What makes a sea wave break, and why are clouds white?

We explore some of the curious questions that science can answer

In shallow water, the water on top of a wave travels faster than the water underneath (Getty)

What makes a sea wave “break”?

In a sense, it trips over its own feet. Waves move more slowly in shallow water than in deep water. This makes the bottom of a large wave travel more slowly than the top. Eventually the crest of the wave gets so far ahead of the water that is supporting it below that it falls over – so the wave breaks.

Why do we get more sunlight in the summer than in the winter?

Because the Earth is tilted, the northern hemisphere points more towards the sun in the summer than in the winter. This means the sun rises higher in the sky in summer, so our days are longer.

What causes earthquakes?

The Earth’s crust is made up of 15 pieces, or “plates”. Currently, we think the plates float on the molten rock underneath and can move around. Earthquakes happen where the plates join – the fault lines – as the plates move and push against each other.

Why is the outer skin of a cactus so fat?

A cactus is basically a big fat stem covered by a thick waxy cuticle. Because the cactus is naturally found in a dry habitat, conservation of water is vital. It has no leaves because they would lose water too easily: the fact that leaves are thin and have a large surface area means evaporation takes place rapidly. The cactus uses its stem to perform the function of the leaves, that is, to absorb sunlight and photosynthesise, making the plant’s food. The thickness of the stem reduces water loss. The waxy cuticle that gives the plant its shiny appearance also prevents water loss.

What is the frequency of a rumble of thunder?

Every thunder rumble has a different frequency. The noise we recognise as thunder is created when lightning superheats the air it travels through. When air is heated it expands and, in a lightning flash, the expansion is so fast that a shock wave is created, causing the sound wave we hear as thunder. Essentially, the frequency of the sound wave depends on just how fast the air expands, which in turn depends on the stability of the air, and other complicated, thermodynamic factors.

But each roll of thunder sounds slightly different. You may have noticed that at the start of a roll of thunder you will hear a sharp clap and then the rest of the thunder rumble continuing after it. The sharp clap has a much higher frequency than the rumble.

The droplets in storm clouds are larger, and absorb and scatter more light, so storm clouds look darker

The reason we hear different types of thunder – the long roll or the short crack – is because of the path the lightning takes.

Imagine a flash that starts a kilometre above our heads and travels diagonally to hit the Earth a kilometre from our feet. Every point on that path is (very roughly) the same distance from us, so the sound from every point of the flash hits us at about the same time. This gives a loud noise like a “crack”. Now, imagine a flash that starts about a kilometre up but ends close to our feet. The sound from the start of the flash’s path has to travel much further than the sound from the end of it, and will, as a result, reach us much later. This creates the long, slow roll of thunder at a lower frequency.

Clouds are made of tiny water droplets. So why are they white and opaque?

Clouds look white (or grey) because of the way light bounces around inside the water droplets. If light travelled straight through the cloud, it would be clear. These water droplets are the sort that form rainbows. Although the sky looks reasonably clear where a rainbow forms, there are in fact enough water drops to refract the light and create a rainbow.

However, in clouds the light is refracted and reflected in the droplets, making the ray bounce around inside the cloud before leaving. There is no coherency in the beam, so the white colour of the light is all that is left; it is rather like looking at a white wall as opposed to a mirror.

The droplets in storm clouds are larger, and absorb and scatter more light, so storm clouds look darker. The thicker a cloud is, the darker it appears, as it is more difficult for the sunlight to pass through.