First, how is it that a fire can gain the ability to move?
A fire requires the following: Oxygen, fuel, and heat. Oxygen being from the atmosphere cannot limit wildfire movement. Plants are the main provider of fuel for fire and due to its common close arrangement allows a fire to spread from one plant to the next. Using the following three main causes of heat: convection, radiation and conduction. Which is also something closely arranged plants provide a fire.
Convection- The transfer of heat caused by the movement of a gas or liquid.
Radiation- Burning objects releasing energy in the form of heat.
Conduction- direct contact with heat.
Their are three standard places where a Forest Fire is common to take place: Underground, on the surface, and in Tree Crowns.
Underground- Where plant debris smolder below the surface of wetlands.
Surface- Burning fuels near ground level such as grass, shrubs, small plants.
Tree Crown- Fires which burn on the tops of trees.
It is common for fires to begin on the surface or underground and hit extremes of tree crowns and spread vigorously.
So now that you know a few facts about how fire works what is it that makes a fire travel faster up then down?
Heat is just energy, when we measure the temperature of heat were actually measuring the amount of energy.
The more energy something has the faster the energy can move, and since the energy is moving faster it takes more space.
And in this logic that which takes up more space will have less density. So hot air rises higher then cold air because it is less dense.
Which is why Fire moves faster uphill then downhill.