Activity for World Meteorological Day 2014: Make Your Own Observations

On 23 March each year, national weather services around the world celebrate World Meteorological Day to commemorate the establishment of the World Meteorological Organization on this day in 1950.

In 2014, the theme for World Meteorological Day is ‘Weather and Climate: Engaging Youth’. We thought this was an ideal opportunity to provide a few do-it-yourself weather projects for the keen weather kids out there.

Why do we need weather observations?

Before weather forecasters can tell us what the weather is going to be doing tomorrow, they need to know what it is doing right now.

Every hour hundreds of weather stations across New Zealand, from Cape Reinga to Stewart Island, send in reports of how the weather is where they are. As well as telling us how windy it is, how warm it is and how much rain has fallen, the weather stations can also tell us where it is raining or snowing, how much cloud there is, and the pressure of the air.

These weather reports are really important to weather forecasters who use them along with information from radars and satellites to find out what the weather is doing across the country. These observations are also shared around the world to help build a global picture of the current conditions.

Not all of us have an official weather station in our back garden or can afford to build our very own weather radar – but that doesn’t mean we can’t take our own measurements using things we find around the house.

The weather station in Wellington’s Botanical gardens

This is the weather station in Wellington’s Botanical gardens. A is the MetService headquarters; B is a receiver on the roof which lets us collect information from satellites as they pass overhead; C is an anemometer which is used to measure wind speeds; D is a device that measures sunshine; E is the Stevenson Screen used to keep our thermometers in the shade; and F is a collection of different rain gauges.


Although we cannot see wind we can see the effects that it has on other things like trees , flags and smoke. The Beaufort scale compares wind speeds with the effects that they have over land and sea. For example, if we notice that only the leaves of a tree are moving in the wind we would describe it as a “light breeze”, but if we noticed that the whole tree was shaking we would call it a “gale”.

You can find out more about the Beaufort scale and download your own copy here:


Measuring how much rain has fallen needs a rain gauge. We can make a simple rain gauge by using a straight sided bottle, some stones and a ruler:

How to make a rain gauge - step 1How to make a rain gauge_2How to make a rain gauge - step 3

The rain gauges that MetService use are called tipping bucket rain gauges. As rain falls into the gauge it fills a little bucket at the base of a tube, then once the bucket is full it tips over and empties out the rain it has collected. A counter keeps track of how many times the bucket tips over and as we know how much rain can fit in the bucket we can work out how much rain has fallen. Because the rain gauge empties itself it never overflows.


The temperature of the air can very a lot over a short distance. To measure the temperature of the air, meteorologists [scientists who study the atmosphere] use thermometers. You might already have a thermometer in your house which will let you measure the temperature.

The thermometers that are used for the official observations are kept in special boxes called Stevenson screens. These white boxes keep the thermometers shaded while the gaps on either side of the box let the air move freely over the thermometer.

Because we share this information around the world, it is important that we are measuring the temperature in the same way as the meteorologists everywhere else in the world. This is also why we have a special calibration lab, to ensure all our measuring equipment performs exactly the same way as the equipment used by other national weather services.


There are lots of different types of weather, so as well as recording things like if it is raining or snowing, observations such as if you can hear a thunderstorm or it is foggy are useful things to note down. Why not keep track of the different types of weather you can observe over a week? You can write them down or draw a picture to show the weather. These are the symbols we use on our website to show the different types of weather – can you name them all?

Weather icons used on


As well as different types of weather there are different types of cloud. Some are high in the atmosphere and made of ice others are low down close to the ground made of large water droplets giving the underside of the cloud a menacing grey colour.  We have some great clouds in New Zealand, and this poster gives some great examples of the ones you might see:

While you are looking at the different clouds outside, try to figure out how much of the sky is covered in cloud. Is it completely covered? Perhaps it is only half the sky that has cloud covering it or perhaps you can’t see any clouds at all.

Weather observers measure the cloud cover in Oktas, or eights. A sky that is completely covered in cloud is called overcast and has 8 oktas of cloud.

How much of the sky do you think is covered in these pictures? Imagine squashing the cloud together and counting the grey coloured squares.

