Gravity Current: An opposing flow of two fluids caused by a density difference.
- Chitty, R - 1994

A gravity current forms when the cooler, denser, fresh air flows under the hot, less dense buoyant gases at an opening into a fire compartment or room. At the interface, there will be a premixing of air and combustible gases. This pre-mixing is an important element of RFD.

A gravity current occurs because the density of the fresh air is higher than the density of the hot smoke inside the compartment. In Figure 18, we see the effects of the gravity current 2 seconds and then 10 seconds after opening a vent to a heated compartment. The cool air (blue) enters and mixes with the hot smoke (red), creating a combustible region (green).