Formation of a forest fire
Most commonly forest fires occur in the semi-arid regions of the earth. The conditions for forest fires are abundant vegetation and an extended drought, which dries out the vegetation. A forest fire can be triggered through lightning or by the carelessness of people (cigarette ends, not extinguished fireplace in the forest). Depending on land-use people often set fire to vegetation intentionally (slash-and-burn cultivation). The fire burns for days or even weeks. Sometimes several square kilometres are on fire.
In particular ecosystems forest fires play an important role for the balance of vegetation. This occurs in the humid savannah or in scrub vegetation areas (e.g., in the Mediterranean Macchiae or along the western coast of Northern America). In those places forest fires are indispensable for the spreading of seeds.
In Switzerland forest fires are mostly of small scale. They occur in the Valais, in the Grisons and quite frequently in the Ticino, where chestnut forests are primarily affected.
What damage can occur?
Forest fires affect mainly vegetation, however, in some instances these fires are desired from an agricultural point of view. In addition, the fires destroy houses or traffic infrastructure as well. In few cases people are isolated by the fire and perish. However, the health of human beings is mainly threatened by smoke and ash. The emissions of forest fires have a strong influence on the atmosphere and thus on climate conditions. A forest fire mainly emits CO2 and CO. This supports the formation of ozone within the lower layers of the atmosphere, thus creating smog-like conditions. In addition, the ash and smoke particles have climatic consequences: they increase the presence of cloud condensation nuclei. As a consequence there are more small-scale water drops instead of fewer large-scale water drops. These small drops are very light and remain floating in the air thus causing more haze and reducing the rainfall probability. In addition, the haze formation has a cooling effect of the earth due to increased reflection of the sunrays.
Increased landslide activity is a secondary effect of forest fires. The destroyed vegetation results in unstable slopes that causes increased landslide hazard.