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.

Wind

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:
http://about.metservice.com/assets/downloads/learning/winds_poster_web.pdf

Rain

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.

Temperature

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.

Weather

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 metservice.com

Clouds

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:
http://about.metservice.com/assets/downloads/learning/clouds_poster_web.pdf

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: http://weather.unisys.com/wxp/Appendices/Formats/SYNOP.html

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

It’s a fine day. Isn’t it?

What do we mean when we say the weather is “fine”?

The word fine is often used to convey the positive attributes of something. It is synonymous with good, well, enjoyable.

How are you? I’m fine!
How was the movie? It was fine.
This is a fine bottle of wine.

When we write weather forecasts we define the term fine to mean that the sun casts sharp shadows. If cloud is thick enough to stop the sun from casting sharp shadows then, even if it doesn’t rain, we don’t think that’s a fine day.

However, New Zealand isn’t known as the “Land of the long white cloud” for nothing, and only infrequently is the sky completely cloud free for a whole day. Cloud often comes and goes. So, when writing forecasts, there are a number of questions to be answered when describing the state of the sky:

  • How much of the sky will be covered by cloud?
  • How thick will the cloud be?
  • How will the amount of cloud vary throughout the day?
  • Is there going to be more or less cloud than the previous/coming days?

Our perception of fine weather also varies with the seasons. In the summer months the sun is stronger and even if there is a lot of thin or high cloud it can still manage to cast sharp shadows. Also, if cloud does block the sun for short periods of time we are less likely to notice because the air is warmer; in fact, it might feel like a relief for a short time!  In the winter when the sun is weaker it may struggle to cast sharp shadows, and the day will feel cooler. If the sun is blocked by cloud, even for a short time, then it can affect the temperature more significantly and make you feel colder.

So, when it’s not a clear-cut blue sky day we consider all these things, as well as how the weather will make people feel. Will they feel it was a fine day? Or a cloudy day?

Auckland on a Fine Day. Photo by Joerg Mueller.

Cloud structures over NZ on 26 July

On Thursday 26 July 2012 a cold southeasterly airstream flowed onto the North Island, around an anticyclone centred just east of the South Island. In this blog post we’ll look at some interesting small-scale cloud structures around the country on this day.

Below is the weather map at midday on Thursday 26 July. The red arrows show the sense of the broad-scale rotation around the anticyclone.

While the North Island was experiencing a southeasterly flow, the isobars were widely spaced over the South Island, indicating little wind there. Take a look at the animation below, based on visible light as received by the MtSat-2 geostationary satellite.

MTSAT-2 visible satellite images, each an hour apart, from 10am to 3pm NZST Thursday 26 July 2012. Images courtesy Japan Meteorological Agency.

As explained in the post on the effect of resolution, the visible satellite image shows all cloud as white or light grey, regardless of how high or low the cloud is. Most of the cloud over New Zealand is on the east coast of the North Island. Just off the Manawatu coast there is a plume-shaped area of cloud that extends northwestwards parallel to the flow. This is low-level cloud in a region where the air flow near the Earth’s surface is coming together, or converging. As the air convergences it is forced to rise and, with enough moisture, cloud forms.

There is a similar process occurring off eastern Bay of Plenty. In this case there is a zone of more concentrated convergence that shapes the clouds into a rope-like appearance, but the orientation of it is still towards the northwest and parallel to the flow.

Most of the South Island is cloud-free, but there is a patch of grey-looking cloud around Lakes Tekapo and Pukaki. This is also low cloud, but it has formed during the night in the valleys and basins. In the afternoon there’s been just enough heat from the weak winter sun to break up and disperse the cloud. The cloud base at Pukaki during the morning was reported as being about 600 metres above the ground - the air temperature had risen from minus 4 overnight to plus 3 by lunchtime.

The Terra polar-orbiting satellite has a very high resolution sensor. Terra passed over New Zealand within the period of the previous images, and I’ve reproduced the image below, split into two colour images.

Very high resolution satellite image within the period of the previous images. Image courtesy of MODIS Rapid Response Project, NASA/GSFC.

