Why has the Grímsvötn ash cloud split into two?
d ~ 370'483'200 km: day 144
Another Icelandic volcano has erupted and ash is back in the headlines. But have you noticed the odd trajectory of the ash cloud? The Met Office has issued maps of where the ash will get to in the next few days and the plume is doing something very strange.
Image © Met Office
Before you curse the winds delivering powdered volcano to the UK, bear in mind that those winds are only bringing half the plume. The rest of it is on its way round the north of Norway to Russia. Why would a massive cloud of floating glass fragments split into two like that?
Wind happens because air is getting pulled by gravity, accelerated by the Coriolis effect and pushed about by pressure differences. For example, where cold polar air touches warm air near the top of the troposphere, the associated pressure difference and the spin of the Earth cause a rapid eastwards wind that we call the jet stream. But the air just above and below that experiences different forces and may be doing slightly different things.
Look up into the sky today and think about where that air might have come from. It turns out that seven days ago the air that is now 1.5 km above the UK was 5km up above the north of Russia, and it travelled to us via Greenland, slowly losing height along the way. The air that is 3km above me today was close to ground level over the mid-Atlantic only two days ago, and it's been rising pretty rapidly since then. We are connected to the rest of the planet by the air above us.
The direction and speed of the air changes with height, and the large patterns like the jet stream only tell part of the story. Hot air balloonists use this complexity to change direction - they can't steer left or right, but they can go up or down to join an airstream which is going in the direction that they want.
So the answer to our question about the ash cloud is that the wind over Iceland is going in different directions at different heights. The plume currently reaches 10-15 km above the surface. The ash that gets up above 8 km has joined a northwards wind which is taking it away over the Arctic ocean. But below 8 km there is a different air stream, carrying the other half of the plume towards us.
Most of the time, we don't see the complexity in our atmosphere because air is transparent. The ash cloud is letting us trace one tiny segment of the air flow around our planet. For me, part of the beauty the atmosphere is that it's always changing. This time, the volcanic eruption and the movement of the atmosphere have lined up to cause problems for travellers. But it won't last long, and the travellers will soon be on their way again. In the mean time, enjoy being able follow part of our atmosphere on its journey around the globe.