How Does Humidity Affect a Fire?

Discover how relative humidity (RH) affects a fire, and why it is very important for taming a fire with expert guidance from the Western Fire Chiefs Association (WFCA).

Understanding the relationship between humidity and fire danger can ensure you take the right steps to prepare under certain conditions.  If you have ever wondered, “How does humidity affect a fire, and how does that impact me?” read on!

What are the Primary Weather Elements that Affect Fire?

The following three weather elements influence fire behavior:

1. Wind
2. Humidity
3. Temperature

In the right proportions, these factors can lead to extreme fire behavior. Fires spread in hot, dry, and windy conditions. When temperatures are warmer and there is lower relative humidity, it makes fuels more receptive to ignition. When winds are strong, they supply more oxygen, preheat fuels in the path of the fire, and transport embers ahead of the flaming front.

What is Relative Humidity (RH)?

Relative humidity (RH) is defined as the ratio of the amount of moisture necessary to saturate the air at the same temperature and pressure. RH is expressed as a percentage, and it is the key to understanding how humidity affects a fire.²

You can obtain the RH from a local weather station as they generate an automated reading. You can also manually measure RH with a wet and dry bulb reading.

How Does Relative Humidity Affect Wildfire?

Relative humidity affects wildfires by either dampening or drying out potential fuel. The air exchanges moisture with the soil and dead vegetation or organic matter. Forest fuels include needles, twigs, shrubs, downed trees, logs, etc. Fuel includes anything and everything that is combustible.

Fuel moisture is measured for both live herbaceous and woody fuels as well as dry, dead fuels. The values represent the approximate moisture content of the fuel. In particular, understanding the dead fuel moisture content is critical in determining fire potential.

When relative humidity decreases, fire behavior increases because fine fuels like grass and pine needles become drier quickly. Heavy fuels are less easily affected by changes in humidity, and it often takes a larger or longer-lasting event to affect the moisture significantly.

Low humidity levels dry out vegetation fuels on a short-term basis, and they can also cause a short-term spike in fire danger. The relative humidity is lowest when the air temperature is high and the dewpoint temperature is low. The dewpoint temperature is the temperature when the air needs to be cooled to become saturated.

Final Thoughts

So, how does humidity affect fire? When there are cooler temperatures and/or high atmospheric moisture levels, it reduces fire danger. Check with your local weather station or fire station to understand the critical relative humidity levels as they vary by region.