Why are storm clouds dark, and how does the Earth produce a magnetic field?

We explore the curious questions that science can answer

Rain stopped play? Larger water droplets absorb and scatter more light (Getty)

Why do storm clouds tend to look darker than normal clouds?

Storm clouds have a lot of water in them in droplets that are larger than those in normal clouds. These larger droplets absorb and scatter more light than smaller ones, so the storm clouds look darker.

Is the ice crust on the polar seas made of falling snow? Also: at what temperature does the sea freeze? And are icebergs made of fresh water or of sea water?

Sea water is very cold and its freezing temperature depends on its salt content. Water gets less dense as it cools because of the bonding structure in the ice lattice pushing molecules further apart than they are when in a liquid state.

The sea freezes at about -2C rather than 0C owing to its salt content. Frozen icebergs, on the other hand, have virtually no salt in them. They are “lumps of ice from the mainland glaciers that have formed from normal snow – that is, fresh water.

Icebergs made of salty water do exist, but they are rare.

Where are the worst levels of radon in the UK?

Radon is a radioactive gas produced naturally from granite rock which contains uranium. The international maximum safe dose of radon in the air is 200 becquerels per cubic metre, but exposure in some homes has been found to be well above 1,000 Bq. Some of the worst affected towns are Buxton in Derbyshire and Northampton and Kettering in Northamptonshire. In Devon, Cornwall and Northamptonshire, 24 per cent of the homes tested were above the danger level.

Why is there more sodium than potassium in the sea when rocks contain as much potassium as sodium?

River beds and ocean floors contain a lot of clay, which is a fine-textured sedimentary deposit that is very good at absorbing potassium. It has spaces within its structure that suit the size and shape of the potassium minerals. In contrast, sodium does not get absorbed by the clay, so more of it is left in solution in the sea and rivers. Rocks on the sea bed that are essentially basalt are weathered just the same as those on land. When weathered, they produce clays which again absorb potassium, making it more scarce in seawater than sodium.

How is the Earth’s magnetic field produced?

The current understanding is that the Earth’s magnetic field is produced by a complex system of electric currents circulating in the molten part of the Earth’s iron core – not all of which is molten.

“Because molten iron is a good electrical conductor, and the core is undergoing convective stirring (as its heat passes to the upper mantle), that generates currents.

Those currents produce a magnetic field just like a standard wire electromagnet, but the picture is complicated because the Earth’s field interacts with the currents themselves, thus changing the convection pattern.

Inside the core, then, the magnetic field is extremely complicated, but fortunately the net effect seen from the outside is not so complicated. Measured at the surface, the Earth’s field is rather like that from a slowly wobbling bar magnet. This is why the direction of magnetic north changes slowly with time.