Why it’s been so dry

For about the last month, anticyclones have dominated the New Zealand area. Many places have had little or no rainfall since early February.

The map below shows the average mean sea level pressure over the New Zealand area over the last month or so. There’s no doubt about the pressure being high and not changing very much. Because this map shows averaged pressures, we don’t see the few troughs that have passed across the New Zealand area in the last 28 or so days.

Mean sea level pressure, averaged over the period 0000UTC 24-Jan-2013 to 1800UTC 23-Feb 2013. Image provided by Physical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, from their Web site at http://www.esrl.noaa.gov/psd/

Mean sea level pressure, averaged over the period 0000UTC 24-Jan-2013 to 1800UTC 23-Feb 2013.
Image provided by Physical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, from their Web site at http://www.esrl.noaa.gov/psd/

There are reasons why the weather gets into “regimes” like this. It has to do with what meteorologists call the “long waves”. If you’ve been reading the MetService blog for any length of time, you might notice that this article is very similar to one published in September 2010 titled “Wave Three“.

The map below is one way of depicting what the average weather pattern about half-way up the troposphere (the troposphere is that part of the atmosphere in which the weather occurs) looked like over the last month or so. In this pattern, there’s nothing that looks like a high, or low, or front. But there is a bunch of wavy black lines … which look kind of regular, but not quite. The upper ridges are marked with red dashed curves. Note that there’s been one over the New Zealand area for most of the last month.

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500hPa height, averaged over the period 0000UTC 24-Jan-2013 to 1800UTC 23-Feb 2013.
Image provided by Physical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, from their Web site at http://www.esrl.noaa.gov/psd/

Now what if those waves could be separated out into longer ones and shorter ones, so we could see what was going on “under the bonnet”? There happens to be a way of doing exactly this; it’s called Fourier analysis.

The picture below is made from a Fourier analysis of the pattern at midnight Saturday 23 February 2013. It reveals that, in New Zealand’s latitudes, the pattern is dominated by a wave which has one ridge (pink) and one trough (blue) wrapped around the hemisphere. Most important to us is the big area of pink shading – the ridge – near New Zealand. Further south, in the Southern Ocean, the pattern has three waves with a remarkably strong ridge south of South America.

The "long" and "medium" waves about half-way up the troposphere at midnight Saturday 23 February 2013.

The “long” and “medium” waves about half-way up the troposphere at midnight Saturday 23 February 2013.

The picture below shows a 12-day history and 5-day expected movement of these longer waves. While shorter waves (some of them corresponding to weather features) are travelling steadily eastwards, the underlying pattern in the New Zealand area remains ridged.

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The Effect of Resolution

New Zealand’s weather is currently dominated by a large anticyclone that’s bringing fine sunny weather and light winds to many areas.

The chart below shows the position of the centre of the anticyclone at midday Tuesday 10 July 2012. The red arrows show the sense of the broad-scale rotation around the system (anticlockwise in the Southern Hemisphere). It also shows the smaller-scale flow around the top of the South Island, and how this is directed onto Manawatu and Wellington. There are some subtle aspects to this flow that I’d like to investigate in this blog post, using satellite imagery.

Weather map at midday 10 July 2012.

The term resolution is used to describe the capability to distinguish between objects sitting next to each other. High resolution means that objects that are close together can be separately identified, whereas lower resolution means you can’t tell that there’s more than one object. The term resolution can refer to the number of pixels in a computer monitor or dots-per-inch of a printer. It also refers to the amount of detail that can be discerned in an image from a weather satellite.

Take a look at the image below, based on visible light as received by the MtSat-2 geostationary meteorological satellite.

MTSAT-2 visible satellite image for 3pm NZST Tuesday 10 July 2012. Image courtesy Japan Meteorological Agency.

The image covers a large area, and it shows patchy cloud over and around New Zealand. Because the image uses visible wavelengths of light, it shows all cloud as white or light grey, regardless of how high or low the cloud is. This differs from infrared images, the ones usually displayed on TV and on websites, which show high cold clouds more prominently than the lower (and warmer) clouds.

