Only a few parts of the world experience the classic four seasons of spring, summer, autumn and winter. Many parts of the world get only two or even one. So, what's going on?

Every day, the Earth spins once on its axis.

But our planet isn't perfectly upright when it spins. Thanks to a few collisions during its formation, the Earth is tilted at an angle of 23.5 degrees.

This means that as the Earth takes it annual trip around the Sun, different areas of the planet face the Sun more directly during their daylight hours at different times of the year.

The tilt also affects the daily amount of light — without it the whole planet would have 12-hour days and nights every day of the year.

Summer and winter

Australia has summer at the end of the year when the southern hemisphere is tilted towards the Sun.

In summer, days are longer because more hours are spent facing the Sun. And they're hotter because we're facing the Sun more head-on — so we get hit by more rays of sunlight than if we were on an angle.

The summer solstice in Australia — about December 22 — is when we have our longest day of the year. On this day the Sun is as far south in the sky as it gets — it passes directly above the Tropic of Capricorn, roughly over Rockhampton.

The seasons are a function of the Earth's tilt. ( ABC: Julie Ramsden )

But while we're busy planning Christmas barbecues, the northern hemisphere is tilted away from the Sun. That means there are fewer daylight hours up there and the light is spread out over a greater surface area, so it doesn't get as warm. Their shortest day — the winter solstice — happens on our longest.

The tables turn six months later, when the Earth is halfway around its orbit of the Sun. The northern hemisphere's summer solstice (longest day) matches our winter solstice around June 22, when the Sun is as far north as it goes — above the Tropic of Cancer.

Spring and autumn

In spring and autumn the planet isn't tilted towards or away from the Sun — it's roughly side-on.

And for two days each year the Earth's tilt is exactly side-on to the Sun. The two days are called equinoxes (equal nights), and they fall in the middle of spring and autumn, usually on September 22 and March 22.

On an equinox, night and day are equal length everywhere on the planet.

But spring and autumn only happen in mid-latitude areas of our planet. It's a different story in the tropics and at the frozen ends of the planet.

Tropics and poles

Some parts of the polar regions are so consistently cold — and the tropics so hot — they could pass for having only one season.

Even the sunniest Antarctic day is as cold as winter in most places. This is because the light reaching the bottom of the planet is at such a low angle it doesn't carry much heat.

On the other hand, the tropics are consistently hot. It doesn't matter if they're tilted towards or away from the Sun, they're still closer to it than anywhere else on Earth and they get plenty of direct light and heat.

But both places have two distinct seasons.

Only the mid-latitudes experience four seasons. ( ABC: Julie Ramsden )

In the polar regions, the main difference comes down to the amount of daylight. During 'summer', the whole area is tilted towards the Sun and flooded with sunlight. Daytime at the poles lasts for half the year.

And the polar night lasts almost as long — making for one very long, dark winter.

In the tropics, the difference between seasons is due to rainfall.

The wet is caused by a permanent belt of storm clouds around the middle of the planet that dumps huge volumes of rain on the land or sea below. Thanks to the tilt of the planet and some super-sized sea breezes, the storm belt doesn't stay in one place. During the northern summer, the hot air over the land rises, sucking the storm belt as far north as the Tropic of Cancer, doling out monsoons wherever it goes.

As the northern summer ends the storms are dragged down towards the Tropic of Capricorn, driving the southern tour of the monsoons.

The belt travels across the equator twice a year, once going south and once on the way back up. If they've got the right combination of mountains, wind and sea temperature, some equatorial areas — such as Kuala Lumpur — can score two wet seasons each year.

Fortunately, the Top End is far enough from the equator to just have the one wet season — imagine how crazy Darwin would get with two build-ups each year…



Thanks to Dr Blair Trevin from the Bureau of Meteorology.