Measuring clouds in oktas

How much cloud do you think there is in the picture of the weather station at the top of the page?

Weather codes

As you can imagine, sending all the weather information back to the weather office and sharing it takes a lot of computer power. To make it easier, all the information is transmitted as a series of codes.

Here’s an example of an observation that gets send from a weather station:
93110 17698 /2311 10206 20137 40238 57006 60004 70300=

At first it looks like a random string of numbers, but each group gives information about different aspects of the weather. The first block gives the station number for Auckland so we know where the observation comes from. The other blocks give information about wind speed, temperature rainfall and weather. From this observation we know that there is 11 knots of wind coming from the southwest; the temperature is 20.6C; the surface pressure is 1023.8hPa; there hasn’t been any rainfall in 24 hours; and there is some cloud developing.

You can find more about decoding the observations here:

We hope you enjoy doing some or all of these weather projects!

Unusual Wind Direction

Last Monday evening, having just arrived home after the short walk from Trentham Station, I remarked to my family that it was unusual for the wind to blow from the northeast in Upper Hutt. Five days later and it’s still blowing from there.

For over a week now, mean sea level pressure has been higher than usual south and east of New Zealand while pressures have been lower than usual over the Tasman Sea (to the northwest). This anomalous pattern is not confined to the surface – it extends up through the troposphere and further. This is known as a blocked pattern, where highs and lows become slow moving or stationary. Such slow moving frontal systems can result in heavy and sometimes intense rainfall, and we have seen some of that in the Severe Weather Warnings affecting south Canterbury, Waikato, Coromandel and Gisborne this week.

Animation of MetService MSL analyis charts at 6 hour intervals over the Tasman Sea and New Zealand area, 28 July to 3 August 2012.

The animation here shows the high pressure to the southeast as either a ridge or an anticyclone. The depression over the Tasman Sea has gone through the complete life cycle from early stages of contrasting air masses on 28 and 29 July, through development and deepening on 30 and 31 July, to mature and filling in on 1 and 2 August. Today (3 August) there is a fairly uniform air mass of rotating belts of convection.

Path of the centre of the low (the point of lowest pressure) starting from midday 28 July (top left of map) and ending 6am 3 August 2012.

This simple map shows the meandering path of the centre of the depression during this time. The track starts near the north boundary of the map and ends towards the western boundary.

The satellite image below shows the old low centre this afternoon (3 August) with bands of moderate convection spiralling out.

Visible satellite image at 3pm 3 August 2012 NZST from the MtSat-2 meteorological satellite, courtesy of Japan Meteorological Agency.

All this time, the wind flow over central and southern New Zealand has been from the northeast. For Wellington this has resulted in an unusually long period of wind from between east and northeast. Winds of this direction are fairly rare as shown in the diagram below, called a wind rose.

This wind rose was constructed from hourly wind observations. The length of the coloured bars is proportional to the hours of wind from that direction, where 360 is North. The colours further partition the wind into speed ranges. The longest bar represents 12.1%, and the range circle at half way represents 6% (of the time).

Diagrams such as these are called wind roses because, for most stations, the wind blows from many directions and the shape of the resulting diagram looks a bit like a flower. The wind rose here is for day-time in winter at Kelburn (central Wellington) and shows that the wind is mostly from the northwest to north or from between southeast and south. It very seldom blows from the east or west.

The graphic below traces the hourly wind speed and direction between midday 30 July and midday today 3 August 2012. The start point is at 030 13 km/h and the end point is at 080 22 km/h. Can you find the start and end points? :-)

Trace of wind speed vs wind direction of hourly wind recorded at Kelburn, Wellington, between midday 30 July and midday 3 August 2012.

Anyway, you can see that a lot of the time during these last four days, the wind at Kelburn (and most of Wellington) was a moderate easterly. And you don’t often get that for such a long period.


The Bomb

It’s been a while since a rapidly-deepening low passed close to, or over, New Zealand. I thought it might be interesting to take a quick look at why the “bomb” low of Saturday 03 March 2012 deepened so quickly and why the winds around it affected the areas they did.

First of all, here is a series of weather maps covering the period 1pm Friday 02 March to 1am Sunday 04 March.

