Shortest day today. Coldest day still to come.

Last night was the longest of the year, and today is the shortest day.

The time from sunrise to sunset is shorter today, by a few seconds, than the surrounding days.

The winter Solstice, or that point in time at which (from our viewpoint) the sun was furthermost to the north, occurred at 5:16am New Zealand local time.

Click here to see a previous MetService blog that has a graph of the sun’s changing position from our viewpoint during the year.

In Coordinated Universal  Time (UTC), this winter solstice was at 17:16 (hours: minutes) on Tuesday 21 June.  Some calendars printed overseas have June 21 marked as the shortest day, but for us it is Wednesday 22 June 2011.

The actual date of the solstice varies a little from year to year, and  wanders towards the 22nd as we  approach a leap year.  After the leap year adjustment is made and the solstice and equinox are corrected back toward their natural date, the 21st.

There is an old saying that goes something like this: “when the days start to get longer, the cold gets stronger”.  To help appreciate what that means, look at the past data page for a site near you from our ‘Towns and Cities’ or ‘Rural’ page.    Auckland is shown below as an example:

On our website you can read individual data points by using the ‘mouse-over’ option.

Notice how the long-term average of the minimum temperature for Auckland,computed monthly, reaches its lowest value of 1.9C in July, while the long-term average of the maximum temperature , computed monthly, reaches its lowest of 17.6C in August.  The coldest sea temperatures of the year in Auckland are just below 14C and usually occur in late August.  For Auckland, the warmest air temperatures usually occur on or within a few days of Waitangi Day and the warmest sea temperatures follow around ten days later.

In New Zealand, the coldest air temperatures of the year occur between mid-July and early August, around 3 to 6 weeks after the solstice.   This seasonal lag varies around the country, and is longer in the north than in the south.  The lag reflects the time required for the sun to warm the earth’s surface. In New Zealand, because of the strong effect of ocean temperature on our climate, the lag is strongly tied to the response of the oceans to changing sunlight.  Click here to read more. However, this lag is also seen in places with a continental climate – i.e. not directly affected by sea surface temperatures – where it reflects the time it takes to warm up the ground.

The graph above shows that, in Auckland, May 2011 was warmer than normal, about as warm as an average April.  So far, June has also been warmer than normal.  This has been caused by the frequent presence of  strong low pressure systems in the Tasman Sea, which have fed persistent, moist, mild northerly winds onto NZ, with very few days of cooler southwest winds.  There is still time for this pattern to change.  The latest seasonal weather outlook for July, located here, indicates that some parts of NZ are likely to see a change in weather pattern over the next few weeks.  For them, the ‘cold will get stronger’.

New Plymouth tornado, Sunday 19 June 2011

The tornado in New Plymouth early on the morning of Sunday 19 June 2011, like every tornado that passes through an area where people observe it, was certainly dramatic.

Like many tornadoes that affect Taranaki, this tornado formed out to sea – not very far to the northwest of New Plymouth – in a line of thundery showers.

Radar imagery

At 4:15am, a mesocyclone (to be explained later in this blog) is identifiable over New Plymouth in imagery from the New Plymouth radar. Radar imagery 7.5 minutes earlier, at around 4:07am, hints vaguely at the mesocyclone’s existence but is far from conclusive. Radar imagery at around 4:22am suggests the mesocyclone either has decayed or is rapidly decaying. In other words, conditions supporting the development of a tornado in the New Plymouth area were favourable only for a very short time.

Reflectivity image from the New Plymouth radar for 4:16am Sunday 19 June 2011. Colours represent how strongly precipitation bounces the radar signal back to the radar.

Colour key for the above image. The further to the right along the scale, the more strongly precipitation reflects the radar signal.

