastrophe is assured.

These connected weather conditions, which make a fire unstoppable, are what experts call the “Rule of three 30’s”: 30ºC temperature, wind of more than 30km/h and humidity below 30%.

A percentage of 5% of fires are caused by purely natural causes, such as the “latent lightning” phenomenon. This was probably the origin of the fire declared last June in Gátova (Valencia, Spain), which affected more than 1,300 hectares of land, 360 of which were within the perimeter of the protected natural area.