Winds Aloft

In my previous blog post I wrote about how the winds high in the sky differ from the winds we usually experience on the surface of the Earth.

We can get an appreciation of the winds aloft by looking at a loop of images from a geostationary satellite. The Japan Meteorological Agency launched the satellite we use most in New Zealand, and it orbits the earth at an altitude of 35,000 kilometres above Papua New Guinea. It’s specially configured to move in an orbit synchronized with the rotation of the Earth, hence the name geostationary (meaning Earth-stationary).

Take a look at this loop of satellite pictures for the week 28 July to 4 August 2008:


The images are similar to what you see on the Maps & Rain Radar page (by selecting Satellite Imagery), TV or newspaper, but with added colour. Firstly, you must be aware that the satellite is “seeing” in the infra-red, like wearing special night-goggles to look at the Earth. We’ve then applied a colour enhancement to highlight what’s happening. With this particular enhancement, cold surfaces are shown as light grey or white and warm surfaces are a darker grey or black; very cold surfaces, typically clouds high in the sky, are coloured red (-50 C) or green (-60 C); there are even a few patches of blue at -70 C, indicating extremely cold cloud that is high above the Earth.

We can learn so much by looking at a loop such as this!

First let’s look at the area across central Australia. Over the dark background (the warmer Australian landmass) there are bits of white and red cloud high up above the land, caught up in a very strong westerly jet-stream of air that’s racing towards the Tasman Sea and New Zealand. A weather balloon flight at a height of 10 km above Wagga Wagga (New South Wales) reported a wind of 356 km/hr around the time of the middle of the loop (1 August 2008). Winds over Sydney peaked at 302 km/hr. You almost never get winds that strong near the Earth’s surface.

Over the Top End of Australia there are filaments of high cirrus that originated near the equator and are moving towards the jet-stream. A bit farther down to the south there are the comma-shaped clouds of lows, plus intervening highs, that we see on weather maps trundling through our patch of the Earth, the mid-latitudes.

If you look at the loop several times, you’ll notice patches of dark grey cloud (near the ground or sea) swirling around in quite different directions (and speeds) from the white or coloured cloud higher up.

As an interesting aside, notice how the Australian landmass seems to pulse from dark to light each day? This happens as the land warms from the sun in the daytime (dark shading) and cools at night (light grey shading). The effect is strongest over the southern states.

The jet-stream isn’t apparent at the very top and bottom of the images, and this is quite typical. Near the equator and near the poles the winds aloft are not as strong as in-between.

I think this loop gives us insight into why the wind is different high in the sky. I’ll continue this thread in a later post.

Severe Thunderstorm Warning Service

From Wednesday 01 July 2009, MetService will provide a Severe Thunderstorm Warning Service. This blog entry explains why we’re now able to do this, why warnings of thunderstorms are different from warnings of broad-scale weather events, which parts of New Zealand they will apply for, how you can receive them and what actions you can take to protect yourself.

Why is MetService introducing a Severe Thunderstorm Warning Service?

Over the last six years or so, four important developments at MetService have together made it possible to provide a warning service for localised severe weather:

  • Expansion and enhancement of the weather radar network
  • “Next generation” radar processing tools
  • Fast, automated tracking tools for severe thunderstorms
  • Thunderstorm forecasting methods and forecaster expertise.

What is “localised severe weather”?

Most of New Zealand’s severe weather is brought about by big weather systems like lows and fronts. But some severe weather events are the result of severe thunderstorms.

Severe weather that is related to thunderstorms usually occurs over a much smaller area, and lasts for a much shorter time, than severe weather related to lows and fronts. This is because thunderstorms are much smaller weather systems than, and last for much less time than, lows and fronts. But the sheer intensity of the rainfall, hail and/or wind associated with some thunderstorms can pose just as big a threat to life and property.

Below is an example of the difference in scale. Broad-scale heavy rainfall over northern New Zealand (the first image) is of far greater extent than a local severe thunderstorm in coastal Bay of Plenty (the second image, circled).

Northland Rain 10 July 2007
Heavy rainfall over almost all of Northland and parts of Auckland and Coromandel, 7:21am 10 July 2007. Overall, this heavy rainfall event lasted at least a day. According to the Insurance Council of New Zealand, about $60 million in insurance claims resulted from this event.
Severe thunderstorm over coastal Bay of Plenty 11 May 2009
Severe thunderstorm just east of Tauranga, 11:59 am 11 May 2009. This event lasted a few hours altogether, and much less than that in any one location. While there is a lot of echo on the radar scope, the small area near Tauranga is the only place where severe weather is occurring. A narrow swath of heavy hail affected the Bay of Plenty coast from near Tauranga to near Opotiki. In locations more than about a kilometre from the thunderstorm, severe weather didn't occur.

