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Altocumulus – a real show-stopper

It’s the show pony of the skies – and if you keep an eye out over the next week as various fronts and low-pressure systems approach New Zealand, you’re bound to see it.

A sheet of altocumulus undulatus ahead of a front.

Altocumulus is a pretty common sight above us. What makes the clouds so interesting is the range of forms they take – from the well-known “mackerel sky”, to the tufts and turrets of the castellanus form, warning of thunderstorms to come, to the weirdly UFO-like lenticular dishes which fly high over eastern parts in northwesterly winds.

A classic “mackerel sky”.

These middle-level clouds generally form ahead of arriving frontal systems. They can be as low down as 8000 feet – when it becomes very difficult to tell them from high stratocumulus – or can reach up to about 20,000 feet and merge with delicate cirrocumulus clouds.

In the undulatus, or “mackerel” variety, generally the smaller the blob the higher the cloud.

The setting sun highlights altocumulus bumps underneath the Canterbury nor’west arch.

The nor’west arch of Canterbury and Otago is composed of high altocumulus which reflects the shape of the winds screaming across the Southern Alps.

Flying through altocumulus can often be a choppy experience, as they mark areas of turbulence. If it looks like cobblestones, then it will probably feel like you’re driving across them.

The plethora of shapes, lumps and heights in sheets of altocumulus also guarantee the most vivid sunrises and sunsets.

In a country where we are blessed with amazing dawns and dusks, these show ponies really know how to put on something memorable.

By Guest Weather Analyst Paul Gorman –


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