Everyone has seen large thunderstorms, with their characterisitic anvil-shaped clouds, but there is more to the spreading anvil than meets the eye! Updrafts within cumulonimbus clouds (Cb) can be very powerful, reaching speeds of 50mph in the UK, sometimes more. It is the result of violent convection, when locally warm parcels of air rise rapidly through the surrounding cold air. Developing cumulus clouds work on exactly the same basis as a simplified lava lamp - next time you see a lava lamp, imagine that the brightly-coloured wax is actually a cumulus cloud. The complex dynamics are very similar in the real world!

A growing cumulus cloud:

Fully-blown cumulonimbus with anvil:

So why is this report called 'Invisible Anvils'? Well, to help explain, the image below was taken in North Wales in the spring of 2003. At face value, it shows a developing thunderstorm with the anvil beginning to spread from left to right at the top of the image. However, on closer inspection, something else is going on.

On this image below, we've highlighted an area of cirrocumulus, which is the highest cloud variety on this photo. About 10 minutes before this image was taken, this patch of cloud was much larger, covering the area of sky where the developing Cb was headed:

On this next image, we've highlighted the growing cumulonimbus cloud, or thunderstorm:

This following image shows what's happening inside the growing cumulonimbus. Updrafts of air are rising at great speed inside the cloud, just like the bubble of wax in a lava lamp (as mentioned earlier):

At the 'lid' of our troposphere (the bit of atmosphere with all our weather in it), is the tropopause. Air temperatures generally decrease the higher up you go, but sometimes you can have inversions. These are where you actually have an increase in temperature with height. The tropopause is basically an inversion, and it quicky gets warmer the higher up you go. When the growing cumulonimbus hits this, it can't go any higher (as warm are cannot rise through even warmer air). So as the updraft continues inside the cloud, it spreads out underneath this ceiling of warmer air, and that creates an anvil:

Finally, the part of the image giving this report its title, the 'invisible anvil'. Air is flying outwards so fast in the anvil, that it's actually ramming into the air ahead of it. This clearer, drier air is then crashing straight into the cirrocumulus clouds, and through rapid evaporation, is wiping them out of the sky (shown here by the red arrows):

This is something the EOTS team have very rarely seen, and despite being a simple mechanism, we've yet to see it again!

Please let us know if you've captured something similar on camera!