Mean sea level analyses from 1pm Friday 02 March 2012 to 1am Sunday 03 March 2012, at six-hourly intervals.

A Bit of Background

Large-scale features on the weather map – that is, those systems which influence the day-to-day weather (highs, lows, fronts) – are driven by processes in the middle and upper parts of the troposphere. Lows become deep and anticyclones become intense when there is strong positive feedback between these processes.

In the case of lows, when the intensification (a change in central pressure) exceeds more than a certain amount over a given time, they are defined as “bombs”. Actually, it’s not quite that simple: if you really want to know more, see “Technical Stuff” at the end of this article.

Between 1am Friday 02 March and 1am Saturday 03 March, the central pressure of this particular low fell from 1002 hPa to 975 hPa: it qualifies.

The term “weather bomb” has come into popular usage in New Zealand to describe dramatic and/or destructive weather events – but very seldom is a “bomb” low the cause. “Bomb” lows aren’t all that common in the New Zealand area.

The Low of Saturday 03 March

Below is the weather map for 1am Saturday 03 March, superimposed on a satellite image. At this stage the low was still west of Taranaki, deepening rapidly and heading more or less straight for Palmerston North (but it never got there). I’ve drawn in some mauve arrowed lines to indicate the axis of strongest winds in the upper troposphere, known as the jet. The relative locations of the low and the jet, and the shape of the jet, strongly favour further deepening of the low and its movement towards the eastsoutheast – which is what happened. Note: this is a simplified explanation ahead of a bit more case work.

Mean sea level analysis 1am Saturday 03 March 2012, together with infra-red satellite image. Satellite image courtesy Japan Meteorological Agency.

A few hours later, at 6am Saturday 03 March, the low had a central pressure of about 971hPa and was not far south of Patea. Below is a portion of the working chart for 6am, drawn by one of MetService’s Severe Weather team. There’s a lot of isobars around the low – and over Taranaki, Wellington and the Marlborough Sounds in particular. The closer together the isobars, the stronger the winds. Also note that there’s a front drawn curling around the low. Tucked in on the southern and western side of this front is a zone of very strong winds, shown by the blue arrow. It’s mostly this zone which did the damage as it moved across the southern part of the North Island.

Hand-drawn mean sea level analysis for 6am Saturday 03 March 2012. This is an internal working chart, drawn by a member of MetService's Severe Weather team. Image copyright Meteorological Service of New Zealand Limited 2012.

Some Interesting Observations

At Hawera, the wind increased quickly during the early morning hours of Saturday 03 March.

Time Wind direction Mean speed (km/h) Highest gust last hour (km/h)
Midnight Fri-02-Mar Northnortheast 35 57
1am Sat-03-Mar North 26 44
2am Sat-03-Mar Northnortheast 24 43
3am Sat-03-Mar Northnortheast 53 83
4am Sat-03-Mar Northnorthwest 53 87
5am Sat-03-Mar Northwest 85 122

No observations were received between 6am and 11am because power to the reporting site was lost.

At Wanganui Airport, the temperature climbed steadily from 12.7 C at midnight Friday 02 March to 17.5 C at 5am Saturday 03 March. (The temperature behaved similarly at Hawera a few hours earlier). This happened because the relatively warm moist air flowing around the northern side of the approaching low was warmed and dried – the Foehn effect - as it passed across the high country to the north of Wanganui.

At Ohakea and Palmerston North, the winds were reasonably strong easterlies for some hours before and after dawn, but blew from the northwest for a while around dawn. Two switch-arounds (not quite 180 degrees) of steady to strong-ish winds in a short time is remarkable.

Between 4am and 3pm Saturday 03 March, the south to southeasterly wind at Brothers Island had a mean speed of 108 km/h. During this time, the highest gust was 142 km/h.

Crossing the Country

I mentioned earlier that at 6am the low was not far south of Patea and heading for Palmerston North but never got there. This is because it didn’t physically cross the North Island. Nevertheless, a graph of mean sea level pressure at Palmerston North (below) for Saturday 03 March 2012 shows that the pressure certainly reached a minimum around dawn before rising steeply again from mid morning.

