Red sky at night…

An old weather saying; and a good one. There are many references to this on the Internet and I haven’t read any of them and I won’t link to any here. This will be yet another discussion of this old adage.

I have often heard the saying repeated, and I have said it myself on occasion. There seem to be several versions. This is the one I learned from my Mum when I was young, and it is the one I use now, and teach to my grand children:

Red sky at night, Shepherd’s delight.
Red sky in the morning, Sailor’s warning.

I’m not going to argue about shepherds and sailors; that’s not important here. The questions are: “Is it a useful saying? Does it work? If it works, why?”

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Red sky in the morning...

In the mid-latitudes weather systems migrate around the hemisphere from west to east. “Red sky in the morning…” is describing how, soon after dawn, the rising sun illuminates the cloud masses of the weather approaching from the west. The whole sky is ablaze with shades of red, orange and pink. It is not enough that a few clouds in the east are red. When this saying works, it is when the whole sky is involved. The red sky warns the sailor of the approaching weather – strong, possibly gale force winds, bad visibility in rain, and possibly thunderstorms. “Shorten sail! Set course for a sheltered cove!”

Red sky at night...

Red sky at night...

“Red sky at night…” refers to the setting sun lighting up the cloud of the weather system as it moves away eastwards. Again, a substantial part of the sky is in red and orange tones, not just the sky in the west. The shepherd can expect a peaceful night in the open, and not be too concerned about wind and rain.

But why red? We know why the daytime sky is blue. It is blue because short wavelength light (blue) is scattered by the gas molecules in the atmosphere more strongly than long wavelength light (red). So, the blue colour is from the short wavelength light that has been scattered from all over the illuminated atmosphere.

The sunlight at sunrise and sunset is red because it is shining through a much thicker slab of atmosphere, and all the shorter wavelength light has been scattered away leaving only the red long wavelengths.

An eggbeater southerly

Today’s weather map shows how this cold southerly is being produced by a combination of a HIGH or anticyclone in the Tasman Sea, and a LOW or depression between Canterbury and the Chatham Islands. For want of a better phrase, we could call this an eggbeater southerly.

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Using the isobars as a proxy for the air flow we can see how the air moving onto New Zealand is coming from around 55 degrees south— and that’s the Southern Ocean, but NOT from Antarctica. This technique doesn’t show the actual trajectory of the air, but is a good-enough first approximation, and it is one of the ways that enthusiastic weather watchers use weather maps to glean more about the weather.

As this cold air moves northwards and encounters warmer surfaces it becomes more buoyant, bubbly and bouncy. It picks up some moisture and turns this into clouds, but, because these clouds are so buoyant, they produce a scattering of showers containing rain, hail, sleet and snow, rather than a canopy of widespread snow.

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This southerly blast is serious enough to warrant a heavy snow warning for Southland and eastern Otago, and has already disrupted transport in the South and across Cook Strait.

The weather maps over the next few days show that this is just the beginning of several days of southerly and, IF THINGS EVOLVE THE WAY THEY ARE GOING, another Low may roll in from the east this weekend and give another, more compact, eggbeater southerly. See our
7-day weather maps.

So, when making plans for your weekend remember to check the latest MetService Severe Weather Outlook.

NOTE that this week’s southerly LACKS THE MOISTURE of a large scale snow storm such as the one that hit Canterbury in 2006— in that event a low pressure system conveyed some moist air aloft from the tropics to the South Island which was undercut by very cold air from the south, and brought a period of rapidly dropping pressure to Canterbury, accumulating a canopy of widespread snow.

For more about southerly outbreaks read our Learning Centre Article on the Old Man Southerly .

Bob McDavitt

Meteorological Aspects of a Recent Trip to the Tararuas

May 4th/5th 2009
I work as a public forecaster and my main tasks include writing regional, urban and mountain forecasts. I either work mornings (which start at 5:50AM and finish mid afternoon), or afternoons (which commence mid afternoon and run until 10:45PM).

A benefit of shiftwork is frequently having weekdays off, which earlier this month allowed me and Marine Forecaster Nicole Ranger to go tramping in the Tararua Forest Park. The state of the weather is an unavoidable facet of the outdoors, and so this first blog post will cover some of the meteorological aspects of this trip. Our destination was Mitre Flats Hut on the eastern side of the range, with an attempt on Mitre (highest peak in the Tararuas at 1571m) the following day.

A front was forecast to race up the South Island overnight and into the next day whilst forcing northwesterlies over the Tararuas. The good news was that this ought to clear up the the remaining cloud (from a dying southerly) on the eastern side of the range. The bad news was the threat of the winds being too strong on the exposed tops, and for Mitre itself to be clouded over.

Frost

The skies cleared on Monday night, resulting in a grass frost the next morning.

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Image 1: Grass frost (Mitre Flats)

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Image 2: Cumulus fragments showing wind direction aloft


Although it appeared to be a fine morning from the hut, sinister, scrappy cumulus fragments could be tracked coming off the main range as evidence that winds were now northwesterly aloft.

On cold nights, like the one we experienced, the surface layer of air “de-couples” from other layers of the atmosphere. This prevents stronger winds higher up from reaching the ground, and leads to the classic still, cold winter night. However, when the sun rises, the surface layer becomes destabilised, and winds can now mix down to the surface. This is just what happened on the frosty morning of May 5, and once the northwesterlies reached ground level the grass frost expired very quickly.

