Explainer: Why don't we get more daylight faster after the winter solstice?

Explainer: We've passed the winter solstice, the shortest day of the year. It's the 1st of July and it should be getting lighter by now, right?

Right?

Unfortunately, it doesn't quite work that way - as MetService meteorologist Katie Lyons explains, despite the solstice on 21 June marking the shortest day, it takes a while for the added hours of daylight to speed up a bit.

"Even though the shortest day of the year has passed, it feels like it takes forever for the days to get any longer. And it does," she said.

In the initial days after the solstice, we may only get a few extra seconds of daylight per day until it starts to ramp up, and that typically comes in the evenings.

Why does daylight come back slowly but start to accelerate as winter goes on? It's all down to the equinox, and where the Earth is in relation to the sun.

What's an equinox again?

Some may think seasons change because we're moving closer or further from the Sun - but actually, it's how Earth leans toward the sun that determines seasons. The Earth is tilted 23.5 degrees on its axis, which is the reason for the seasons in the first place.

As the year rolls by, the axis tilt of the Earth rotates around the sun, until we reach what's called the equinox twice a year during spring and autumn.

It's the time when part of the Earth is tilted neither toward or away from the sun, giving nearly equal periods of daylight and night.

This illustration shows the basic mechanics of it:

And what is the solstice?

The world solstice actually means "sun stands still," basically - it's derived from the latin sol for sun and sistere, to stand still.

At the winter solstice, the Southern Hemisphere is tilted the furthest away from the sun, meaning less daylight, which is why we often talk about it being the "shortest day."

The further south you are, the darker it gets, which is why Antarctica stumbles through months of darkness in winter.

Earth Sciences New Zealand recorded the shortest daylight hours on this year's 21 June solstice in Invercargill with a grim 8 hours, 35 minutes of daylight, while Kerikeri in Northland by comparison basked in 9 hours, 46 minutes of sunshine.

Think of the solstice as a turning point, but not a radical shift. We'll really start seeing more daylight the closer we get to the Spring Equinox, which is on 23 September.

So why is daylight so slow to increase?

Lyons explains it as following a curve where things slow down at the centre and endpoints but start to ramp up as the curve rises.

"The daylight hours follow a sinusoidal type curve (think back to maths at school), meaning around the peaks and troughs (longest and shortest days) the changes are smallest, but around the middle, or the inflection point, (equinoxes) the changes are biggest."

In an article for The Conversation, University of Melbourne researcher Tanya Hill noted that sunrise may actually be slightly later for a short time after the winter solstice.

"Our days are still getting longer, just the extra daylight is added to our afternoons, not our mornings," she wrote.

For instance in Wellington, there were only an extra three minutes of daylight from 21 June to 30 June.

But from 21 June until 7 August (the halfway point until equinox), there will be a 58-minute growth in daylight, while after the halfway point, it really speeds up, and between 7 August to 23 September we'll get a whopping two hours more of daylight.

MetService's handy graphic lays out exactly how that curve works:

And then Daylight Savings Time kicks in and changes the clocks forward by an hour just to make it even more disorienting. It will start at 2am on Sunday 27 September.

Why 'winter' can have more than one meaning

We think of "winter" as one monolithic entity, but there's several ways to define it. The main two are meteorological winter, which begins 1 June and runs to 31 August, and astronomical winter, which begins on that solstice when Earth's axis is tilted furthest from the sun and will end on the equinox in late September.

The two don't line up exactly due to a concept called "seasonal lag," Lyons said.

"Even though the days are slowly getting longer at the moment, the Earth is still losing more heat overnight than it is gaining during the day and therefore continues to cool for a while. The same concept applies for summer but in reverse."

At MetService, of course, they're looking first and foremost at the weather, not the stars.

"Astronomical winter is really all centred on the sun's position and therefore daylight hours. Whereas, meteorological winter is centred on temperatures, the coldest vs hottest periods of the year."

"When we talk about 'winter' we are almost always referring to meteorological winter," Lyons said.

It's "just a reminder to ourselves every year around this time that even after the winter solstice we've still got most of the cold winter temperatures to go, and the recovery of daylight is slow going," she said.

Look, I'm cold and it's too dark in the morning, what can we do about that?

Not a lot, to be honest.

"The slow increase in daylight hours after the solstice can be tough, especially since on average the coldest temperatures are felt well after the shortest day of the year," Lyons said.

"So sometimes when we hear the shortest day has passed, we think the worst of winter is over, but that's not necessarily (and usually isn't) the case. Still got lots of winter to come!"