Mean sea level pressure at Palmerston North.

While lows have distinct structures, they’re best not thought of as rotating solid bodies of air – because they’re not. Rather they are the manifestation, at the Earth’s surface, of processes which have produced a local minimum of pressure. Looking back to the working chart above, we see that there are two lows at 6am: one south of Patea, and another just east of Hawke’s Bay. The low south of Patea came ashore east of Wanganui and then decayed, while the “new” low continued to deepen and move away to the east. This decay/development happens because the “driving” atmospheric processes are largely above the Earth’s surface and moving with the general flow: they left the Patea low behind and powered the development of the “new” low east of the North Island.

Technical Stuff

Finally, the definition of a “bomb” low is technical and not whimsical. As far as I know, the first mention of the term “bomb” was in a paper by two distinguished meteorological researchers, Fred Sanders and John Gyakum, titled “Synoptic-Dynamic Climatology of the “Bomb”", published in the October 1980 issue of Weather and Forecasting, a journal of the American Meteorological Society. Because of their destructive potential, rapidly deepening lows have been the subject of many a research paper.

A Southerly ‘Buster’

On Monday 28th November, a south to southwest change swept its way northwards across Otago and Canterbury during the afternoon.   Temperatures soared to 28 C preceding this change then rapidly plummeted to around 16.  This was a good example of what is known in Australasia as a ‘buster’.

Temperature traces on 28 November in degrees Celcius. DNA=Dunedin, OUA=Oamaru, TUA=Timaru, CHA=Christchurch and KIA=Kaikoura. Timestamp is in UTC; 27 0000 is 1pm NZDT.

The weather map for 1pm Monday 28 November 2011 showed a typical trough moving across New Zealand. The last of a series of fronts within this trough was the one responsible for this dramatic drop in temperature.

Weather map for 1pm NZDT Monday 28 November

The reasons for temperatures soaring to between 26 and 28 C ahead of this southerly change are:
•    Northwest winds ahead of the trough warmed by around 5 to 10 degrees Celsius as they descended down the eastern slopes of the Southern Alps
•    Sunny conditions in the relatively clear skies over the Canterbury Plains – on a date less than one month ahead of the longest day.

These warm temperatures combined with falling air pressure to produce a zone of relatively low density. Higher density air in the cooler southerly flow that followed this cold front accelerated into this zone of low density air producing a squally “gust front” with the wind change. This “gust front” built in size and intensity during the afternoon as can be seen from the tweets sent from @metservice during the afternoon

  • Southwest change arrived Dunedin Airport around 11:50am. Temp dropped from 22 to 14 C , gusts to 50 kph , and its on its way north. ^BM
  • Southerly change got to Oamaru about 1:30 pm, temp. dropped from 22 to 14 C, initial gusts were 54 kph . South Canterbury’s next ^BM
  • Southwest change arrives in #Timaru just before School’s out, Temperature drops from 28 C at 2:30 pm to 16 C by 3 pm , Gusts to 70 kph ^BM
  • Southerly change reached #Ashburton between 3:30 and 3:45 pm, temp. dropped from 26 to 16 C, gusts 70 kph see ^BM
  • Southwest reached #Christchurch at 5 pm in time for evening commute, temps 28 C to 17 C in 20 minutes. with wind gusts 75 kph , ^BM
  • Hi #Christchurch be quick and look at wind blown dust of that southerly change on MetService radar at past hour ^BM

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September to November is the season for the strongest of these southerly busters (but they can occur at any time of the year).  Spring brings the strongest westerly winds of the year to South Island and it is also a time of relative cold offshore sea temperatures.  The temperature difference between the heated air over the Canterbury Plains and seas in the Canterbury Bight is what feeds the wind gusts of a buster. The coldest sea temperatures of the year occur in early spring, and they only just start rising in November.   You can find the latest reading from by clicking on ‘marine’ and then ‘beach’ and selecting a suitable site.  The one shown below is Jack’s Point near Timaru (timestamp is 10am Wed, 30 Nov 2011).

