It’s a fine day. Isn’t it?

What do we mean when we say the weather is “fine”?

The word fine is often used to convey the positive attributes of something. It is synonymous with good, well, enjoyable.

How are you? I’m fine!
How was the movie? It was fine.
This is a fine bottle of wine.

When we write weather forecasts we define the term fine to mean that the sun casts sharp shadows. If cloud is thick enough to stop the sun from casting sharp shadows then, even if it doesn’t rain, we don’t think that’s a fine day.

However, New Zealand isn’t known as the “Land of the long white cloud” for nothing, and only infrequently is the sky completely cloud free for a whole day. Cloud often comes and goes. So, when writing forecasts, there are a number of questions to be answered when describing the state of the sky:

  • How much of the sky will be covered by cloud?
  • How thick will the cloud be?
  • How will the amount of cloud vary throughout the day?
  • Is there going to be more or less cloud than the previous/coming days?

Our perception of fine weather also varies with the seasons. In the summer months the sun is stronger and even if there is a lot of thin or high cloud it can still manage to cast sharp shadows. Also, if cloud does block the sun for short periods of time we are less likely to notice because the air is warmer; in fact, it might feel like a relief for a short time!  In the winter when the sun is weaker it may struggle to cast sharp shadows, and the day will feel cooler. If the sun is blocked by cloud, even for a short time, then it can affect the temperature more significantly and make you feel colder.

So, when it’s not a clear-cut blue sky day we consider all these things, as well as how the weather will make people feel. Will they feel it was a fine day? Or a cloudy day?

Auckland on a Fine Day. Photo by Joerg Mueller.

Mt Taranaki Kármán Vortex street

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.



Clouds: Their changing shapes often occur over a time-scale and space-scale that we humans can not always fully appreciate unless we use time-lapse photography.

In the international cloud naming scheme used to describe and identify clouds, there are ten basic characteristic cloud FORMS or TYPES or genera (nouns):  Cirrus, Cirrostratus, Cirrocumulus, Altostratus, Altocumulus, Nimbostratus, Stratocumulus, Stratus, Cumulus, and Cumulonimbus.

To further describe clouds there are several accepted and defined adjectives covering 14 cloud species (shape and structure), 9 varieties (arrangement and transparency),  9 supplementary features, and two words (genitus, mutatus) describing growth.   Click here for a table of these words.

There has recently been a call from the Cloud Appreciation Society of the United Kingdom to ask The Royal Meteorological Society to apply to the World Meteorological Organization to officially add a new variety or species of cloud to the international scheme.  The new word is “ASPERATUS”, after the Latin word for rough, and is intended to be used as an adjective to describe those clouds whose underbellies look like the surface of choppy sea.

This story was covered in the UK here and here and then in NZ by Radio NZ National on the Panel.


Photo credit to Bill Slater, taken near Hanmer Springs on 2 March 2005 and winner of the Met Society Photo competition, shows an example of what ASPERATUS implies.  Bill explains “It was a fine day and we first noticed some round disc like clouds at fairly high altitude. We commented that they were like flying saucers. Then as we reached Hanmer Springs we started to see these swirls and dangling clouds, looking back towards the Lewis Pass no rain ever fell.”

“Asperatus” clouds form when there are two (or more) layers of air of differing density, one sitting on the other.  The cooler and higher layer is cloudy and the other layer is clear.  The boundary between these layers may occasionally get knocked up, but will return downwards thanks to gravity and then may go further down but will return back up thanks to buoyancy.  This creates a wave-like surface along the cloud base, and we call these gravity waves because the returning force is gravity and buoyancy.

Yes, the waves on the surface of the sea are a good example of this process.

Another good example is when moist air blows over a range of mountains and makes a system of mountain wave clouds.  In New Zealand this often happens, and people in Canterbury call the mountain wave clouds  “the northwest arch”.

At first individual Altocumulus lenticularis clouds form,  but as a front approaches, upper-level moisture increases and middle and high clouds combine to produce an arch cloud comprising Altocumulus, Altostratus, and Cirrostratus.  This arch cloud displays a very sharp edge near the mountains and often there is an arch of clear sky immediately downstream of the mountain divide.

We can cope with the current naming scheme and use Altocumulus lenticularis to describe the NW arch clouds, but it would also be useful to have the extra variety or species word “ASPERATUS” especially when there are undulations in the cloud base.


The MetService cloud poster already has a special photo devoted to the NW arch cloud.  At present it is just classified as “Northwest Arch    Ac/As/Cs”, but if the word “ASPERATUS” is officially accepted then we are ready and waiting.

If you are coming to the National Fieldays at Mystery Creek 10-13 June then pass by the MetService display in the main pavilion and ask for your own complimentary full-sized cloud poster.

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?”


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.

Regional Cloudscapes

Clouds. When I’m away from home in various parts of the country I am always interested in the cloudscapes I find there. The thing is that they are often typical of that region and different from other regions. This may seem obvious and trivial, however it is interesting to consider the influences that contribute to the cloudscape and how they work in different atmospheric conditions. I live in the relatively hilly and mountainous region of Wellington, and the important cloud forming processes are related to the arrangement of the hills and the higher mountains, and the all-important (for Wellington) wind direction. The terrain of other regions influences the cloudscapes of those regions, and that is how the differences arise.

There is a very useful booklet called “Cloud Forms” by my ex-colleague, Ray Smith. This is now out of print, but a few copies are still about. In it he says that there are basically three types of cloud; heap, layer and streak. These are the result of different cloud formation processes. From another approach, although there is an infinite range of cloud shapes and sizes, any cloud observer can identify these three types. The classical cloud naming system was proposed by the London pharmacist Luke Howard in 1803, and many people are familiar with the basic cloud names; “cumulus” forms for heap types, “stratus” forms for the layer types, and “cirrus” forms for the streak types. With these simple categories and some elementary meteorological knowledge, you can observe the clouds and work out what is going on in the atmosphere around you. There are also low, middle and high cloud categories, and the cloud forms that appear in those altitude ranges depend on the meteorological conditions that occur there.

To interpret the cloud forms you see, the basic meteorological elements of knowledge you need are that air cools as it rises, and that if it cools enough that the temperature of the air reaches its dew point, the water vapour will condense and become visible – a cloud. It is interesting to realise that with perfectly clean air (only the constituent gases and no aerosols) the temperature needs to decrease well below the dew point before condensation will occur. In the real atmosphere there are plenty of condensation nuclei among the aerosols that are naturally present, so condensation readily occurs when saturation is reached; that is when the air temperature is the same as the dew point and the Relative Humidity is near 100%.

In Blogs that follow I will write a bit about the cloudscapes of some regions of New Zealand, as well as some special types of cloud.