When does spring actually start? And what can we expect this season?

Does spring begin on the first day of September? Or does it start on September 23 - the perfect middle point between the winter and summer equinox?

Principal scientist at the National Institute of Water and Atmospheric Research (NIWA) Chris Brandolino is American, and sees spring as starting on 23 September.

"We kind of follow the astronomical definition of the seasons, which is marked by the sun crossings of [an] imaginary line on the Earth, whereas other parts of the world, including New Zealand ... they go by the meteorological or climatological definition of the season, so there's two sets of definitions."

Why do we see it differently here? Well, following meteorological dates and dividing seasons into three-month chunks keeps things tidy and is helpful we want to look at the average temperature and rainfall during the season, Brandolino told Nights.

The so-called spring equinox in the Northern Hemisphere goes right down to the minute, with this year's one starting September 23, 2024 at 12.43am.

The seasons in this definition are determined by the sun crossing an imaginary line across the equator at 23.5 degrees, either north or south, twice a year - giving us the shoulder seasons known as autumn and spring.

Why do we care so much about seasons?

Seasons are a human construct that give structure to our way of life and have historically been useful for determining things like harvesting.

But the seasons in terms of their practical sense are getting longer and shorter, largely because of climate change, Brandolino said.

"The winter season, that's getting shorter. We're seeing frosts happen later, the first frost, and we're seeing the latest frost happen sooner. So what happens is that it's kind of shrinking on both ends and then this is a long-term trend.

"Whatever definition you subscribe to, astronomical or meteorological ... we are seeing that the types of weather we are experiencing during those seasons evolve with time."

Still, that doesn't mean you wouldn't expect the coldest temperatures to be around the same time of the year in the future, he said.

"If we hop on a time machine and we exit the time machine 70 years from now, we will still have the coldest temperatures, if you were to kind of graph when the warmest weather occurs versus the coolest weather, it'll be in the winter season, it just won't be as cold."

So what can you expect this spring?

This outlook for spring was the 'theme' for the season, not a forecast, Brandolino advised.

"Weather is your mood, climate is your personality. So what I'm describing here is Mother Nature, is basically her personality for the next three months. Now, on any given day during the next three months, her mood may be incongruent with her personality."

NIWA is confident about a warmer than average spring for eastern parts of the North and South islands. Other parts of the country could experience a warm lean too, but NIWA's confidence on that is lower.

"So we're thinking either average or above average, and that's about equal chances. But to put differently, a colder than average spring is pretty darn unlikely over the next three months," Brandolino said.

"So days where we get this unusual warmth, maybe more common than what say typically spring offers, which is a moody season. It's the teenager of seasons because you have these wild swings between winter which we've just exited and summer which we are approaching."

The eastern side of both islands are also leaning towards a dry lean, but there is some uncertainty there so it's unclear if rainfall will be average or below average at this point.

The west and top of the South Island and the west of the North Island was leaning towards being more wet, with normal or above normal rainfall. And the top of the North Island could see near normal rainfall for the seasons.