Convection is a process of heat transfer in a flowing medium, which are notably liquids and gases, since solids are unable to flow due to their fixed molecular structure. The hotter, and thus less dense, mediums rise up to be replaced by the cooler and more dense mediums. So what has this got to do with the behavior of fires? The upward flow of hotter gases form the basic theory of heat movement in a fire, which tends to travel upwards. In large fires, convection plays a very destructive role. A firestorm is created when the convection currents get really strong. Lighter fuels on the ground are drawn into the fire, while the upward convection currents are huge enough to lift the burning contents and debris of the fire up into the air causing them to fall downwind.
Radiation is a form of heat transfer in the form of waves. All matter that is above absolute zero radiates heat energy in the form of electromagnetic waves. Electromagnetic waves are a form of energy and hence are even able to travel in a vacuum. Below 500oC, radiation of heat is in the form of infrared radiation, but above it, the heat waves start to fall under the electromagnetic spectrum that our eyes can detect. Radiant heat is a particular hazard especially when the fire is very hot. Radiant heat can pose as an obstacle for firefighting as equipment and the firefighters themselves get heated up by heat radiation. Another hazard is the tendency for fuels facing the fire to absorb the heat radiation and thus causing the surface temperature to rise, possibly to the ignition temperature of the fuel. This has been the cause for buildings and vehicles igniting near a huge fire source when there seems to be no apparent connection to the fire, thus complicating both the investigation process and the process of firefighting.
Direct Flame impingement
Direct flame impingement, technically speaking, is a combination of both convection and radiation. This form of heat transfer plays a main role in the spread of fire rather than the cause of fire since a flame must first be present. When the flame is in contact with the new fuel, the fuel gets heated by convection currents and radiation until the fuel pyrolyzes and generates gaseous fuels that become ignited by the flame. This flame is able to extend outwards and upwards with the addition of the gaseous fuel. The expanding flame is then able to seek out more fuel to pyrolyze provided the supply of fresh air is maintained.