Severe Weather Forecaster John Crouch has extracted radar data from the bottom few “sweeps” of the radar beam. The mesocyclone was sufficiently close to the New Plymouth radar for these sweeps to provide a reaonable view of it. Here they are below, animated. Each time step is 33 seconds; the first sweep is at about 100 metres above the ground near downtown New Plymouth, while the last is about 1000 metres above the ground. So, we’re looking at successively higher layers of the mesocyclone as time goes forward; the reflectivity pattern can be seen wrapping around the circulation associated with the mesocyclone.

Radar reflectivity loop covering the (just over) three-minute period from 4:15am to 4:18am Sunday 19 June 2011. A hook shape can be seen moving from the northwest across New Plymouth.

It’s important to note that this doesn’t mean it’s possible to produce 33-second radar imagery routinely, or in “operational time”. It’s only when phenomena very close to the radar are moving and evolving sufficiently rapidly that data from subsequent sweeps of the radar beam might be coherent enough for animations of them to be meaningful.

Life cycle

Note, in the above animation, how quickly the hook-shaped pattern moves and changes. It suggests strongly that the whole tornado event was over in a couple of minutes, which appears to be consistent with reports in the media. In the animation, there’s evidence of only one hook-shaped pattern in the New Plymouth area: it may be that there was only one tornado, and it wasn’t always reaching down to the surface along its path.

Warm seas to the northwest of New Plymouth provided some of the “fuel” for the thundery showers which passed across Taranaki in the early hours of the morning of Sunday 19 June 2011. This is very likely why the tornado was short-lived: when the mesocyclone came ashore, its fuel supply was cut off.

A little bit about mesocyclones

In brief, a mesocyclone is a local rotation and ascent of air about a vertical axis.

Hook-shaped patterns in radar reflectivity imagery are not uncommon – but are nothing like conclusive evidence of the presence of tornadoes. There also needs to be strong, coincident, rotation: this is one of the reasons why weather radars measure the inbound / outbound speed of the echo using the Doppler Effect.

In the case of the tornado in New Plymouth early on the morning of Sunday 19 June 2011, there was a strong velocity couplet observable at 4:15am, but not either side of this time.

Further reading

For more on tornadoes in New Zealand, see the blog about the Albany tornado of Tuesday 3 May 2011.

Volcanic ash cloud in the New Zealand area


The Wellington Volcanic Ash Advisory Centre (VAAC), operated by MetService on behalf of the New Zealand Civil Aviation Authority, is one of nine VAACs that operate under the International Airways Volcano Watch. Wellington VAAC is supported through the collaborative effort of MetService, GNS, Airways New Zealand and aircraft operators. For more information about the overall system, see Bob McDavitt’s blog of 21 April 2010.

ICAO, the International Civil Aviation Organization, coordinates the International Airways Volcano Watch. The function of each of its nine centres is to respond to reports of volcanic ash within their region and provide forecasts to the aviation community of ash cloud extent and movement. Observations may come from ground stations and volcano observatories, aircraft in flight or orbiting satellites.

Volcanic Ash in the New Zealand area

To quote from a media release from the Civil Aviation Authority on Saturday 11 June, “The Cordón Caulle eruption began on 04 June 2011 with the initial ash plume reaching above 50,000ft. Volcanic ash particles come in a range of sizes and while the biggest will fall to the ground quickly, very small particles take a long time to settle out of the atmosphere. This eruption ejected these small particles very high into the atmosphere, where strong winds have carried them great distances to the east.”

Monitoring and Forecasting

MetService is monitoring, and forecasting the position of, the plume of very small ash particles from the Cordón Caulle eruption which are high in the Earth’s troposphere.

Monitoring: Satellite Imagery

The primary way of monitoring atmospheric volcanic ash is by using weather satellite imagery. To certain sensing instruments on weather satellites, volcanic ash in the atmosphere “appears” different from cloud. These differences in appearance can be used to make ash cloud more easily distinguishable from water and ice clouds, thereby making tracking possible.

Below is a visible satellite image for 11:00am Sunday 12 June. The volcanic ash cloud can be seen, but it’s not altogether clear where it begins and ends. And visible imagery is only available during local daytime.

