How does a wildfire make its own weather?

By: T.M. Giannaros, K. Lagouvardos, V. Kotroni

Every wildfire has the potential to make its own weather, which may result in unpredictable and often erratic fire behaviour. This was vividly manifested in the Kechries 2020 wildfire, which broke out on July 22 2020 in Corinthia, Greece. In a matter of only a few minutes (~30 min) following the ignition of the wildfire, a pyrocumulus was captured to form. Such clouds form as a result of the intense heating caused by the burning fuels and clearly indicate that the wildfire is making its own weather.

Photo taken by John Kalatzis on 22.07.2020, showing the formation of a pyrocumulus above the Kechries wildfire.

How do pyrocumulus clouds form?

The intense heat released by the burning fuels warms the air, which consequently begins rising rapidly in the smoke plume. Hot and buoyant, the rising air is also very turbulent. As a result, cooler air from outside the plume is drawn in, and the plume begins cooling. As the plume keeps rising higher, the decreasing atmospheric pressure causes the plume’s air to expand and cool even more. Once cooled sufficiently, water vapour present in the plume condenses on the ash particles, forming a cumulus cloud above the plume. Since it comes from the wildfire, this cloud is called a pyrocumulus.

What do these clouds mean to fire behaviour?

The formation of a pyrocumulus above a wildfire is a clear indication that the wildfire is making its own weather. Fire-induced winds should be expected, as air from surrounding regions begins flowing towards the wildfire in order to compensate for the rising hot air. These winds may be significantly stronger than the prevailing, ambient wind. In addition, the updraft of the rising air may enhance spotting activity, by carrying burning embers over long distances and setting up new ignitions.