Wednesday morning at 9:18am marked a special point in our orbit around the sun. That was when the overhead sun shifted from the northern hemisphere to the southern hemisphere, or in other words the September equinox.

In the southern hemisphere this is also referred to as the vernal (spring) equinox. For the northern hemisphere it is the autumnal equinox.   It is only at the equinox that the sun rises due east and sets due west.


In the above diagram Declination is used to describe the latitude where the sun appears to be directly overhead.  Positive declination is used for latitudes in the northern hemisphere, and negative declination for latitudes in the southern hemisphere.  This varies in a regular way throughout the year and can be taken as a marker of our seasons.

Astronomers, logically, time the seasons using the solstices and equinoxes of earth’s orbit around the sun.  By this reckoning, spring in the southern hemisphere starts with the vernal equinox.  Note that this makes our winter last 93 days and summer 89 days because the earth’s orbit is slightly elliptical and the earth speeds up a bit during our summer.

For practical purposes climatologists measure climate data averaged over each calendar month, and by this reckoning the months of September to November are taken as the southern hemisphere spring.  Note that by this reckoning summer lasts 90 days (91 days in a leap year).  In New Zealand this method of marking the seasons is more popular than using the equinox and solstice.  Fair enough, it adds 1 or 2 days to summer  :)

I have received an email asking why the sunrise and sunset times given at metservice.com (and in other media) don’t have night and day the same length on the day of the equinox.   Typically daytime seems to be around 7 minutes longer than twelve hours.   This is because sunrise is defined at the time the top limb of the sun is just visible on the horizon, and similarly sunset is defined when the top limb of the sun disappears below the horizon.    Due to refraction of the atmosphere, and the fact the sun is not just a point, the sun appears and disappears when its centre is slightly below the horizon.  This explains the “extra” daylight.    The term used to describe the date when time between sunrise and sunset crosses twelve hours is the equilux.

Over the next few weeks, as we get to notice the longer days (thanks in part to our switch to Daylight Saving this weekend) there will be extra warmth reaching the southern ocean.  This helps to activate the generally westerly quarter winds found there, and usually causes these winds to expand onto New Zealand producing fronts and squally winds.  This period of “equinoctial gales” normally reaches its peak in October and November.

Ridge-Top Winds

I started tramping as a teenager with the expectation of rain in the hills about two days out of three. So we were always prepared to change plans if confronted by a river in flood. Likewise, conditions on the tops could send us scurrying back below the bush line, as the wind over the ridge crests was sometimes strong enough to throw an adult carrying a heavy pack off their feet.

Once, after returning from a trip when this happened, I checked the wind measurements made by the weather balloon released nearby at the same time. The balloon flight found only 30 knots at the height of the ridge but on the ridge top itself the wind would have been around 60 knots.

This sort of enhancement of the wind strength is greatest when the wind blows perpendicular to the ridgeline – a northwest wind over many mountains in New Zealand. It is also helped by the presence of a temperature inversion in the atmosphere above the ridge.

An inversion occurs when the air temperature increases with height through a shallow layer of the atmosphere. Normally, the temperature decreases the higher you go – which is why there is snow on the tops of mountains. However, when an anticyclone develops, it creates a temperature inversion that lowers to a kilometre or so above sea-level. The colder air below the inversion is denser than the warmer air above and so buoyancy forces make it hard for the cold air to rise.

If an inversion lies just a short distance above the ridge then the air accelerates through the narrow gap between the ridge and the inversion – a bit like the way a thumb held over a trickling tap can squirt water across a room.

Frequently, these extreme winds over the ridge tops are accompanied by dense fog with visibility down to five metres or even less. Fog this thick is caused by the speed with which the air rises to reach the ridge top. As air rises, it experiences lower surrounding air pressure, which causes it to expand, which, in turn, causes its temperature to fall. The amount of water gas the air can contain depends on its temperature. Warmer air is able to hold more water gas than colder air.

Once the rising air cools to the point that it reaches 100% humidity, some of the water gas condenses to form the tiny liquid droplets that clouds are made of. The faster the air rises, the quicker the water droplets form and the denser the cloud or fog becomes.