When do severe thunderstorms occur?

In eastern and inland parts of New Zealand, severe thunderstorms primarily occur on some afternoons and evenings in spring and summer. In western parts of New Zealand, though, they can occur any time. For example, there were tornadoes in New Plymouth on 4 and 5 July 2007 – the middle of winter. And they can occur in the middle of the night.

The Warning System

The size and lifetime of a weather system (for example, low, front, thunderstorm) influences how far ahead in time the weather related to it can be forecast. The warning system for small-scale severe weather mimics that for broad-scale severe weather, as shown below. But when small-scale severe weather is expected, the forecast has much less lead time.


How often do severe thunderstorms occur?

Thunderstorms occur reasonably often in New Zealand. Severe thunderstorms are much less common, occurring perhaps once a month or less. Your chances of experiencing one are fairly small. But if you are under or near one, it is important that you know what to do to try and keep yourself and your property safe.

What can I do with less than 60 minutes’ notice?

Quite a lot. A few simple examples are:

  • Torrential Rain: If you’re in a narrow watercourse or working in a stormwater drain, get out of it
  • Very strong winds: If you’re up on the roof, get down, secure loose roofing iron and other potentially dangerous flying objects
  • Lightning: Go inside, or at least stay away from trees which are out in the open
  • Hail: If you’re driving, be ready to slow down or stop

How does MetService detect thunderstorms?

Weather radar is essential for the detection and forecasting of small-scale severe weather events. The amount and quality of data decreases with distance from a weather radar. For these reasons, warnings of severe thunderstorms will only be available within about 150 kilometres of a weather radar.

How many weather radars does MetService have, and where are they?

In the maps below, the bright blue areas are both within 150 km of a weather radar and contain enough weather radar information to provide Severe Thunderstorm Warnings. Siting weather radars in New Zealand is a challenge because hills reduce, or in some cases completely block, the radar signal. This is why, for example, the Canterbury radar provides no coverage of the South Island West Coast.

MetService weather radar network as of June 2009
Intended radar coverage in about 2012 (with the exception of Otago, for which funding has not yet been secured). Over the next three or so years, weather radars will be installed on Mahia Peninsula (third quarter of 2009), in western Bay of Plenty, Northland and on the South Island West Coast. Sites for radars in western Bay of Plenty, Northland and on the South Island West Coast are being pursued but have not yet been finalised.

Who does the warnings?

MetService has a group of very experienced forecasters whose focus is severe weather. Every day, one of the things they do is issue the Severe Weather Outlook and the Severe Thunderstorm Outlook. The Severe Weather Forecasters issue Watches or Warnings when they expect the warning criteria to be met.

Severe Thunderstorm Warnings are a bit different …

Because they are not based on radar information, the Severe Thunderstorm Outlook and Severe Thunderstorm Watch are available New Zealand-wide.

In contrast, Severe Thunderstorm Warnings are referenced to radar areas. Further, no Severe Thunderstorm Warning will be issued for a location greater than 180 km from any of New Zealand’s weather radars.

A Severe Thunderstorm Warning will only be issued once a severe thunderstorm exists and can be tracked. This might be 30 minutes after it developed. In other words, by the time you receive a Severe Thunderstorm Warning, there is already a severe thunderstorm.

How can I receive Severe Thunderstorm Warnings?

Because Severe Thunderstorm Warnings will have short lifetimes, they need to be delivered as quickly as possible. They will be available

  • On
  • By email (words only; no picture) via publicly subscribable email list
  • To the media

Severe Thunderstorm Warnings will be based around weather radars rather than around districts. That is, the occurrence of a severe thunderstorm within, say, the Wellington radar area will prompt the issue of a warning for the Wellington radar area. Prominent in the warning heading will be the location it applies to (e.g., Blenheim, Paraparaumu, Upper Hutt, etc).

Appendix I: Example



Issued by MetService at 5:53 pm Saturday 03 January 2009

Valid until 6:37 pm Saturday 03 January 2009

This warning affects people in the following local government areas:


At 5:37pm, MetService weather radar detected severe thunderstorms near MOTUNAU BEACH, PEGASUS BAY, SCARGILL and GRETA VALLEY.

