Author Archives: Chris Noble

About Chris Noble

Chris Noble is a Severe Weather Forecaster at MetService.

Layers and streets of cloud

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Clouds come in many different types and are characterised and named according to both their shape and height in the atmosphere.  While a single snapshot in time at a given location may only contain one type of cloud, there are many days when multiple cloud types can be observed in the sky at once. On Saturday, 3 July 2010, we had a nice example in satellite imagery of different cloud types sitting at different levels above Waikato.

Here’s what the satellite image looked like Saturday afternoon for southwestern parts of Waikato near Kawhia Harbour:

(Image courtesy of MODIS Rapid Response Project at NASA/GSFC.)

Looking at that image you can probably spot three different cloud types quite easily. The broad, flat-looking cloud near the top right of the image is an area of fog sitting on the ground. Above this, and casting shadows on the fog layer, are lines or “streets” of puffy looking cumulus cloud. Higher still are thin wisps of cirrus both over the sea and extending onto the land, some of which you can see through to the cumulus and coastline below.

To see how these clouds changed during the afternoon, here’s a short animation of hourly images covering the period from midday to 3pm:

You should be able to spot the area of fog slowly shrinking, while cumulus forms near the coast in warm updrafts over the land eventually spreading inland across Waikato as streets of cloud.  The lower quality of this animation makes the small wisps of cirrus hard to spot.

So how do these cloud streets form?
One might be tempted to compare each individual cloud street to a smoking chimney, and that is perhaps partially true. The cloud continuously forms at a stationary point, over a coastal hill for example (acting like a chimney), and is then blown away by the wind – a southwesterly in this case. However, this doesn’t completely explain the situation as you might expect the cloud to form all the way along the coast, and then move inland as a solid layer, not as lines of cloud with gaps between them. The answer is clearly a little more complicated and is best illustrated by the following image from Wikipedia:

(Image courtesy Daniel Tyndall, Department of Meteorology, University of Utah, via Wikipedia)

Clouds form when rising air cools and reaches its saturation point, causing the moisture in the air to condense into water drops. Here we see lines of rising air or updrafts between counter-rotating tubes or rolls of air that are aligned with the flow. In between the updrafts are lines of sinking air in which cloud can’t form. The result is narrow lines of cloud, separated by narrow lines of cloud-free air.

Note that this is quite a different process to lines of cloud that are often observed downstream and parallel to a mountain range and perpendicular to the wind flow, but that story can wait for another blog post …

Thunderstorms!

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The last week in January 2010 will be remembered by many over the central North Island for the frequent thunderstorms that developed in the afternoon, often lasting well into the evening. Conditions changed little during the week with a slack pressure gradient over the North Island allowing afternoon sea breezes to combine with abundant low level moisture, triggering heavy showers and thunderstorms inland. Many of these storms were slow moving, prompting a number of Severe Thunderstorm Warnings as radar detected torrential rain and hail in some cells.

To illustrate the impressive number of lightning strikes that resulted, and just how widespread the thunderstorms were, here’s the North Island strike maps (courtesy MetService and Transpower) covering the afternoon and evening periods (all about 12 hours) from Monday 25th through to Thursday 28th January.

Monday 25th - more than 9,900 strikes:

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Tuesday 26th – more than 32,200 strikes:

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Wednesday 27th – more than 18,900 strikes:

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Thursday 28th – more than 17,100 strikes:

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The colours of the lightning strikes give an indication of when the strikes occurred, with each colour on these maps representing two hours worth of lightning. The times are pink and blue for early to mid-afternoon (about midday to 4pm), green and brown for late afternoon and early evening (about 4pm to 8pm) and orange and red for late evening and night (about 8pm to midnight). Armed with this knowledge you can now deduce where the lightning started in the afternoons and where the last strikes were in the evening. For example, on Wednesday some of the first strikes occurred on a line from South Taranaki to Gisborne with the last strikes of the day occurring in Waikato near Hamilton.

In addition to the colours for each lightning strike, you’ll also note different symbols on the maps. These tell us something about the nature of each strike including the charge. Cloud-to-ground strikes are plotted as a ‘plus’ or a ‘circle’ for positive and negative strikes respectively, while cloud-to-cloud strikes are plotted as a ‘square’.

While it’s not unusual to get afternoon and evening thunderstorms in summer, this week certainly stands out as a particularly active period with a large number of strikes on four consecutive days. And as I write this on Friday afternoon we are again expecting more thunderstorms inland over the North Island today (but perhaps not as many strikes as the above examples).

Lastly, I’ll leave you with a nice example of a large thunderstorm with anvil seen over the Wairarapa on Wednesday 27 January. I took this  panorama from the roof of the MetService building at about 7:25pm:

Click for full size image.

Wellington Rainbow

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With a superb view over Wellington and the harbour from the MetService building in Kelburn, we’re often (and quite appropriately) treated to some fantastic weather related vistas. Here’s a little sample, snapped on Monday 26 October as a few light showers passed over the city in a northerly flow late afternoon.

Wellington rainbow

The view here is looking east, and with a clear western horizon behind us the sun angle was just right for some rainbow spotting over the city. The main rainbow, caused by a double refraction and single reflection of sunlight by raindrops, is clearly visible. If you squint hard enough towards the upper left of the image you might just make out the fainter secondary bow (resulting from two reflections of sunlight inside the raindrops).

For more information about the technical side of rainbows and how they are formed, head over to the Rainbow article at Wikipedia.

Mt Taranaki Kármán Vortex street

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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.

Canterbury Snow, 10 May 2009

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With clear skies over most of Canterbury on Monday, we got a good look at the fresh snow that fell on Sunday (10th May).  Here’s the view late Monday morning (around 10:30am) from NASA’s Earth Observing System Terra Satellite,

2009-05-10-2225

Fresh snow on the Alps and Canterbury foothills - Monday 11 May 2009. (Image courtesy of MODIS Rapid Response Project at NASA/GSFC.)

Based on the coverage in that image and reports from snow observers, the bulk of the snow in South Canterbury fell above about 300 metres, although some places lower down, especially near the foothills, may still have had light snow that didn’t settle appreciably.

While this wasn’t the first cold outbreak of the year, Sunday’s snow event over the lower South Island (including Fiordland, Southland and Otago) was certainly the most significant of 2009 to date.  If you have any tales of how you were affected that you’d like to share, please feel free to leave a comment below.