The features discussed above are very apparent. The area of convergence off Manawatu evidently has a double structure, and it is striking how this high resolution image shows individual cumulus clouds as white dots (as discussed in the previous post). There are cloud streets over the central North Island from the Kaweka to the Raukumara ranges.

Over the Pacific ocean there is a lot of cloud having a cellular structure. This is typical of a cold body of air that moves onto relatively warmer water. The air bubbles up into cumulus clouds that tend to clump together into ring-shaped clusters.

The low cloud over Lakes Tekapo and Pukaki (below) has a flat appearance typical of layered stratus cloud. It extends its fingers into the valleys between the peaks of the surrounding ranges.

As previous image, but for South Island.

The cloud over the ocean east of Canterbury is stratocumulus, a combination of the lumpy texture of cumulus cloud and the layering of stratus cloud. In many respects there was nothing particularly unusual about our weather on 26 July, but the satellite images were still able to reveal some fascinating and beautiful cloud structures.

 

A Winter Storm

Since Wednesday 6 July, stormy westerly conditions have affected New Zealand. In this blog, we’ll look at why.

The “Long Waves”

Below is the mean sea level analysis – the weather map – for 6am Sunday 10 July. In between big highs over the mid South Pacific and south of western Australia is a really large trough; it’s the area shaded light blue. The weather map has looked like this, more or less, since Wednesday 6 July: that is, the big features on it aren’t moving much.

MetService mean sea level analysis for 6am Sunday 10 July 2011.

There’s good reasons why these big features aren’t moving much. They reflect the so-called “long waves” in the troposphere (the troposphere is that part of the Earth’s atmosphere in which the weather occurs), which are stationary at the moment. Below is an image made from a Fourier analysis of the wave pattern in the Southern Hemisphere at midnight Saturday 09 July. There’s a trough (blue) in this wave pattern more or less in the same place as the one shaded light blue on the weather map above. And either side of this trough, there are ridges (pink) in about the same place as the big highs on the weather map above. For more about this, see my blog post on Wave Three.

The "long" and "medium" waves about half-way up the troposphere at midnight Saturday 09 July 2011.

Polar Air

Below is a plot of where the air arriving on New Zealand’s west coast at midnight Saturday 09 July came from. The air is from the Antarctic.

"Backward trajectories" of the air at mean sea level arriving at Cape Reinga, Farewell Spit and Secretary Island at midnight Saturday 09 July 2011. Each red line traces the path of the air over the period of a week. That is, the air arriving on New Zealand's west coast at midnight Saturday 09 July left the Antarctic about a week previously. Data courtesy NOAA Air Resources Laboratory.

On its way to New Zealand, this air travelled over a long stretch of ocean. It will have been colder than the surface of the sea it passed over, and will therefore have taken up heat from the sea surface. As heat transfers from the sea surface to the air immediately above it, “blobs” of air become warmer than their surroundings and rise upwards. This process is known as convection (see Chris Webster’s blog post about predictability and popcorn). If convection continues for long enough, showers and/or thunderstorms are the result. In the satellite picture below, more or less all of the clouds over the Tasman Sea, New Zealand and the seas to the south of the country are “blobs” of air which is rising (or has recently risen) convectively.

MTSAT-1R infra-red satellite image for midnight Saturday 09 July 2011. Image courtesy Japan Meteorological Agency.

The Jet

A major flood of air out of the Antarctic region, like this, has other consequences. The northern boundary of the cold air pushes against the warmer air further north. Thus, the north-south temperature contrast increases and simultaneously the strength of the westerly winds increases – not just at the Earth’s surface, but throughout the depth of the troposphere. (How this works might be the subject of a future blog post). This is at the heart of why many places have been windy over the last few days.

The axis of the strongest winds, in the mid- to upper troposphere, is known as the jet stream. In the plot below (for midnight Saturday 09 July), the polar jet has two branches (arrows in black): one curving across the south Tasman Sea and over the South Island, and the other crossing the south of the North Island. Over central New Zealand, wind speeds above 10,000ft were generally 80 kt (150 km/hr) or more.

