Below which relative humidity is the fire danger considered critical?

The designation of critical fire danger is often associated with very low levels of relative humidity, which can significantly affect fire behavior and the spread of wildfires. Relative humidity refers to the amount of moisture in the air compared to the maximum amount of moisture the air can hold at a given temperature.

When relative humidity drops below 10%, it indicates extremely dry conditions, making vegetation more susceptible to ignition. Under such circumstances, the combination of low humidity, dry fuels, and potentially high temperatures creates an environment conducive to rapid fire spread and intense burning. As humidity levels decrease, the likelihood of fire occurrence and the severity of its behavior increase dramatically, and conditions can be described as critical for wildfire danger.

While levels of 35% and 50% present some fire danger, they do not pose the same critical threat as humidity levels falling below 10%. Therefore, identifying the threshold of 10% as the critical level for fire danger is rooted in the principles of fire behavior and environmental conditions that promote wildfire ignition and spread.