Central Australia is largely cloud-free, and there are variations in the shades of grey caused by the varying texture of the land surface there. Over Queensland there is cloud with a wispy, fibrous texture. This is cirrus, composed of ice crystals which give it this appearance. It’s not possible to see much detail in the cloud over New Zealand. Note that the southeast corner of this image is darkened, due to the setting sun there.

The same satellite can provide us with higher resolution visible imagery over New Zealand as below, valid for the same time as the previous image.

MTSAT-2 NZ visible satellite image, same time as previous image. Image courtesy Japan Meteorological Agency.

We can now see that there’s quite a bit of cloud over Fiordland and Southland, and a few patches around north Westland, Manawatu and Wellington. Most of this cloud is at low-levels, typically stratocumulus and low-topped cumulus. Notice how the cloud seems to swirl around Farewell Spit and then towards the lower North Island. You might just be able to pick up a little of the structure of the few clouds that there are over Wairarapa.

The Aqua polar-orbiting satellite has a very high resolution sensor, capable of providing visible imagery to a resolution of 250 metres. Because this satellite orbits the Earth from Pole to Pole, the images are only available for places that the satellite passes over. Fortunately, Aqua passed over New Zealand at about the same time as the previous images, and I’ve reproduced the image below (converted to black and white to aid comparison). The richness of detail is striking.

Very high resolution satellite image for approximately same time as previous images. Image courtesy of MODIS Rapid Response Project, NASA/GSFC.

Stratocumulus covers Manawatu, and transitions into a lumpier looking cumulus structure to the north near Wanganui. There is banding in the cumulus as it approaches the gap between the Ruahine and Tararua ranges, the area surrounding the Manawatu Gorge. The stratocumulus and cumulus cloud has been enhanced by the air having been forced to rise on the western (windward) side of the Tararua ranges (see post on Foehn wind). On the eastern (leeward) side the skies are mostly cloud free, indicating a sunny winter’s day.

The smaller Puketoi range east of the Gorge provides another smaller-scale environment for enhancing the cloud on the windward (western) side. The image also shows snow lying on the tops of the Ruahine and Tararua ranges.

The banding I talked about earlier is called “cloud streets”: the cumulus clouds are organised into longitudinal rolls by the low-level wind-flow. There’s further evidence of banding east and northeast of the Puketoi range, with curvature indicating the shape of the wind-flow as it exits the Gorge bottle-neck and spreads out. The resolution of this image is so high that you can see (or resolve) individual cumulus clouds as small white dots.

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.

The Structure of Highs

After writing two blog posts about lows or depressions, I thought it would be a good idea to also write something about highs or anticyclones. After all, one is just the opposite of the other, isn’t it?? Well, in some ways yes, but there are some important differences too.

If you have a barometer you’re probably aware of one of the differences. Look at the display, especially if it’s an older piece, and you’ll see words like stormy or rain when the air pressure is low (in or near a depression), and finedry or settled when the air pressure is high (in or near an anticyclone). But it’s not as simple as that.

Let’s list some of the differences between highs and lows.

1. Wind direction

Around a low on a weather map (mid-latitude or tropical low, it doesn’t matter which) the wind blows clockwise around its centre in the southern hemisphere. I had to add that last bit about the hemisphere because for our friends in the northern hemisphere the wind blows the other way.

Around a southern anticyclone the wind blows anticlockwise. That’s easy to remember in the southern hemisphere :-).

High_2D

A typical anticyclone moving onto NZ

2. Wind speed

There are some subtle differences in speed too – check out the post on Year 12 Maths. If an anticyclone is close to you, the wind around you will be gentle. For a depression this may not be the case although some depressions are weak and the wind is still light.

3. Cloud and rain

Here’s the biggest difference between highs and lows. The reason is again to do with air motion, but not the familiar wind that blows horizontally. More important is up and down motion, and it’s perhaps hard to visualise just from 2-D weather maps - you need to think 3-D as I explained in my post about the Nor’west storm of August 1975.

Up and down air motion is much smaller than horizontal, by a factor of a hundred or more. It’s very difficult to measure too, although there are indirect ways of estimating it.

When air rises it cools, as Erick Brenstrum explained in his post on Ridge-Top Winds. With enough moisture, cloud will form and possibly rain. Conversely, when air sinks or subsides, it warms and any cloud or rain dissipates. Guess what the typical vertical motion is in highs…sinking or subsidence. That’s also why an inversion forms (as Erick explained).