Wind

Heading up to Mitre, the wind was quite gusty in the bush, but it was much more so when we reached the exposed tops. The main peaks to our west had also clouded over in the northwest flow.

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Image 3: Northwesterly winds ahead of the front were forced to rise over the Tararuas, forming clouds, and hence keeping the main spine of the range "clagged-in".


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Image 4: Meanwhile, the Wairarapa is fine- thanks to the sheltering effect of the Tararua Range.


Unfortunately, we were not able to make it to Mitre this day.

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Image 5: At about 1200m, before turning back. Back row - Altocumulus; Middle row - Marine Forecaster Nicole Ranger, Public Forecaster Jon Tunster; Front row - Holly the Dog


Despite our retreat, this was not the end of the interesting meteorology. The hint is in Image 5, with the quite impressive altocumulus behind us, which is aligned perpendicular to the northwesterly wind. This suggests significant “downslope winds”, which bring strong winds from aloft down to the surface in the form of gusts. When we returned to the hut (at about 400m above sea level), the wind was very gusty. A sharp-edged altocumulus to our east matched the theory of downslope winds.

A few days later, at the end of a shift, I found an archived satellite picture from that time of the day (1PM, May 5th). The cloud signature was indicative of strong downslope winds in the lee of the Tararuas. Had I been on shift that day and seen such a satellite image, I would have expected this phenomena to be occurring- but being in the middle of it was much more fun…

Downslope Winds

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In the satellite picture, I’ve drawn the red arrow showing wind direction. Air in the northwesterly was forced up the hills, formed clouds and then descended on the lee side. This relatively dry, descending air was responsible for the strong wind gusts we experienced. Downstream, the air rises again, and forms the sharp edged line of altocumulus parallel to the Tararuas. It is this cloud formation which you can see in both the photograph from the ground and the satellite image from 36000km above. The clear area between the hills and the altocumulus is a variety of “foehn gap”- which is where I was standing when I took the photo. This cloud formation, seen either from the ground or on a satellite image, allows a good estimate to be made of where there may be especially strong and gusty downslope winds.

Jon Tunster, Public Forecaster May 17th 2009

Variable 10 knots

New Zealanders are well-travelled people by world standards. I’ve travelled overseas a few times and, as a meteorologist, have always taken an interest in the local sky conditions. It wasn’t until I visited the northern hemisphere that I understood why visitors to our country often remark on the brilliance, or “punch” the sunlight has here. Most of New Zealand is very lucky in this regard.

Several years ago, while on a trip to the UK, I noticed something of meteorological interest that was not in the sky. I was outside Westminster Abbey, one of the world’s greatest  landmarks, and the burial place of many famous people. Sir Isaac Newton, a man who had a profound influence on all branches of science (including atmospheric science) being one such esteemed individual. As I walked around the Abbey, I noticed a plaque that nicely describes what mariners might call “variable 10 knots”:

Westminster Abbey Church

Westminster Abbey Church, click image to enlarge.

“The wind goeth towards the south and turneth about unto the north – it whirleth about continually and the wind returneth again according to His circuits.” Ecclesiastes IV 6

This is a quote from one of the books of the Old Testament, and I believe it is the best description I have seen of the variable nature of the wind near the earth’s surface. It would seem that the variable characteristics of the wind have been observed and known for some time!

Westminster Abbey

Click image to enlarge.

If people many centuries ago (perhaps even Newton) could have experienced winds higher up in the sky, I think they would have been surprised at how different these winds are from winds near the Earth’s surface. Climbers who have ascended some of the higher peaks around the world have had direct experience of this difference. Nearer sea level, where most of us actually live, the wind is much more variable and unpredictable.

There are many reasons the wind behaves as it does, such as the rotation of the Earth, and the nature of the sun’s heating. In future blog posts I will write a little about the hows and whys of the wind around us.

Canterbury Snow, 10 May 2009

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,

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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.

Brrrrr…….

The arrival of colder southerlies over the past few days has made many of us move into our winter mode … turning up the home heating, putting an extra blanket on the bed, etc. It has certainly put an end to the golden weather that Waikato, Bay of Plenty, Gisborne and Hawke’s Bay have been experiencing.

brrr

Image from Neil Gordon: Winter mood near Featherston, Wairarapa, Fri 8 May 2009

My barometer here in Auckland is reading 988 hecto-Pascals and our latest weather map shows that much of the country has values below 1000. What has happened is that this low pressure centre has filled itself with air that has come from as far as 55 South….brrrr…. that’s cold air. At those latitudes it is a dry cold, but when the southerlies shovel this air towards New Zealand it picks up moisture and drops this as flurries of snow on places exposed to the south. As the cold air moves north across the Tasman Sea it encounters warmer temperatures and bubbles up, making bands of squally showers that affect places exposed to the south.

weathermap

Click image to expand

The large, complex and multi-centred low took several days to build to this extent and will take a few days to weaken. One of the reasons this large low is here is to do with the gap between Highs — there is a large High out at 140 West and another weaker High in the Australian Bight, and so New Zealand is caught between two Highs at present.
Today’s weather map shows that this link to the southern ocean is now weakening, and so this large area of cold air should now start to gradually warm up.
Bob McDavitt