As the anticyclone intensifies, the inversion usually lowers below the level of the ridge tops, the wind speed drops and the fog evaporates. Then the ridge lines are ok for tramping, although strong gales may still continue around the ends of the mountain chains, in places like Cook Strait or south of Fiordland.

Wind accelerates between ridge top and inversion
Wind accelerates between ridge top and inversion

Late frosts

Earlier in the month many parts of New Zealand had frosts. Since we are now into the beginning of spring, it got me thinking about the impact that late season frosts can have on the delicate buds sprouting on trees and vines around the country.

September blossom on a nectarine tree
September blossom on a nectarine tree

Let’s clarify what a frost actually is. A frost occurs when the temperature falls to below 0°C, and it becomes visible when there is moisture to make ice crystals. Of course, your freezer compartment is in a state of permanent frost (I remember as a boy helping my parents by “defrosting” the freezer!).

An air frost is when the air temperature a metre or two above the ground falls below zero. A ground frost is when the air touching the ground cools below zero. A ground frost is more common than an air frost because the strongest cooling at night is usually at the ground surface.

The recent frosts were, as usual, when we were under the influence of a big anticyclone with generally clear skies and light winds. These factors are necessary for frost in NZ, as well as time of year. Let’s take a closer look at these factors:

The Effect of Cloud

If it’s cloudy at night then the air near the ground won’t cool enough for a frost to form (although the ground may still get damp with dew). This is because the cloud radiates heat down towards the ground, acting a bit like a blanket over the land. Low clouds like stratocumulus or stratus are very effective at keeping the ground  temperature up at night – you may have noticed this effect when you’ve been outside on a cloudy winter night.

Of course the opposite happens in the daytime – if the cloud hangs around through the morning and into the afternoon, it screens off some of the incoming heat from the sun, and the maximum day-time temperature won’t get very high.

The low cloud types mentioned above are common in anticylonic weather, especially in areas affected by moist flows coming in from the sea.

The Effect of Wind

The animation below shows what happens when the wind is light enough for frost to occur.

Starting at 2pm, we’ll let the clock go forward: When it gets dark on a clear night the ground cools and so the air just above doesn’t mix very easily with the air higher up. Near the ground the wind becomes gentle, and the cooling is concentrated in the air that surrounds the tender buds. By the way, in the graph the lines of constant temperature slope towards the top left – I am happy to explain this graphical technique further if you’re interested.

If the wind aloft is strong, it forces its way down to the ground, lifting the cold air and mixing it up through a deep layer. The night-time cooling gets spread through such a depth of air that the temperature near the ground doesn’t fall enough for a frost to occur. These strong winds can happen whenever the isobars are closely packed – e.g., when a deep depression is nearby.

As noted in the blog post on year 12 maths, the wind is always light near the centre of an anticyclone. So in anticyclones there isn’t much forcing from the wind to disrupt the night-time cooling.

The Season

In summer we don’t get frosts (apart from up in the mountains). This is because of the accumulated heating from the sun: the days are long and the sun heats the ground powerfully during the day. The heat is then stored in the sub-soil and in the air above. Extra moisture in the form of  water vapour in the air over summer also slows down the night-time cooling.

On clear calm summer nights the temperature falls but, before it gets anywhere near zero, the dawn of a new day begins. So any tender young plants are safe from the cold during the summer months.

In addition to the factors above, there’s the type of terrain to consider. For example, different soil types will cool at different rates. And a sloping surface won’t cool as much as a horizontal one  – these could be fertile topics for a future blog post :-).

As we get further into spring the likelihood of frosts decreases. But watch out for a southerly that pushes pre-chilled air onto the land, then peters out as an anticyclone moves on. If the skies clear too, we will be susceptible to a late season frost.

Waikato Stadium Weather

After a week of sunny weather, it appears that rain will dampen Waikato Stadium before this weekend’s Tri Nation rugby game starts there at 7:35pm on Saturday.

Waikato Stadium
Waikato Stadium

This clash between the Springboks and the All Blacks is the first Tri Nations game to be held at Waikato Stadium (capacity 25,800).