Jack's Point on the Marine & Surf section. Wind, Wave ands Sea conditions are now available on


The showers with this buster occurred mainly along the coast and at sea.   In the drier air over the Canterbury Plains the southwest wind change picked up dust and dirt, especially over the Rakaia River, and lifted and blew these as a “dust storm” into Christchurch.  This can clearly be seen on the animation below, taken from the high frequency Christchurch rain radar site at

The Big Chill

2:30pm Sunday 14 August 2011

It’s early days in a weather event which is likely to be memorable for its coldness.

Below is a satellite image for midday Sunday. The wind flow over New Zealand is generally from the southwest; the coldest showery air has made its way onto Fiordland, Southland, Otago and south Canterbury.

It’s not raining or snowing over all of southern New Zealand because the wind flow is more or less lined up with the South Island, thereby sheltering some places, and because the precipitation is showery.

Infra-red satellite image for midday Sunday 14 August 2011. Image courtesy Japan Meteorological Agency.

The surface temperature doesn’t necessarily tell the whole story. Below is a graph of the air temperature for the 24 hours from 1:00pm Saturday 13 August to 1:00pm Sunday 14 August. Note how the air temperature at Invercargill Airport and Nugget Point, both fairly open to the southwest, has been on a pretty steady downward trend. At 2:00pm Sunday, the temperature at Nugget Point was 1.0 C and the wind speed was 85 km/h. Brrr.

At Timaru Airport, on the other hand, the temperature rose sharply when the southwest change arrived mid Sunday morning – the reverse of what might be expected. The period overnight Saturday to dawn Sunday at Timaru Airport was one of clear skies and fairly light winds, so the quickly-cooling land surface during this period cooled the air immediately above it and an inversion formed. When the southwest change arrived, the air once again became well-mixed. But this southwest air is expected to also become steadily colder.

Air temperatures at Invercargill Airport, Nugget Point (Otago coast) and Timaru Airport for the period 1:00pm Saturday 13 August 2011 to 1:00pm Sunday 14 August 2011.

4:15pm Sunday 14 August 2011

Here’s another look at why the surface temperature doesn’t necessarily tell the whole story.

In my blog post about the winter storm of early July 2011, I partially explained how showers may form in cold air moving over a warmer sea surface. The (relatively) warm sea heats “blobs” of the air immediately above it; these blobs then ascend because they are less dense than surrounding air. For the ascending process to continue, the surrounding air must remain relatively cooler than the ascending warm blobs. Thus, it is important to have information – that is, observations and forecasts – about the vertical temperature structure of the atmosphere.

Observations of the temperature structure of the atmosphere are primarily made using weather balloons. Below is a graph of the temperature at three levels in the atmosphere above Invercargill, obtained from radiosonde balloon flights. The blue line is the temperature at about 5000ft, the red line is the temperature at about 10000ft, and the green line is the temperature at about 18000ft. These heights are approximate; the height of a given pressure level varies with the air temperature; here, we should probably discuss the idea of the thickness of an atmospheric layer – but I think we’ll do that some other time.

Anyway, the graph shows that the atmosphere above Invercargill has been cooling off steadily since the middle of Saturday 13 August. In depth, it is now very cold.

"Upper air" temperatures obtained from balloon soundings at Invercargill Airport at midnight Fri-12-Aug, midday Sat-13-Aug, midnight Sat-13-Aug and midday Sun-14-Aug.

6:15pm Sunday 14 August 2011

It snowed quite heavily in Wellington City, above about 100 metres, from approximately 4:30pm for at least an hour. This is the heaviest and most widespread snowfall in Wellington City for at least 30 50 years.

Snow in Clifford Road, Johnsonville (altitude approximately 200 metres) early evening, Sunday 14 August 2011. Snow depth 3 to 5 cm.

At midday Sunday 14 August the freezing level around Wellington, obtained from the Paraparaumu radiosonde balloon flight, was just over 1000 metres and falling (it was around 1600 metres at midnight Saturday 13 August). But late on Sunday afternoon, it would still have been well above the level to which snow fell in Wellington. Snow starts melting once it falls below the freezing level – but the melting process draws heat from the surrounding air, which lowers its temperature; thus, the melting snow “drags” the freezing level down with it, at least for a while. How far the freezing level within the area of falling snow is dragged towards the ground depends mostly on the intensity of the snowfall and the vertical variation of temperature and humidity of the air it is falling into.