Wind speed at 500 hPa (approximately 18,000 ft) in the New Zealand region at midnight Saturday 09 July 2011. Warmer colours are stronger winds. Data courtesy European Centre for Medium-range Weather Forecasting.

Thunderstorms and Tornadoes

Air ascending into convective clouds (showers and thunderstorms) comes down again. When a convective cloud collapses, the resulting downdraft hits the Earth’s surface and spreads out, much like water does when tipped from a bucket onto the ground. When the showers and thunderstorms are themselves fast-moving (on the afternoon of Saturday 09 July, storm motions on the Kapiti Coast were 70 to 90 km/hr), the winds near the Earth’s surface can become very strong when downdrafts occur: this is very likely to be the cause of some of the wind damage on the Kapiti Coast on the afternoon of Saturday 09 July. And this is one of the reasons why the various Severe Thunderstorm Outlooks, Watches and Warnings issued over the last few days have included the mention of damaging wind gusts.

Imagery (see below) from the Wellington radar for 4:00pm Saturday 09 July shows a line of thunderstorms extending across Cook Strait onto the Kapiti Coast. The tornado is very likely to have been associated with the strong thunderstorm shown just east of Waikanae at 4:00pm; eye witness reports suggest that the tornado crossed State Highway 1 around 3:55pm. The tornado’s genesis remains unclear: it may be that a low-level vortex was “spun off” the northern end of Kapiti Island just at the time that this thunderstorm passed by, and the strong ascending motion in the thunderstorm developed it into a tornado between there and landfall on the Kapiti Coast. Once again, it is remarkable that there was no loss of life.

Reflectivity image from the Wellington radar, 4pm Saturday 09 July 2011. Colours represent how strongly precipitation bounces the radar signal back to the radar.

Snow

The Southern Alps are a significant barrier: in westerly airstreams, generally only a moderate amount of precipitation falls any distance east of the Divide. How much precipitation falls east of the Divide, and how far east of the Divide it reaches, depends on a number of factors. Not the least of these is the Foehn Effect. Overnight Saturday 09 July and on the morning of Sunday 10 July, a reasonable amount of snow fell east of the Divide, in parts of Otago, to below 500 metres, in a northwesterly airstream. This is an uncommon occurrence, and reflects how deeply cold and showery the air passing across Otago was at the time.

Large Sea Waves

Since about the middle of the first week of July, sea waves arriving on New Zealand’s western coasts have been notably large. In some places, they have probably attained heights observed only once every year or two – and will remain high until late in the week ending Fri-15-Jul. The weather map below, from about the time large waves began arriving, shows why:

  • The fetch – that is, the expanse of ocean over which waves arriving on New Zealand’s western coasts have been generated (pink arrow) – is very long
  • The waves are still growing as they reach New Zealand’s western coasts, because the winds across New Zealand are themselves strong.

MetService mean sea level analysis for midnight Thursday 07 July 2011. The red arrow shows the approximate path sea waves arriving on New Zealand's western coasts have travelled.

Weird Taranaki Cloud

Addendum added at end of this post on  20 Dec 2010

Lesley of New Plymouth recently sent in some cloud images taken by cellphone at New Plymouth.  They were taken at around 7:50am on Thursday 18 November 2010 and look a bit like someone or something had been slicing the clouds.  The first, below, was taken from the Waimea Street/Brois Street roundabout, looking roughly east-south-east  (to the right of the morning sun).

The second, above, was taken on Huatoki Street outside Vogeltown School facing in the same direction.

The bearing of sunrise at New Plymouth on 18 November was 116 degrees true, and by 7:50 am the bearing was around 99 degrees, at an elevation of 20 degrees above the horizon.

Rough map of details in photo.

Explanation:

Let’s divide the image into three interesting parts.

And look at the weather map and satellite image

On 18 November the weather map shows that an anticyclone (or high) was departing to the east of New Zealand and a warm-frontal zone was approaching from the north.  The barometer was steady at around 1020 hPa, but the clouds above Taranaki were in a state of flux.  There was still some sinking air aloft caused by the high, but also some layers of rising moist air caused by the approaching front.

