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Fire Science: How to identify dry fuels in Colorado to avoid starting a fire

Wildfire Fuel Types
Posted at 6:01 PM, Apr 12, 2024
and last updated 2024-04-12 21:18:18-04

SOUTHERN COLORADO — Fire danger peaks in southern Colorado in Spring. April is a particularly busy month for southern Colorado firefighters, with the most Fire Weather Warning days of any month all year - averaging slightly more than 9.

What you may also not know: all 20, of the top 20 wildfires on record in Colorado have occurred in the last 20 years. The science of fire fuels is important: knowing what fuels Colorado's fires can help you avoid starting one, and understand how fire forecasters figure out what a high risk day will be.


You may know this - but let's review it again. We need three things to produce high fire risk weather-wise:

  1. Dry Air
  2. Strong Winds
  3. Dry Fuels
Fire danger is highest when weather conditions are dry and windy - and fuels are able to dry below a critical threshold

Dry air and strong winds are self explanatory. The top-10 Colorado wildfires in terms of acres burned all featured wind gusts at some point of at least 35 mph, or a rapid shift in wind direction. So - your first clue to a big time fire weather day is strong winds and in general the stronger the winds, the greater the risk.

But, what about the fuels?


There are three types of fire fuels - each behaves differently, and peaks at a different time of year in Colorado. Meteorologists and firefighters classify these fuels into three buckets - slow, medium, and fast, based on how quickly the fuels dry out in response to dry air.

Wildfire Fuel Types
Like a campfire, wildfires have three types of fuels. Firefighters group them into "slow", "medium", and "fast".

Of the three, the flash fuels are the most dangerous both to you and firefighters. They ignite easily, burn hot and fast, and can spread very rapidly. You can easily identify them. Short, brown looking grasses and small shrubs with dry withered leaves, in a dense configuration. Particularly if they've been in direct sunlight - even for just an hour. Grassland fires tend to be most frequent during spring but are a year-round issue.

Quick burning fuels are generally narrow - whether it's a twig, grass, or mulch, the width is typically well under an inch. This allows the fuel to dry quickly.

Mid-duration fuels include bigger shrubs — about the height of you — that take between a half day and 4 days to dry out. These are an issue when we have several dry days in a row. If you hike in the foothills - you've seen plenty of these.

Long duration fuels are generally dead trees and stumps. They take more than a week to dry out...sometimes several weeks. As a result, they become a bigger issue during droughts. When they do burn, they can be devastating due to the intensity at which they burn - giving off incredible amounts of heat. Colorado sees this risk more during the late summer.

When weather forecasters evaluate fire risk, they look for each of these fuels to contain less than 30% water. Forecasts and warnings are issued based on those criteria - which is why we don't always see a fire weather warning when we have a dry and windy day.

If you're outside on a high fire danger day, or you're driving someplace, the small fuels are the biggest thing to be aware of and be careful of. Particularly during spring. If you do find yourself in this terrain, preventing a fire is simple. Be extra aware not only of the obvious (an open flame), but something on your car that could drag, such as a chain - which can produce sparks.

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