Wellington radar image, 5:05pm Sunday 14 August 2011.

1:00pm Monday 15 August 2011

So far in this blog, I’ve been talking quite a bit about the temperature throughout the depth of the troposphere (the troposphere is the part of the atmosphere in which weather systems exist). Time, now for a picture. Below is a plot of:

  • Forecast temperature (colours) at the 500 hPa level (roughly 18,000 ft, or about halfway up the troposphere)
  • Forecast wind speed (black lines) at the 250 hPa level (near the top of the troposphere)

… for midday Monday 15 August.

Plot of (a) forecast temperature (colours) at the 500 hPa level (roughly 18,000 ft, or about halfway up the troposphere), and (b) forecast wind speed (black lines) at the 250 hPa level (near the top of the troposphere), for midday Monday 15 August 2011. Data courtesy European Centre for Medium-Range Weather Forecasting.

The colours in this plot are forecast temperature; over most of New Zealand, the temperature at around 18,000 ft was forecast to be -30 C or lower. The important thing to note is that a large mass of Antarctic air covers almost all of New Zealand.

The red arrow on this plot shows the forecast position of the axis of strongest winds, near the top of the troposphere, at midday Monday 15 August. This is the polar jet, on the border between the deep pool of Antarctic air over New Zealand and the warmer mid-latitude air around it.

Incidentally, the forecast temperatures compare very well with the observed temperatures at midday Monday 15 August, as shown in the table below.

Forecast 500 hPa temperature (C) Observed 500 hPa temperature (C)
Whenuapai -29.3 -29.9
Paraparaumu -38.9 -39.1
Invercargill -34.4 -35.1


4:30pm Monday 15 August 2011

Here’s a graph of how the freezing level over New Zealand has changed over the last few days.

As of midday Monday 15 August, the freezing level varied between about 1000 ft at Invercargill to about 2000 ft at Whenuapai. Snow has fallen to sea level in many parts of southern and central New Zealand – that is, to at least 1000 ft below the freezing level.

This is a classic example of the melting effect (see the post made at 6:15pm Sunday 14 August 2011, above). Over the last few days, MetService’s Severe Weather Forecasters have spent a lot of time considering how far below the freezing level snow would fall. This requires a good understanding of cloud physics.

Freezing levels at Whenuapai, Paraparaumu and Invercargill from midnight Friday 12 August 2011 to midday Monday 15 August 2011, derived from radiosonde flights.

9:30am Tuesday 16 August 2011

Here’s a few photos from the Wellington snow of June 1976.

Wellington snow of June 1976: looking approximately southwest from the MetService building.

Wellington snow of June 1976: Karori Road, looking southwest.

Wellington snow, June 1976: MetService building, from Salamanca Road. Look at those cars ...

10:30am Tuesday 16 August 2011

In southerly flows, the West Coast of the South Island is well sheltered by the Southern Alps. Since the southerly took hold on Sunday, the air on the West Coast has been very dry because of the Foehn Effect.

Below is a graph of the dew point temperature (the temperature which air must be cooled to for water vapour to condense into water liquid or water solid) at Hokitika Airport from 10am Sunday 14 August to 10am Tuesday 16 August. On the afternoon and evening of Sunday 14 August, there’s a huge change in dew point (around 13 degrees), down to around -10 C. Since then, the dew point has remained negative, generally fluctuating between about -3 C in the morning and -7 C in the afternoon. Such a low dew point makes the air feel much colder than its temperature would suggest. We take the dew point into account when calculating the “feels like” temperature.

Dew point temperature at Hokitika Airport, 10am Sunday 14 August 2011 to 10am Tuesday 16 August 2011.

2:30pm Tuesday 16 August 2011

This event has been characterised by many places having low daytime (maximum) temperatures.