One of these layers was indicated by a layer of cloud called Altocumulus (Alto=middle, Cumulus= in heaps).  This is sometimes called a mackerel sky because it looks similar to the scale patterns seen on a fish.  This cloud layer had a base that was measured by MetService at New Plymouth airport to vary between 17,000 and 21,000 feet (in aviation meteorology altitude is measured in feet and horizontal visibility is measured in metres), well above the top of Mount Taranaki (8,260 feet).

The droplets in this cloud would have been formed from water vapour condensing onto small aerosols (dust or salt particles) into little liquid balls as the layer of air rose and cooled.   Even though these cloud droplets had floated higher in the sky than the freezing level, they were still liquid – such cloud droplets are given the name super-cooled.

The zoom (above)  taken from the  8am image from the MTSAT  geostationary satellite shows a cloud gash orientated north/south and located east of Stratford. It also seems to show some possible steps up and down within the cloud deck, oriented east-north-east to west-south-west.

1. Cloud step or gap

This deck of cloud has a sort of step feature or crack in it running from lower right to upper left in these photos.  These features are sometimes seen in other cloud decks at lower and higher levels – possibly due to some sort of disruption in the rising layer as the cloud forms, but there is insufficient information in this case for any useful conjectures.

The zoomed image above shows that the air is clear along this gap.   A hint of shadow seems to indicate that there may be a jump in height from left to right across this gap.

2. Long gash

The striking gash line from left to lower right is an example of a process colloquially called a “hole punch”.

To form a “hole punch”, basically, requires a disturbance such as the addition of some frozen nuclei – it could be ice or soot or something solid with a sub-zero temperature, possibly falling from a cirrus cloud higher up.   This causes the supercooled water droplets that it touches to freeze.  The frozen ice particles then grow by absorbing water vapour from the air around them.  This lowers the relative humidity of this air and increases the evaporation rate of the remaining liquid water droplets so that they dissipate as a result of a diffusion process.  The growing ice starts to fall in streaks and a hole opens up.

Eventually the water vapour cannot diffuse fast enough from the edges of the hole and the process stops.  It’s a race between diffusion and gravity, and gravity wins.

In this case the fall streaks encountered drier air below the original cloud and mostly evaporated, but sometimes they can form a long wispy cloud just beneath the hole. These falling and evaporating clouds are called virga.

We can only guess as to the cause of the disturbance that led to this hole punch.  Since it seems to have a straight track, it could likely have been a passing aircraft.  The exhaust fumes of a jet engine wake contain water vapour, carbon dioxide, and small amounts of soot and other combustion products. Soot may have descended into the cloud and started the freezing process.  Jet engines leave a trail of frozen nuclei in their wake that, given the right conditions, can seed cloud growth and make a contrail (condensation trail), or punch a hole in a cloud and make a distrail (dissipation trail).

This cloud gash matches the flight path for aircraft between Auckland and Wellington.

There may have been other sources for the original freezing nuclei.

Here are some local examples of clouds showing a hole punch:

Colin Langley, Hauraki Gulf, Auckland

Sheryl Logan, Maungaturoto, Northland

Ron Ovenden, Whatawhata

Dave Swarbrick, Auckland

And here are some links from Wired Science and NASA explaining the processes involved

3.  A spot of colour

The above zoomed image has been highlighted to show a coloured spot in the image taken from Huatoki Street.

This is likely to be iridescence (sometimes called irisation), with sun side-lighting a thin cloud and producing a coloured pattern similar to that seen in soap bubbles.

The likely process here is called diffraction (click here for more information).

The simple way of explaining diffraction is to say that light is bent as it passes around the thin sharp edge of an object, and some wavelengths or colours are bent by different amounts, so that sunlight is turned into a rainbow of colour.  The cloud needs to be thin and all the droplets need to be all of similar size and stay coherent.  This occurs in newly formed clouds, especially in mountain wave clouds (altocumulus lenticularis), cumulus clouds, and in contrails.   Iridescence occurs best when viewed at only a small angle from the sun, with the sun low in the sky and covered.

Mathematically, diffraction is best described as being an interference pattern, where different streams of light get superimposed and combine like waves, adding here and cancelling there.

