OpenLearn Live puts free learning at the heart of your world. This page will be updated across the day.
And on a personal note: farewell to Matthew Harrington, who is leaving the OpenLearn team to head off and do interesting things with equally lovely people elsewhere in the university. Matthew, we'll never forget you, or your gravity-defying chair.
It's Friday, which means during the course of today we'll be publishing the second in our series of Shakespeare Speaks videos, produced in association with BBC Learning and designed to show how the Swan of Avon influenced the English language.
Rounding off our week of strange things that the weather can do, let's start with a quick reminder about those events you might have missed:
- The Novaya Zemla effect, which creates a rectangular sun
- Derechos, like unrolled tornados
- Heat bursts, which can fry crops on the ground
- The zud, which devastates Morocco farmers
We're ending the week with Mammatus clouds. Lovely, blobby old clouds like this:
Here's a video of some scuttling across the sky:
You won't be surprised to know they're also called cotton ball clouds. They're normally a good sign - an indication that a thunderstorm has passed. They're caused by a thunderstorm anvil cloud collapsing - the updrafts which carry the 'wet' air to the top of the anvil start to lose their momentum, and the water-filled air spreads out. Because the wet air is heavier than the air around it, as the head of the anvil falls in, it sinks back towards Earth. The sinking warms the air, melting any ice and evaporating the water - but this change of state absorbs the energy that had been moving the air. But if enough energy is being absorbed by the changing of state, the heat will go out of the air and, being colder, the air will continue to sink. The rounded pouches are caused as a result.
What makes them really amazing is if the sun is around to bounce of their bottoms...