created with NetLogo
view/download model file: Fire.nlogo
This project simulates the spread of a fire through a forest. It shows that the fire's chance of reaching the right edge of the forest depends critically on the density of trees.
The fire starts on the left edge of the forest, and spreads to neighboring trees. The fire spreads in four directions: north, east, south, and west.
The model assumes there is no wind. So, the fire must have trees along its path in order to advance. That is, the fire cannot skip over an unwooded area (patch), so such a patch blocks the fire's motion in that direction.
Click the SETUP button to set up the trees (green) and fire (red on the left-hand side).
Click the GO button to start the simulation.
The DENSITY slider controls the density of trees in the forest. (Note: Changes in the DENSITY slider do not take effect until the next SETUP.)
Set the density of trees to 55%. At this setting, there is virtually no chance that the fire will reach the right edge of the forest. Set the density of trees to 70%. At this setting, it is almost certain that the fire will reach the right edge. There is a sharp transition around 59% density. At 59% density, the fire has a 50/50 chance of reaching the right edge.
Each turtle that represents a piece of the fire is born and then dies without ever moving. Because no turtles are moving, one could say that the motion of the fire is a construction in our mind. This is an example of an emergent phenomena: each tree is either burning or not burning, but these collective local behaviors amount to an overall effect. The effect (moving fire) is created by the agents, even though it is not experienced by the agents. So is an emergent phenomenon the same as an optical illusion? That is, do you think that the motion of fire is an optical illusion? Is this the same as the illusion of motion running through a sequence of flashing neon lights? If not, why not? Arguably, there is no "thing" running along the neon lights -- just lights flashing. So, is fire a "thing" at all? If not, then how can it move? Moreover, how can it burn?
What if the fire could spread in eight directions (including diagonals)? To do that, use "neighbors" instead of "neighbors4". How would that change the fire's chances of reaching the right edge? In this model, what "critical density" of trees is needed for the fire to propagate?
Add wind to the model so that the fire can "jump" greater distances in certain directions.
Unburned trees are represented by green patches; burning trees are represented by turtles. Two breeds of turtles are used, "fires" and "embers". When a tree catches fire, a new fire turtle is created; a fire turns into an ember on the next turn. Notice how the program gradually darkens the color of embers to achieve the visual effect of burning out.
The "neighbors4" primitive is used to spread the fire.
You could also write the model without turtles, by just having the patches spread the fire. Written that way, the model would run much slower, since all of the patches would always be active. By using turtles, it's much easier to restrict the model's activity to just the area around the leading edge of the fire.
See the "CA 1D Rule 30" and "CA 1D Rule 30 Turtle" for an example of a model written both with and without turtles.
Percolation, Rumor Mill
This model was developed at the MIT Media Lab. See Resnick, M. (1994) "Turtles, Termites and Traffic Jams: Explorations in Massively Parallel Microworlds." Cambridge, MA: MIT Press. Adapted to StarLogoT, 1997, as part of the Connected Mathematics Project. Adapted to NetLogo, 2001, as part of the Participatory Simulations Project.
To refer to this model in academic publications, please use: Wilensky, U. (1998). NetLogo Fire model. http://ccl.northwestern.edu/netlogo/models/Fire. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.