These severe thunderstorms are moving towards the northeast, and are expected to lie near CHEVIOT, DOMETT and BLYTHE VALLEY at 06:07 pm and near CHEVIOT and OFFSHORE HURUNUI at 06:37 pm.

These thunderstorms are expected to be accompanied by very heavy rain and large hail.

Very heavy rain can cause surface and/or flash flooding about streams, gullies and other low lying areas, as well as make driving conditions extremely hazardous.

Large hail can cause significant damage to crops, orchards, vines, glasshouses and vehicles, as well as make driving conditions hazardous.

Very heavy rain associated with this storm has been reported earlier from CHRISTCHURCH RADAR BUT THE TREND IS NOW FOR WEAKENING.

Large hail associated with this storm has been reported earlier from CHRISTCHURCH RADAR TO AROUND 20MM DIAMETER BUT SIZE IS LIKELY NOW DIMINISHING.


MetService advises that as storms approach people should:

  • Take shelter, preferably indoors away from windows
  • Avoid sheltering under trees if outside
  • Move cars under cover or away from trees
  • Secure any loose objects in the vicinity of your home
  • Check that drains and gutters are clear

During and after the storm, people should:

  • Beware of fallen trees and power lines
  • Avoid streams and drains as you may be swept away in flash flooding.

This warning is due to be updated or lifted by 06:37 pm Saturday 03 January 2009.

NOTE: Only severe thunderstorm tracks are shown in the graphic. Other thunderstorms may exist in the area.

MEDIA PLEASE NOTE: This warning will be updated within the next hour or so. MetService would appreciate it being broadcast regularly during this period.

Appendix II: What is a thunderstorm, anyway?

A thunderstorm is a local storm produced by a cumulonimbus cloud. In the photograph on the right, the tall cloud in the middle of the frame with an anvil-shaped top is a cumulonimbus cloud. (And without weather radar, there is no way of knowing whether this is a severe thunderstorm … unless you’re underneath it).

Impressive cumulonimbus (thunderstorm) seen from MetService. Image courtesy Allister Gorman, MetService.

Thunderstorms are the result of strong updraught of air throughout a considerable depth of the atmosphere but over a small area. In New Zealand, thunderstorms tend to be associated with

  • Vigorous, fast-moving cold fronts moving from west to east across New Zealand. These occur at any time of the year, night or day.
  • Southerly changes along the east coast, especially Canterbury, and especially during the warmer part of the year.
  • In both these situations, the thunderstorms are commonly arranged in lines along or near the front / southerly change.

  • Afternoon and evening “build-ups” inland during the warmer part of the year.
  • Humid north or northwest flows driven up over high ground.
  • In these situations, the thunderstorms occur more randomly.

Appendix III: Severe Thunderstorm Criteria

A Severe Thunderstorm Warning will be issued when one or more of the following is expected:

  • Heavy rain (from thunderstorms):
    • Rainfall of 25 millimetres per hour, or more.
  • Large hail:
    • Hailstones 20 millimetres in diameter, or larger.
  • Strong wind gusts (from thunderstorms):
    • Gusts of 110 kilometres per hour (60 knots) or stronger.
  • Damaging tornadoes:
    • Fujita F1 (wind speeds greater than 116 kilometres per hour (63 knots)) or stronger.

Appendix IV: Thunderstorm-Related Hazards

  • Rainfall intense enough to cause localised or flash flooding – perhaps some miles away from the storm, where it might not even be raining
  • Lightning
  • Hail
    • Large enough to cause injury
    • In sufficient quantity to cause drifts
    • Individual storm swaths which are generally tens to a hundred or two metres wide and at least a few kilometres long
  • Winds (straight-line or tornadic) strong enough to
    • Cause structural damage, blow trees over, etc.
    • Generate dangerous flying debris
    • Blow people off their feet
    • Blow vehicles over
    • Damage swaths as for hail.

Mt Taranaki Kármán Vortex street

MetService weather forecasters naturally spend a lot of time looking at satellite imagery and every so often are treated to some fascinating cloud patterns in the airflows around New Zealand. One pattern I’ve always liked seeing is the Kármán Vortex street, most frequently observed near our shores to the west of the North Island, generated by Mt Taranaki in a south to southeast flow.