Location Maximum temperature on
Mon-15-Aug 2011
Lowest daily maximum temperature on record Month / year occurred in Record starts Monday’s max temperature is the lowest since …
Auckland Airport
8.1 8.1 15 August 2011 1966 New record
New Plymouth Airport
8.8 6.0 July 1951 1945 25 July 2011 and 12 July 1951
Napier (Nelson Park)
7.8 3.6 June 1969 1940 June 2006
Paraparaumu Airport
6.1 5.4 August 2004 1972 July 1986
Kelburn, Wellington 5.1 4.5 August 1938 1931 25 July 2011 and 17 July 1995
Christchurch Airport
4.6 1.7 August 1992 1954 July 2007

Monday night / Tuesday morning was very cold in some places, though. Here’s a few notable overnight minima from MetService automatic weather stations.

  Waiouru Automatic Weather Station     -7.7 C (new record for August)  
  Blenheim Airport Automatic Weather Station     -6.2 C (new record)  
  Rotorua Airport Automatic Weather Station     -5.2 C (equals record)  
  Taupo Airport Automatic Weather Station     -5.1 C  

5:00pm Tuesday 16 August 2011

Below is a plot of where the air arriving at an altitude of 500 metres above Auckland at midday Monday 15 August came from. Four days previously, it was over the Antarctic landmass; two days previously, it was still over the Antarctic sea ice. The Antarctic sea ice edge is close to its northern-most extent and is near latitude 60 degrees South. Thus, the air arriving at Auckland passed very quickly over the relatively warm ocean between the Antarctic ice edge and New Zealand. In contrast, the air from the Southern Ocean which arrived over Auckland on Saturday 9 July (see my blog post on the stormy period of early July 2011) had travelled over a much longer stretch of ocean, over a longer period of time, and consequently was warmer and moister.

"Backward trajectories" of the air at 500 metres above mean sea level arriving at Auckland midday Monday 15 August 2011. The red line traces the path of the air over the period from midday Friday 12 August to midday Monday 15 August. Triangles denote the location of the air mass at 12-hourly intervals. Data courtesy NOAA Air Resources Laboratory.

6:00pm Tuesday 16 August 2011

As of 2:00pm Tuesday 16 August 2011, the extent of snowfall in this storm is as shown in the image below.

The scalloped areas delineate where significant snowfall is known (or believed) to have occurred. The dashed areas delineate where lesser amounts of snow have been observed and where little or no snow is believed to by lying. The period is from the beginning of the event on Sunday 14 August 2011 to 2:00pm Tuesday 16 August 2011.

4:00pm Wednesday 17 August 2011

During the next few days, while an anticyclone advances onto the country, the general wind flow will decrease in strength and the depth of cloud along eastern coasts gradually reduce. Near sea level the air over New Zealand remains very cold, and the advancing anticyclone more or less “traps” it in place. Very cold air, clear skies and light winds overnight are a recipe for hard frosts.

Hopefully, the diagram below – known technically as a tephigram – helps illustrate this. It is a plot, in the vertical, of the air temperature and the dew point temperature derived from the radiosonde balloon flight at Invercargill at midday Wednesday 17 August. At Invercargill there is already a large mass of sinking, warming (and drying) air above about 5000 ft (see text in red on diagram). This sinking air presses on the (relatively) colder air beneath it, trapping it near the Earth’s surface. In this particular case, the zone of transition between the two different air masses is known as a subsidence inversion. I’ve marked the subsidence inversion on the diagram; it’s the broad blue horizontal bar near the bottom.

The very cold air trapped below about 5000 ft at Invercargill is much less inclined to move around than the air further up in the atmosphere. As I’ve explained above, this is partly because of the advancing anticyclone. But it’s also partly because cold air is less “runny” than warm air. (Treacle flows much more readily when warm than cold). On the right of the diagram below are the winds in the vertical, as they were above Invercargill at midday Wednesday 17 August. Clearly (see text in green on diagram), the one wind barb shown below 5000 ft indicates quite a different flow from all the winds above 5000 ft: the flow near the surface has become decoupled from that above.

Invercargill tephigram for midday Wednesday 17 August 2011.

In a general sense, this vertical temperature and wind structure is expected to spread over eastern parts of New Zealand during the next few days as the anticyclone moves closer and pressures over the country rise.

Forecast surface pressure field for midnight Wednesday 17 August 2011. Forecast surface pressure field for midnight Thursday 18 August 2011.

