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What is La Niña: The weather phenomenon about to hit Australia

After a rainy summer last year, the weather phenomenon which caused it is back for a second drenching.

A La Niña has formed in the Pacific Ocean, the Bureau of Meteorology has confirmed.

But what is a La Niña, and what does it mean for our summer?

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The 2010-12 La Nina cycle was powerful and devastating.

What is La Niña?

La Niña, which is Spanish for The Girl, is a complex weather pattern that pushes warm water towards the western side of the Pacific, including Australia and Asia.

As a consequence of the warmer water, more evaporation means more rainfall over Australia.

It also leads to cyclones being formed with more intensity.

It is the counterpart to El Niño, Spanish for The Boy, a weather pattern which leads to warmer weather and drought conditions.

Because the La Niña and El Niño system covers the entire Pacific, which accounts for close to half the Earth's surface, a La Niña year will likely reduce global temperatures on average, for a few months at least.

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The powerful La Nina fuelled the powerful Cyclone Yasi which devastated north Queensland.

What does a La Niña mean for Australia?

Because of La Niña, the eastern half of Australia can be expected to have a cooler and wetter summer than usual.

It is the second year in a row that a La Niña has hit Australia.

The bureau's Head of Operational Climate Services Dr Andrew Watkins said back-to-back La Niñas were common.

"La Niña also increases the chance of cooler than average daytime temperatures for large parts of Australia and can increase the number of tropical cyclones that form," Dr Watkins said.

"La Niña is also associated with earlier first rains of the northern wet season, as we've observed across much of tropical Australia this year."

READ MORE: La Niña officially declared over after wet and wild summer

The last substantial La Niña to hit Australia was in 2010 to 2012.

That prolonged weather event brought with in intense floods in Victoria, NSW and Queensland.

It also created Cyclone Yasi, which cost an estimated $3.5 billion in damage in northern Queensland.

But the bureau forecasts that this year's La Niña will be much weaker than the 2010-2012 cycle, and perhaps weaker than this past year.

Is La Niña good or bad?

It depends on how strong it is, and your situation in life.

If you are spending your summer at the beach or by the barbecue, rainier conditions are of course less than ideal.

Though a cooler summer may be welcome compared to the scorching heat of El Niño conditions.

La Niña is also associated with fewer extremes than El Niño. A mild La Niña means it's unlikely many weather records will be broken this summer.

The La Niña cycle also reduces the risk of a bad bushfire season.

And a mild La Niña is likely good news for farmers still recovering after a terrible drought.

"Every La Niña has different impacts, as it is not the only climate driver to affect Australia at any one time," Dr Watkins said.

A mild La Niña probably means just a few more rainy days over summer than usual, and a few less scorchers.

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How long will La Niña last?

This La Niña is forecast to last until at least the end of January next year.

Consequently, the sun should be out by the time the kids need to be back at school.