Spectacular contrail outbreak over Wellington

On the afternoon of Monday 19 July 2010, a neat set of four persistent contrails moved across the Cook Strait area.

 NASA’s  MODIS Rapid Response System captured the contrails in the image stream from the Aqua space craft which was over Wellington at about 2:20 pm. You can see the image on the MODIS web site here: http://rapidfire.sci.gsfc.nasa.gov/subsets/?subset=NewZealand.2010200.aqua.1km

 Here’s part of the MODIS image.

MODIS Aqua satellite image in true colour at about 2:20 pm 19 July 2010.

Several things to notice: 

  • There is a contrail in the upper left (labelled NZ152) oriented more or less west-east. I think this was made by Air New Zealand Flight NZ152 which arrived in Wellington at 2:30 pm that afternoon from Melbourne.
  • Then there are four contrails oriented northeast – southwest over Marlborough and Cook Strait. These are labelled 1, 2, 3, 4.  Contrails 2 (and possibly 4) are casting shadows on the ground and sea. The shadows are labelled 2s and 4s.
  • The contrail furthest west (labelled 1) must have come from an aircraft which was on a different route from those that made the other three because it is not parallel with them. This contrail also extends southwest into north Canterbury.
  • The two furthest east appear to converge slightly towards the north. I suspect this is because the (westerly) wind was increasing to the south and so the contrails are being rotated anticlockwise a little.

I took some photos of these contrails from the roof of the MetService building in Kelburn, Wellington at 2:45 pm Monday 19 July 2010, just 25 minutes after this MODIS image. Also, MetService’s web-cam at Christchurch Airport caught the southern end of the easternmost contrail (labelled 1 in the MODIS image) as it moved across Canterbury Plains. 

Two contrails over Wellington Harbour at 2:45 pm on Monday 19 July 2010. This view is towards the north. One contrail (the eastern one on the right) is partly obscured behind the lower cumulus cloud. These two contrails are the eastern most ones in the satellite image.

These two contrails are the western two in the satellite image. This photo was taken at 2:45 pm on Monday 19 July 2010.

Christchurch Airport web-cam photos looking northeast. This is an animation of images at 10 minute intervals from 2 pm to 4:40 pm.

Around Wellington, the wind at the level of the contrails was quite a strong westerly, which explains why the contrails moved quickly across the sky from west to east. The wind in the atmosphere was also increasing with height; this spreads the contrails out enough to make them visible in satellite images. These contrails could still be seen in a lower resolution infra-red satellite image at 5:00 pm when they were 230 km east of Wellington.

In New Zealand, the long distance air routes are all more or less northeast – southwest. Thus, it is easy to see how several aircraft travelling these routes can create a set of parallel contrails. In Europe and North America the air routes are in all directions, and regular grid patterns of contrails are sometimes seen. Sometimes, the contrails in these grid patterns spread out to form a big sheet of high cloud: cirrus or cirrostratus.

This was one of the best and most persistent contrail outbreaks I have observed for some time. Contrails are interesting to watch and sometimes, as with these, there are fascinating details and patterns in the ice clouds as they evolve.

Keep watching.

Wellington Rainbow

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.

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.

Thunderstorms in Wellington

Cumulonimbus (CB, thunderstorm) cloud is not a common part of the Wellington cloudscape, and certainly is not in response to diurnal heating as happens in other regions like Waikato and Bay of Plenty. When Wellington gets thunder and lightning it is because of the development of cumulonimbus other than diurnal heating. CB will develop in a cold unstable air mass, and can be triggered by low level convergence usually caused by the wind flow interacting with the land forms. This is why they can occur at any time of the day in Wellington, and it is what happened on Wednesday morning (6 May 2009) when the southern suburbs were rudely awoken by crashing thunder and the sky lit up with spectacular lightning.

It started at 5:45am when a small trough arrived in Cook Strait. The convergence was between the westerlies ahead, and the southerlies behind the trough being squeezed into the harbour entrance. That was the trigger and away it went! Half an hour later it was over and the trough moved away to the northeast.

lightning_6may2009_wellingt

The map is from our Lightning Detection Network and shows the lightning strikes between 5:45 and 6:10am as red dots concentrated over the southern suburbs and south of Island Bay and the harbour entrance.

wnradar_dbz_decluttered_200

The radar scan at 6am shows the position of the trough line (by its rain echoes) near Wellington and extending out to the east.

I was in Upper Hutt at this time, just getting out of bed. I heard nothing. While I was waiting for the train at 6:40am I could see the top of a CB to the south. “Hmmm. Interesting. Palliser Bay perhaps?”. When I got into the office here, the place was buzzing with excitement, and tales of lightning, hail, and deafening thunder.

Here is a great video posted by Matt Kovesdi. Thanks, Matt.
There is a growing collection of New Zealand weather videos on the video section of our website.