High_3D

As previous map but in 3-D. The descending arrows are not to scale - the troposphere is only about 10 km thick so the arrows would be much shorter than drawn

So anticyclones tend to bring clear skies. But sometimes it can be cloudy even when the barometer needle is up high. That’s because the sinking air doesn’t make it down to the ground and moisture is trapped at low levels – forming stratocumulus cloud. If the cloud is thick enough you might even get some drizzle. Perhaps you’ve been in an aircraft taking off on an overcast day and quickly pierced through the stratocumulus to completely blue sky?

Finally I should mention that, if you use your barometer, pressure changes are more important than the actual value of the pressure. It’s possible to have rain when the air pressure is higher than 1020 hPa – for instance, if there’s an area of relatively low pressure between two large highs. Conversely, it’s possible to have a sunny day when the pressure is lower than 990 hPa – for example, in a nor’wester along our east coasts with a deep low to the south.

But if the air pressure is rising strongly, then an anticyclone or a ridge of high pressure is heading your way, and any rain is likely to dissipate, especially if the flow is offshore to help along that down motion of the air.

How Lows and Highs move

In my blog post about winds aloft there is a loop of satellite images for a week in winter 2008.  It shows that the big cloud features in the mid-latitudes typically travel from west to east. In other words, the features you see on weather maps affecting New Zealand have usually started out roughly in the area of southern Australia.

There are exceptions to this, and a notable one that comes to mind is when tropical cyclones or sub-tropical depressions move southwards onto the Tasman Sea/New Zealand area. A couple of famous examples are tropical cyclone Giselle in April 1968 (the “Wahine” Storm that devastated Wellington and many other parts of NZ) and tropical cyclone Bola in March 1988 (that badly flooded the Gisborne region).

Sometimes, too, the Highs or Lows seem to grind to a halt and this is another variation on the westerly pattern. But despite these exceptions it is true that most of our weather does come from the west. This result also relates to the “red sky at night” saying that Ross Marsden discussed a couple of months ago.

I’d like to present you with another interesting aspect to this typical movement, and if you’ve looked at a lot of sequences of weather maps you may have spotted it. Have you ever noticed that the depressions (“L”s) usually drift slightly southwards within the westerly pattern? Conversely, anticyclones (“H”s) usually drift slightly northwards within the westerlies.

I’ve put together an animated sequence of weather maps for you to see for yourself. There’s some jumpiness due to small scale effects, but I hope you can pick up the general trend.

 

The maps are Mean Sea Level analyses (fronts omitted) from 14 to 20 August 2009.

There are exceptions; e.g., small depressions that wrap around bigger ones and therefore take a path like a loop around the larger Low. But the drift I’ve mentioned does generally occur.

The first and best explanation I have found of this effect comes from a brilliant Englishman called Reginald Sutcliffe. He lived in the middle of the 1900s and, as well as being an active researcher, applied his scientific nous during the second World War. You can find out more about him on the internet; e.g., on Wikipedia.  He died in 1991, and I would have loved to have had the privilege of meeting him.

The slight drift comes about by first noting how a developing depression in the middle latitudes has colder air on its western side and warmer air to the east. You’re probably already familiar with this notion, since it explains why we tend to have milder weather as depressions approach us (when we are on the eastern side of the L) and colder weather as they move away (when we are on the western side).

Sutcliffe showed that this typical temperature pattern steers the depression towards the southeast - the precise direction depends on just how much cold air there is relative to warm air (which can also be deduced). For an anticyclone, the converse applies and it is steered towards the northeast.

Of course Sutcliffe’s research was originally based in the northern hemisphere, but the result is just as valid in the southern hemisphere which is how I’ve stated it here by applying a mirror image of the effect. With his explanation, Reginald Sutcliffe devised one of the first objective methods for weather forecasting.

Many of the depressions that affect NZ form near the New South Wales coast. This typical drift within the westerlies takes the developing systems on a direct path towards NZ!

As with the example in my post about maths and meteorology, the steering result can be demonstrated using equations. But the beauty and simplicity of it can also be observed on most sequences of weather maps.