Put simply, a Kármán Vortex street is a series of vortices (or eddies) generated in the flow past an obstacle. When wind, cloud and stability conditions combine “just right” over the west of the North Island, the result can be a spiral pattern in the cloud moving away from Mt Taranaki. Yesterday (Sunday, 21 June) we saw an example of this:

Kármán Vortex street west of the North Island, 10:15am 21 June 2009.<br /> (Image courtesy of MODIS Rapid Response Project at NASA/GSFC.)
Kármán Vortex street west of the North Island, 21 June 2009 (Image courtesy of MODIS Rapid Response Project at NASA/GSFC.)

For more information about Kármán Vortices, head over to this Wikipedia page where you’ll also find this nice little animation demonstrating the phenomenon:

Karman Vortex street animation
Kármán Vortex street animation (image courtesy Cesareo de La Rosa Siqueira, via Wikipedia)

Note in the animation above that successive vortices are spun off each side of the obstacle and then move downstream in the flow (left to right in this case).

To help visualise the large scale weather pattern yesterday, here’s the analysis map from midday showing a high southwest of the South Island and a generally southerly flow over the country:

MSL analysis, midday 21 June 2009.
MSL analysis, midday 21 June 2009.

And to dive further into the situation, here’s a “QuikSCAT” image from earlier yesterday morning showing wind barbs over the ocean – depicting the strength (in knots) and direction of the sea-surface winds downstream (north and northwest) of Mt Taranaki:

QuikSCAT image showing surface winds. (Image from NOAA/NESDIS)
QuikSCAT image showing surface winds. (Image from NOAA/NESDIS)

While these winds were measured around two hours before the satellite image the situation didn’t change much so will be representative of the low level wind flow resulting in the Kármán Vortex street. In this case the surface wind downstream of Mt Taranaki was south-southeast at around 20 to 25 knots.

For some more dramatic examples of cloud vortices, head over to the MODIS Rapid Response System website where their handpicked gallery features a number of vortex images.

National Fieldays 2009

MetService forecaster James Lunny (second from right) manning our trade display at last week’s National Fieldays.

MetService has been attending Fieldays for the past 15 years, and this year our display proved as popular as ever, attracting several thousand people.

This year we had a prize draw for a wireless weather station inviting people to subscribe, for free, to our new auto-email service for our seasonal outlook.

This service is almost ready to be launched so keep an eye out for it.  You can access this on our website here

There were 250 entries into the draw and the winner was CRAIG BROWN OF MORRINSVILLE. Congratulations Craig, your new wireless weather station should help on the farm.

Also available at our show stand, as you can see on the floor in the image, were show bags containing full colour posters with information on winds and clouds, as seen on the wall to left.  These are available here.  During the show we got to see in the sky almost all of the cloud types shown on cloud poster, and it was great to discuss the clouds with other cloud watchers, commenting on bumpy bottoms and bumpy tops.

There were occasional wet periods at Fieldays this year, and we noted that when it rained more people came inside and chatted with us.  Since we had online access to our weather radars at the show we knew exactly when the rain would be coming and going  :)

Attending Fieldays gives us an opportunity to speak with rural people whose livelihood depends on the weather, and listen to theirs wants and needs.  This helps MetService as we plan to change things on our website, concentrating on doing what we can to match what you want.  Our Rural Weather section started out as the result of a survey we held at National Fieldays 2006 and has been tweaked following feedback from last year’s National Fieldays and Royal Show.  As Weather Ambassador, I gave several talks at Fieldays about weather maps and how the winning formula for using a weather forecast involves tweaking our ideas with your own local knowledge.  In similar fashion, our winning formula is to tweak our web site with the valuable feedback we picked up at this show.


MetService forecaster James Lunny showing Weatherscape and at Fieldays 2009.

Bob McDavitt



Clouds: Their changing shapes often occur over a time-scale and space-scale that we humans can not always fully appreciate unless we use time-lapse photography.

In the international cloud naming scheme used to describe and identify clouds, there are ten basic characteristic cloud FORMS or TYPES or genera (nouns):  Cirrus, Cirrostratus, Cirrocumulus, Altostratus, Altocumulus, Nimbostratus, Stratocumulus, Stratus, Cumulus, and Cumulonimbus.

To further describe clouds there are several accepted and defined adjectives covering 14 cloud species (shape and structure), 9 varieties (arrangement and transparency),  9 supplementary features, and two words (genitus, mutatus) describing growth.   Click here for a table of these words.