Conclusion

Our thanks to Lesley for sharing this weird and wonderful cloud image with us.  It helps to highlight some of the natural (and man-made) effects that occur around us.  It also gives us an opportunity to share our current understanding of the physical processes at work. Taranaki, with its conical mountain, is a great place for looking for weird clouds. So keep that camera ready!

Addendum

Malcolm Potts has sent in some more images of this weird cloud,  capturing more of the striking iridescence. He has permitted these to be added to this blog.

Thanks Malcolm :

Malcolm Potts

Malcolm Potts

Breaking waves in the sky

Here is an image taken by Norman Robinson’s cellphone of some unusual clouds seen from Ophir in Central Otago last  May.  Norman adds “We have some really interesting cloud patterns here at times, but this was one of the more unusual ones.”

Taken by Norman Robinson, 8 May 2010

MetService’s Consultant meteorologist, Ross Marsden, has been able to ascertain from metadata encoded in the iPhone image file that it was taken on 8 May 2010 at 16:16 at latitude 45.11S, longitude 169.60E at altitude 302 metres,  which is, of course, Ophir.

Here is a colour-enhanced detail from the above image:

This image shows some very nice Kelvin-Helmholtz instability made visible by cloud.

The weather map for that day shows a large low pressure system slowly making its way across the north end of North island, and a rather flat ridge of high pressure on the south of this cradled across Central Otago.

Courtesy of NASA, the MODIS satellite image (for close to the time the cloud image was taken) shows some filaments of cirrus clouds streamed out by the upper winds which were from the northwest over Central Otago.    Click on the image here to link to the full scale image

Satellite image from MODIS, 8 May 2010

Here are the zoomed-in details over Central Otago with a yellow arrow showing the vicinity where the image was taken.

These clouds show waves rolling along and breaking in the sky,  similar to the way that waves behave on the sea.  It occurs when the change in wind across the boundary between two fluids is so much that steady simple ‘laminar’ flow breaks down and becomes turbulent, and the two fluids mix, with regularly spaced eddies.   The name that has been given to this phenomenon is Kelvin-Helmholtz instability, after Lord Kelvin (William Thomson, 1824-1907)  and Hermann von Helmholtz (1821-1894).   This is just one of many types of turbulence in the atmosphere,.

The following section is for meteorological boffins:

The weather balloon released from Invercargill at noon that day took a sounding that measured the temperature, moisture, density and wind profiles.  MetService aviation forecasters use a specially designed diagram called a Tephigram to help grasp the impact of all this data, and the one attached below has a yellow-red vertical bar attached.  This bar uses a derived parameter, Richardson’s Number or Ri, to show the likelihood of turbulence. Red zones have low Ri and here the wind changes are sufficient to dislodge stratified layers and thus be turbulent.  It is unlikely that the balloon went near the cloud seen in the image above, but the sounding can be taken as representative of the whole region, and so the red bar near 350 hPa can be taken as a indicator that the Kelvin-Helmholtz instability layer was at a height of around 8km above ground level.

Invercargill weather balloon noon 8 May 2010 NZST. Temperature trace on right, dewpoint to left, wind plot is north up, each barb=10 knots, pennant=50 knots.

These breaking waves were made visible thanks to the layer of cirrus cloud.  They can occur whenever the wind and air density change fast enough, as found near a jetstream –  a river of rapid moving air aloft.  Often these breaking waves occur in areas which are cloud free.  Airline pilots have a healthy respect for such areas and call them CAT, or Clear Air Turbulence.   Special aviation weather maps are routinely prepared by MetService to forecast hazards such as CAT, and when a pilot encounters a CAT area a report is sent to alert all aviators.

Thanks to Norman Robinson for agreeing to share this image.  Cloud-watching can be a fascinating and useful hobby.  Identifying the processes at work in the changing sky, and linking these to changes in the barometer and isobars on the weather map, allows us to understand what is happening now, all the better to predict what will happen next.

Addendum:

Here is another wonderful image of breaking waves in the sky – this was sent in by Carol Diehl and seen from west Timaru on 22 Sep 2010

22 Sep 2010