"When you talk about 'fuels' and 'fuel load' what exactly do you mean?"
With forest and brush fires each fuel type plays a crucial role in the size, shape, and behavior of a fire. The types of fuel the fire burns has a direct impact on not only the makeup of the fire itself, but on how we can prepare for and fight actively burning fires. For example, large trees do not catch fire as quickly as grass or shrubs do, but they burn longer and hotter than grass. Grassland fires are easier to put out using water while large forest fires require the creation of fire lines and often times using aircraft with water and retardant to control. The types of fuels present are extremely important when determining how likely and dangerous a fire will be.
Types of fuel:
Surface Fuel (sometimes also called ground fuel) is made up all the things found on the forest floor such as leaves and pine needles, grasses, roots, and small plants and shrubs. Usually surface fuel plays an important role in starting and growing wildfires. Highly combustible materials like leaves and dead branches can catch fire easily and in turn ignite the surface fuel around them causing a surface fire. A surface fire is described as a fire that burns on or close to the ground.
Ladder Fuel is defined as combustible material that provides vertical continuity between vegetation and allows fire to climb into the crowns of trees or shrubs with relative ease. In other words, ladder fuel includes things like trees, tall shrubs, and low hanging branches that can help a fire spread not only horizontally but vertically.
Understanding these two types of fuels is important because it helps us to understand the best ways to go about preventing and combating fires.
"What is the criteria for Class 4 Day?"
Fire Danger Classes:
Class 5 Day - EXTREME - Fires start quickly, and burn intensely. All fires are potentially serious. Development into high intensity burning will usually be faster and occur from smaller fires than in very high fire danger class. Every fire that is started has the potential to become large. Expect extreme, erratic behavior. Outdoor burning should not take place in areas with extreme fire behavior. Fire restrictions are generally in effect on a class 5 day.
Class 4 Day - VERY HIGH - Fires start easily from all causes and immediately after ignition spread rapidly and increase quickly. Spot fires are a constant danger. Fires burning in light fuels may quickly develop high intensity characteristics such as long distance spotting and fire whirlwinds when they burn in heavier fuels. Both suppression and mop-up will require an extended and very thorough effort. Outdoor burning is not recommended during this day class. Fire restrictions may be in effect.
Class 3 Day - HIGH - All fine dead fuels ignite readily and fires start easily from most causes. Unattended brush and camp fires are likely to escape. Fires spread rapidly and short-distance spotting is common. Fires may become serious and their control difficult unless they are attacked successfully while small. Outdoor burning should be restricted to early and late evening hours.
Class 2 Day - MODERATE - Fires can start from most accidental causes, but with the exception of lightning fires in some areas, the number of starts is generally low. Expect moderate flame length and rate of spread. Short distance spotting may occur, but is not persistent. Fires are not likely to become serious and control is relatively easy. Although controlled burning can be done without creating a hazard, routine caution should be taken.
Class 1 Day - LOW - Fuels do not ignite readily from small firebrands although a more intense heat source, such as lightning may start fires in duff or punky wood. Weather and fuel conditions will lead to slow fire spread, low intensity and relatively easy control with light mop-up. There is little danger of spotting. Controlled burns can usually be executed with reasonable safety.
RED FLAG WARNING - A short term, temporary warning indicating the presence of dangerous combinations of temperature, wind, relative humidity, fuel or drought conditions which can contribute to new fires or rapid spread of existing fires. Can be issued at any Fire Danger level.
Resources: Communities For Healthy Forests, Society of American Foresters, Massachusetts Bureau of Forest Fire Control, Monson Fire + Emergency Services Department, About Geography: Wildland Fires