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.


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

Wet and windy around Friday the 13th

The wet and windy conditions that battered parts of New Zealand around Friday 13 August were produced by what looks on the weather maps to be just a ordinary passing front.

Click on the weather map below to watch the weather sequence.

Click to view full sequence of weather maps from 11 to 15 August 2010

Severe gales

In our brief mountain forecast issued on Wednesday 11 August, MetService increased the forecast winds for Thursday over the Canterbury alps to severe gale, the top of the scale for mountains.  Indeed, the winds whistled to over 200 kph at Mount Hutt.

Weather map for 12 Aug 2010

The explanation can be gleaned to some extent  from the weather maps.  Winds rotating counter-clockwise around the large high were being flung outwards at low levels (see rules number 3 and 4 for reading weather maps), encountering the winds rotating clockwise around the deepening low in the Tasman Sea.

These flows combined something like the arms of a kitchen eggbeater, making an accelerating northerly wind reach the Canterbury alps.  Also, the high-pressure system left behind layers of stable air which acted like a lid,  pressing down on that northerly wind as it  encountered the mountains.

When this wind found a gap between the mountain tops it shot out like water does through the nozzle of a hose on power flow.

Deluging rain

On Wednesday 11 August, MetService started a sequence of Severe Warning Watches and Warnings for heavy rain associated with an incoming active front.  Warnings for heavy rain were issued for western and northern areas of both the North and South Islands, as well as for Coromandel, Bay of Plenty and Gisborne mountains north of Ruatoria, with peak daily amounts of around 200 mm for the Westland and eastern Bay of Plenty ranges.

The map above shows the accumulation of rain measured in the MetService network for the week ending 9am Mon 16 August.  Most of the rain in that period was from the passing front.   MetService was picking early on that this front would be able to deliver heavier than normal amounts of rain.   This was because it was given a boost of water vapour from an unusual source —

Click on this image to watch loop of ten days ending 15 August (1.6Mb mpg).

The above image is from The University of Wisconsin Space Science and engineering centre data centre web site and shows a global composite of imagery from the water vapour channel of several meteorological satellites.  Clouds are full white and other parts of the atmosphere are shown in shades of grey according to the amount of water vapour they carry (black=dry air)..  Energy for our weather comes from the sun in part by evaporating surface water.  This imagery shows how evaporated water vapour makes its way around the planet in moving rivers of air, acting as fuel lines for weather.  The red circled area above, on day 222 (10 August) shows moist air departing the Coral Sea region and heading for the Tasman Sea.  It was this water vapour that made last week’s front somewhat out of the ordinary, as picked up in MetService’s warnings.

Saturday’s frontal lull

Winds observed Noon Sat 15 August. Each barb=10knots, wind flies from barbs to point.

Notice that as the front crossed the Auckland region on Saturday it started to buckle and twist. This buckle seemed to start when the front encountered Coromandel.  Winds measured at noon on Saturday were gale force about Great Barrier but there was a lull in the wind just following the front in the Hauraki Gulf.  This buckle could easily have stepped back towards Auckland bringing the gales with it, so a marine wind warning was kept in force for Hauraki Gulf for that afternoon, in spite of the frontal lull.   A fleet of 137 started the Shorthanded Sailing Association’s Shorthaul and Longhaul 60 yacht race, but this frontal lull killed their wind and only 29 could finish in the allotted time.  Such is the fun we get with sailing.

On Saturday afternoon, in particular, it was interesting watching the front via the MetService Tauranga web cam (click on web cam and move mouse left-right to time-track).

So this front posed several problems to people in both their work and play.  Sometimes a debrief such as this, after the event, is a useful adjunct to help our understanding of the weather.

A row of teeth

Email received at

Hi there good people from the met service….

The attached photo was one I took the other day while out between jobs.

I was intrigued with the formation as I’d never seen anything like this before & it looked like a row of teeth.

We live in Blenheim & it’s taken from the northern side of Blenheim looking roughly ESE towards the White Bluffs. The forecast was for southerlies arriving later that day.

This is one of those curiosity killed the cat type questions – but I’ve never seen this cloud formation before and keen to find out the reasons and weather that formed this cloud formation.



Row of teeth - Al Hendrickson

Response from Bob McDavitt of MetService as part of our “enquiries” team:

Hi there Al, and thanks for your photo.  I think we are looking here at a layer of air that is showing signs of getting cooler and/or more moist due to changes in the winds aloft.

The middle clouds in the background are Altostratus and show that the air is generally stable and flat, as when an anticyclone is moving across the region.

Here’s my conjecture.  The row of teeth started off as row of cumulus clouds — with a base well above ground level — not sure how high but I think they are near or above the “freezing level”.   So that would make them Altocumulus  or Ac (Alto means middle , as in the alto voice in a choir).   The clouds are forming in a layer where the air is so buoyant that it is able to rise, and there are clear gaps in-between the clouds where there are counterbalancing sinking zones.

The Ac clouds can only rise a limited way and then they show signs of leveling out— this indicates a layer on top of them of warm/dry air.

But inside each cloud there is sufficient moisture to allow some to fall out in the form of ice crystals *.    As these fall below the base of the cloud they encounter some dry air and evaporate away…. What is left is a “fall streak” or a downward growing protrusion from the cloud called virga. Some of this has been perhaps been caught up in the wind along the base of the clouds and strung out along them, marking the “gum line” — or this could be just what appears from this viewpoint.

* I say ice crystals because they appear so white — we can get virga from ordinary cumulus clouds that form rain (below the freezing level) but that virga is usually greyish-looking


Thanks again to Al Henrickson for sharing this unusual cloud formation with us all.

If you spot some unusual clouds and would like to seek an explanation then send an email to  or go to our contact page.

Bob McDavitt