Why does wildfire smoke swirl only one way in the air?

A: Sometimes wildfire smoke in the stratosphere collects into a compact bubble of smoke that spins in a coherent vortex, clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere.

Two new studies, published in Weather and Climate Dynamics and presented at a recent meeting of the American Meteorological Society, have found why. The smoke particles absorb sunlight and warm the air around them. That makes the air buoyant, and it rises through the smoky core, pushing the clump of smoke particles up over time.

Earth’s atmosphere is rotating and has many layers. If you warmed one patch of stratospheric air and kept the warming at the same height, the air just above will start swirling one way and the air just below, the other way.

Because the smoke particles are rising, the heating pattern also moves up with the smoke. This matters because the atmosphere’s ‘push’ to make the air rotate also moves upwards. As the warm core passes through a layer, it will briefly nudge the air into rotating one way. Once it has moved on, the later push in that same layer will undo much of the earlier change. As a result the most coherent rotation is wrapped around the smoke bubble itself, like a collar that travels upwards with it.

The rotating bubble also acts like a container, keeping the warmer smoke concentrated near its centre, rather than mixing with the surroundings, and allowing it to keep rising.