There has recently been a call from the Cloud Appreciation Society of the United Kingdom to ask The Royal Meteorological Society to apply to the World Meteorological Organization to officially add a new variety or species of cloud to the international scheme.  The new word is “ASPERATUS”, after the Latin word for rough, and is intended to be used as an adjective to describe those clouds whose underbellies look like the surface of choppy sea.

This story was covered in the UK here and here and then in NZ by Radio NZ National on the Panel.


Photo credit to Bill Slater, taken near Hanmer Springs on 2 March 2005 and winner of the Met Society Photo competition, shows an example of what ASPERATUS implies.  Bill explains “It was a fine day and we first noticed some round disc like clouds at fairly high altitude. We commented that they were like flying saucers. Then as we reached Hanmer Springs we started to see these swirls and dangling clouds, looking back towards the Lewis Pass no rain ever fell.”

“Asperatus” clouds form when there are two (or more) layers of air of differing density, one sitting on the other.  The cooler and higher layer is cloudy and the other layer is clear.  The boundary between these layers may occasionally get knocked up, but will return downwards thanks to gravity and then may go further down but will return back up thanks to buoyancy.  This creates a wave-like surface along the cloud base, and we call these gravity waves because the returning force is gravity and buoyancy.

Yes, the waves on the surface of the sea are a good example of this process.

Another good example is when moist air blows over a range of mountains and makes a system of mountain wave clouds.  In New Zealand this often happens, and people in Canterbury call the mountain wave clouds  “the northwest arch”.

At first individual Altocumulus lenticularis clouds form,  but as a front approaches, upper-level moisture increases and middle and high clouds combine to produce an arch cloud comprising Altocumulus, Altostratus, and Cirrostratus.  This arch cloud displays a very sharp edge near the mountains and often there is an arch of clear sky immediately downstream of the mountain divide.

We can cope with the current naming scheme and use Altocumulus lenticularis to describe the NW arch clouds, but it would also be useful to have the extra variety or species word “ASPERATUS” especially when there are undulations in the cloud base.


The MetService cloud poster already has a special photo devoted to the NW arch cloud.  At present it is just classified as “Northwest Arch    Ac/As/Cs”, but if the word “ASPERATUS” is officially accepted then we are ready and waiting.

If you are coming to the National Fieldays at Mystery Creek 10-13 June then pass by the MetService display in the main pavilion and ask for your own complimentary full-sized cloud poster.

The Wind-sock of the Lower North Island

In my previous blog post I wrote about how much the winds high in the sky differ from the winds that we are accustomed to nearer sea level. The winds aloft are usually much stronger than those near the earth’s surface, this difference being  especially true in New Zealand. Another difference (which I mentioned in the last blog post) is that the wind we experience every day is more variable, suggesting that the wind aloft is more unchanging. This is true, with westerlies being most common at altitude over all parts of the country.

Mount Kaukau in Wellington
Mount Kaukau in Wellington

This difference in wind is apparent even between sea level and just a few hundred metres up. For example, consider Mount Kaukau, a hill standing about 500 metres above Wellington. Mount Kaukau has a wind-speed recorder (called an anemometer) that consistently reports much stronger sustained winds than at Wellington airport near sea-level. An interesting side-effect of this is that the wind at the airport is, paradoxically, more gusty!

Typical variation of wind with height
Typical variation of wind with height. If the wind is strong, air flows through the orange "sock" making it nearly horizontal.

When I worked in Manawatu a pilot once told me that they thought of the anemometer on Mount Kaukau as “the wind-sock of the lower North Island”. There is a lot of meaning and understanding behind this statement.

The wind aloft is more predictable than the wind nearer sea level. This, together with the interaction between the winds at different levels, suggests that the upper wind can be used as a predictor of what the wind is (or will be) like lower down. An astute pilot can make valid inferences about differences in local weather in the lower North Island based on subtle changes in wind direction at the “wind-sock of the lower NI”; Mount Kaukau winds are also a good guide to winds in Cook Strait. Such inferences are especially useful in New Zealand where the interaction between wind flow and topography has a really big effect on the weather.

You can make your own deductions about the variations in wind by viewing our ski field page over the coming season. Compare the winds at the ski fields with those at lower-lying nearby stations (by selecting a North or South Island station).

We may next want to ask: why is it that the wind is so different high up in the sky? I will discuss that issue in a later post.