MTSAT-1R visible image for 11:00am Sunday 12 June 2011. Data courtesy Japan Meteorological Agency.

Now here’s an image, for the same time, “tuned” for volcanic ash. The ash cloud stands out much more clearly: it’s shaded purple and blue. This image has been made from several others, exploiting the particular characteristics of the ash cloud. But even this “tuned” imagery does not provide conclusive evidence of the presence or absence of ash, because in low concentrations the satellite sensing instruments “see” right through it.

MTSAT-1R image for 11:00am Sunday 12 June 2011, highlighting the location of volcanic ash cloud. Data courtesy Japan Meteorological Agency.

And below is the infra-red satellite image for 11:00am Sunday 12 June. Forearmed with knowledge of the above volcanic ash image, it might be possible to spot the ash cloud. Without such knowledge, there is no way of determining, from this type of imagery, what is volcanic ash and what isn’t.

MTSAT-1R infra-red image for 11:00am Sunday 12 June 2011. Data courtesy Japan Meteorological Agency.

Pilot reports of volcanic ash are also very useful in the monitoring process. All the reports in the Zealand region over the last few days have been of the ash cloud from a safe distance below it: aircraft are avoiding the ash cloud itself.

We’ve also had access to LIDAR data from NIWA’s Lauder Atmospheric Research Station. This has proven most useful in determining the base and top of the ash cloud.

Forecasting: Plume and Trajectory Modelling

In the case of a volcanic eruption in the New Zealand region, MetService uses a local plume model to forecast future locations of the volcanic ash cloud.

However, because the eruption occurred in Chile, we’re employing other tools; for example, we have access to a global plume modelling system. But just as importantly, we’re using our knowledge of the ash cloud’s current location, the expected winds in the upper atmosphere, and some atmospheric trajectory modelling, to forecast where its “edges” will be at set times in the future. These forecasts are available from the links “text(VAA)” and “graphic (VAG)” on the page

Coordination with Australia

Winds in the upper troposphere have carried the volcanic ash plume from the Cordón Caulle eruption eastwards from Chile, across the southern Atlantic and Indian Oceans, then across the Great Australian Bight, and across New Zealand’s skies. In the Australian region, monitoring and forecasting it is the responsibility of the Darwin Volcanic Ash Advisory Centre operated by the Australian Bureau of Meteorology. MetService and the Australian Bureau of Meteorology are working closely together on determining the depth, height and horizontal extent of the plume.

What ash cloud looks like in the New Zealand sky

Near sunrise and sunset, the sky has a streaky look. The low sun angle illuminates the underside of the ash cloud quite nicely, as can be seen in the photo below.

Sunrise over Wellington Harbour, Wednesday 15 June 2011. Image copyright Ross Marsden, 2011.

When the sun is higher the ash cloud has a smoother and more subtle appearance, though it still looks streaky. And near the sun, the sky has a yellowish milky look.

Ash cloud in the Wellington sky, early afternoon of Wednesday 15 June 2011. The sun is left of the field of view. Most, if not all, of the "streakiness" in the sky is caused by ash cloud. Image copyright Ross Marsden, 2011.

Another satellite image

Below is a visible satellite image (click here for the image source) made from two “passes” of the MODIS “Aqua” satellite during the afternoon of Wednesday 15 June. Ash cloud can be seen: it is the light grey streaks from northwest Nelson across Wanganui, Manawatu, Hawkes Bay and Wairarapa.

Visible image created from five-minute polar-orbiting Aqua swath data at 02:00 UTC, 02:05 UTC, 03:40 UTC and 03:45 UTC 15-Jun-2011. Image courtesy NASA/GSFC, MODIS Rapid Response.

It is important to note that, in the above image, there is other ash cloud that can’t be seen. It is masked by the water and ice clouds that cover much of New Zealand.