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.

Contrail over Tauranga

I received an inquiry by way of the Editor of the Bay of Plenty Times from Laurie Sanders of Tauranga with some photographs of an interesting cloud formation. Laurie saw it as he was going to work southeast-bound on 15th Avenue at the Cameron Street traffic lights at 7:40 am on Wednesday, 9 December 2009. He took several photos, and I have reproduced two of them here with his permission.

I think the cloud formation is a contrail from the Aerolineas Argentinas flight AR1182 from Buenos Aires to Auckland, which landed at 8:23 am on Wednesday 9 December 2009. The aircraft used on this route is the 4-engine Airbus A340. The great circle route from Buenos Aires to Auckland dips well south, and the approach to Auckland is from the southeast. The view direction of the photo is towards the southeast along 15th Avenue, and the continuation of the trail would cross the southwestern part of the sky towards the northwest and Auckland. Both the timing and the orientation of the contrail are consistent with its being from that flight.

For contrails to form, the ambient air temperature needs to be lower than a certain temperature. This critical temperature mostly depends on the altitude and the relative humidity. It depends on the jet engine type, and engine efficiency to a lesser degree. Usually contrails dissipate a few plane-lengths behind the aircraft, but if the air is moist enough to be ice-saturated they will remain in the sky and behave like any other cirrus cloud. Persistent contrails eventually dissipate or merge with other cirrus.

Atmospheric measurements by balloon-borne instruments released from Chatham Islands, Paraparaumu and Whenuapai indicate that conditions at jet cruising altitude were suitable for the formation of contrails. Also, the humidity was high enough that the air there was ice-supersaturated, meaning that contrails would not dissipate as they do in drier conditions. The aircraft probably commenced its descent into Auckland somewhere over Rotorua and out of the persistent-contrail layer. This is why the trail in the photo stops abruptly. It is possible the plane could have been observed to be making a short contrail for a few more kilometres after the point where the persistent trail stopped, but Laurie says he did not see it.

In the first photo, further back along the track of the aircraft, where the contrail is older, the wind is distorting the trail and spreading it a little. In the second photo, taken only 30 seconds later, the youngest part of the trail is also being visibly distorted compared to its appearance in the first photo.

Aerolineas Argentinas has recently increased the frequency of its services. There are now five flights each week, scheduled to arrive in Auckland at 7:55 am on Tuesday to Friday, and on Sunday. The return flight is on the same days and departs from Auckland at 7:30 pm, having been to Sydney and back in the meantime. Interested observers could watch for these in the future. Be aware that, due to the long duration of the flight and the variability of winds en route, the arrival time over New Zealand may be ahead or behind schedule by some time. This particular flight was about 30 minutes behind schedule; on other days it could be that far ahead.

Lan Chile and QANTAS also have flights to and from Santiago on six days each week (all except Wednesday), and they operate the same aircraft type. The scheduled arrival time at Auckland is 4:20 am, so observers will need to be up early to see anything of it. Any contrails will be visible in the night sky if the moon has enough of its disk illuminated. They return to Santiago the same days at 4:40 pm and, as with the Aerolineas Argentinas service, have been to Sydney during the day.

Because Buenos Aires is further east than Santiago, the Aerolineas Argentinas flights will usually be further south in the sky over the Bay of Plenty and Gisborne. The initial great circle path heading from Auckland to Buenos Aires is 138°, and to Santiago it is 130°; however, the route flown on any given day will vary depending on forecast wind conditions.