Sydney has been pelted with hailstones the size of tennis balls and golf balls during a fierce summer storm, but some pictures shared on social media showed hailstones resembling a cauliflower.

To explain how hail takes its shape and colour, it's important to look at its structure and how it was formed.

Is it wet growth or dry growth?

Clear hailstones are often very spiky and are formed during wet growth, and the whiter, more opaque hailstones are often more round and are formed during dry growth.

Wet growth structures appear more spiky with icicles, whereas dry structures appears opaque and round. ( Twitter: Tungsten_W )

Monash University researcher Dr Joshua Soderholm said all hail starts as round embryos, about 1 centimetre in diameter.

"As it starts to get bigger, you start to get icicles forming in every direction … that's the wet-growth phase," Dr Soderholm said.

As the hail forms during wet growth, its "lobes" are separated by porous ice, with little spaces that become filled with liquid water.

When this water freezes, it forms radial channels of almost clear ice — like icicles.

"When [hailstones] form during dry growth, the water starts filling in gaps and that's when you start getting the round, very white type," Dr Soderholm said.

So what growth is this?

Here we can see the anatomy of how this hail stone formed. ( Twitter: @Gemma_Stiles )

If we look at the cauliflower-shaped hailstones that battered Sydney on Thursday afternoon, like the one above, we can see the embryo formed as dry growth — that's the white ball in the middle.

Then it went through some wet growth where small icicles formed, and then those icicles were then filled up by lots of dry white growth.

Et voila! Cauliflower-shaped hailstones, although they are scientifically referred to as cusped lobe structure forms.

But as Dr Soderholm observed, the hailstone in this picture appears to have melted as it fell, giving us the full view of the embryo.

Sometimes hail can melt as it falls because of its shape — if they are round or jagged, they tumble; if they are a wide disc shape, they fall without tumbling.

"There's also a chance this particular stone shattered while falling, colliding with other stones, or when it hit the ground," Dr Soderholm said.

"Five-centimetre-diameter hail falls at about 115 kilometres per hour, so it can take quite a hit [and] 8-centimetre hail falls at about 175 kilometres per hour."

Cauliflower floret, anyone? ( Twitter: @Gemma_Stiles )

Dr Soderhold said the change between wet and dry growth will depend on the change in temperature and moisture in a storm cell typical for this time of year.

"A supercell is a severe thunderstorm where the updraught is rotating," Dr Soderholm explained.

"This rotation allows the thunderstorm to become much more organised and well-run, and they become more intense and live a lot longer."