3:30pm Friday 19 August 2011

Finally today, cloud over the south of the South Island has cleared enough to reveal the extent of snow cover there.

Below are two visible satellite images. The first is for around 10:00am on the morning of Wednesday 10 August, some days before this extraordinary cold outbreak. The second is for around 10:00am on the morning of Friday 19 August. Nearly all of the white over Canterbury, Otago, Southland and Fiordland is snow. The imagery only shows the extent of the snow, not its depth.

Before: MODIS-Terra visible image for about 10:00am on Wednesday 10 August 2011. Data courtesy NASA/GSFC, Rapid Response.

After: MODIS-Terra visible image for about 10:00am on Friday 19 August 2011. Data courtesy NASA/GSFC, Rapid Response.

New Plymouth tornado, Sunday 19 June 2011

The tornado in New Plymouth early on the morning of Sunday 19 June 2011, like every tornado that passes through an area where people observe it, was certainly dramatic.

Like many tornadoes that affect Taranaki, this tornado formed out to sea – not very far to the northwest of New Plymouth – in a line of thundery showers.

Radar imagery

At 4:15am, a mesocyclone (to be explained later in this blog) is identifiable over New Plymouth in imagery from the New Plymouth radar. Radar imagery 7.5 minutes earlier, at around 4:07am, hints vaguely at the mesocyclone’s existence but is far from conclusive. Radar imagery at around 4:22am suggests the mesocyclone either has decayed or is rapidly decaying. In other words, conditions supporting the development of a tornado in the New Plymouth area were favourable only for a very short time.

Reflectivity image from the New Plymouth radar for 4:16am Sunday 19 June 2011. Colours represent how strongly precipitation bounces the radar signal back to the radar.

Colour key for the above image. The further to the right along the scale, the more strongly precipitation reflects the radar signal.

Severe Weather Forecaster John Crouch has extracted radar data from the bottom few “sweeps” of the radar beam. The mesocyclone was sufficiently close to the New Plymouth radar for these sweeps to provide a reaonable view of it. Here they are below, animated. Each time step is 33 seconds; the first sweep is at about 100 metres above the ground near downtown New Plymouth, while the last is about 1000 metres above the ground. So, we’re looking at successively higher layers of the mesocyclone as time goes forward; the reflectivity pattern can be seen wrapping around the circulation associated with the mesocyclone.

Radar reflectivity loop covering the (just over) three-minute period from 4:15am to 4:18am Sunday 19 June 2011. A hook shape can be seen moving from the northwest across New Plymouth.

It’s important to note that this doesn’t mean it’s possible to produce 33-second radar imagery routinely, or in “operational time”. It’s only when phenomena very close to the radar are moving and evolving sufficiently rapidly that data from subsequent sweeps of the radar beam might be coherent enough for animations of them to be meaningful.

Life cycle

Note, in the above animation, how quickly the hook-shaped pattern moves and changes. It suggests strongly that the whole tornado event was over in a couple of minutes, which appears to be consistent with reports in the media. In the animation, there’s evidence of only one hook-shaped pattern in the New Plymouth area: it may be that there was only one tornado, and it wasn’t always reaching down to the surface along its path.

Warm seas to the northwest of New Plymouth provided some of the “fuel” for the thundery showers which passed across Taranaki in the early hours of the morning of Sunday 19 June 2011. This is very likely why the tornado was short-lived: when the mesocyclone came ashore, its fuel supply was cut off.

A little bit about mesocyclones

In brief, a mesocyclone is a local rotation and ascent of air about a vertical axis.

Hook-shaped patterns in radar reflectivity imagery are not uncommon – but are nothing like conclusive evidence of the presence of tornadoes. There also needs to be strong, coincident, rotation: this is one of the reasons why weather radars measure the inbound / outbound speed of the echo using the Doppler Effect.

In the case of the tornado in New Plymouth early on the morning of Sunday 19 June 2011, there was a strong velocity couplet observable at 4:15am, but not either side of this time.

Further reading

For more on tornadoes in New Zealand, see the blog about the Albany tornado of Tuesday 3 May 2011.