If the All Blacks win this game and score four tries and the bonus point, there is still a chance of their winning this year’s Tri Nations.  The game is likely to be played in wet conditions, with perhaps 20mm of rain falling in Hamilton on Friday and Saturday.  This equates to something like 20 litres per square metre  of water  – that’s around 10 tonnes falling on the half-hectare of  playing surface inside Waikato Stadium.

20mm of rain will be worth a bonus point as far as Waikato farmers are concerned. After a week of dry weather, that’s about the right amount of water to keep the soil moisture levels up and running, producing optimum pasture growth.

So if the All Blacks can score two points for every millimetre of rain delivered to Hamilton by this front, we will all be smiling.

Will it be raining during the game? Quite possibly. But with a light northerly and an air temperature of around 12 degrees, it won’t be too cold. Scarves are optional.

When it comes to an important event such as this, you can check out TWO of the weather models we use  here (3-day) and here (7-day). These models have different calculation schemes and treat the physics of the atmosphere diifferently, which is why their predictions don’t always agree. Our skilled meteorologists take these and other data into account to produce a forecast that is the most likely one to replicate the real world.

For Hamilton go here, or for your place go to www.metservice.com and click on your place-name.


Our weather in New Zealand is greatly modified by the shape of the land. There are many parts of the country where the air is channeled through gaps in the terrain, and I thought I would write a little about this. Especially since it relates to the thread of my earlier posts on wind.

I was at a school activity last week with one of my daughters and, when it was time to leave, a couple of hundred people had to exit through a single door. It was a classic bottle-neck effect: we started slowly towards the door, accelerating as we got closer and closer, then quickly went through followed by an equally rapid deceleration outside the building. You’ve probably noticed this yourself when leaving a venue with a lot of other people at the same time, e.g. exiting a movie theatre or sports event.

Here’s a graphic that illustrates the effect – perhaps you can think of other everyday situations where this process occurs.

The effect of a bottle-neck
Objects speeding up and slowing down as they move through a narrow passage or "bottle-neck"

As I indicated above, there is a meteorological application of this effect when air is forced through a narrow gap between individual hills or, on a grander scale, through passages between mountain ranges. Examples are between North and South Islands, between South Island and Stewart Island, and through the Manawatu Gorge (although this is a complex example due to the Puketoi range downstream). Another perhaps less obvious example is the channeling that occurs between Banks Peninsula and the eastern foothills of the Southern Alps – this is one of the reasons that Christchurch is predisposed to its nor’easter wind.

By the way, in the animation above, I have intentionally drawn flattened objects to represent the moving air. This is to recognise that the troposphere (the part of our atmosphere in which our weather occurs) is very thin compared to the size of the Earth.

Have a look at the two images below, based on satellite-derived elevation data from the NOAA National Geophysical Data Centre. In the first image we see the familiar outline of New Zealand…

NZ topographic map
NZ topographic map

…but in the second image I have blacked out the low-lands to help highlight the high ground responsible for modifying the flow.

NZ topography with low-lands blacked out
NZ topography with low-lands blacked out


Comparing these two images we can appreciate where the terrain is likely to channel and hence speed up the wind. The areas I gave as examples above are all gaps (of various sizes) in the high ground.

As an interesting variation, the bottle-neck effect also occurs to some extent when the flow is around the side of a single hill or range. For example, the wind will speed up around East Cape (top end of Gisborne) and Puysegur Point (bottom end of Fiordland) in northwesterly and southeasterly flows.

There is a complication to all this. We have to remember, as I mentioned in my previous post about the Great Northwesterly Storm of August 1975, that the wind blows in three dimensions, so air may also rise and fall as it goes through the gaps. And, as in my post about the Wind-sock of the Lower North Island, the wind will not only be stronger in the bottle-necks, but also generally over the high ground.

I think the maps also show just how “bumpy” the NZ topography is, which partly explains why we get such a variety of weather conditions in this country. I hope you’ve found these ideas thought-provoking – I don’t seem to be running out of things to discuss about wind!