What's that cloud? Something fishy here...

Charles Turner captured this distinct ‘mackerel sky’ in Surrey over the weekend, but what actually is a mackerel sky, and what does it mean?

A mackerel sky, is the name given to a sky filled with rows of cirrocumulus or altocumulus clouds and has achieved its name due to its similar appearance to fish scales.

Clouds are classified by their height and appearance; ‘cumulus’ refers to their appearance, puffy and rounded, whilst the prefix refers to their height.

‘Cirro-' refers to the high, wispy clouds and as an exact translation means ‘curl of hair’. These clouds have bases above around 6000m, in sub-zero temperatures.

Alternatively, ‘alto-' means mid-level, and so these clouds form at slightly lower altitudes than the ‘cirro-' variety.

Altocumulus are much more commonly observed, tend to appear larger in size given their lower altitude, and can be made up of grey tones, rather than just white.

More unusual to spot, are cirrocumulus, as seen in Charles’ photograph in Surrey.

Of course, there are then further classifications in terms of their features, with these clouds labelled cirrcocumulus stratiformis, for their horizontal, layer-like form.

They do also appear to have a wavelike pattern, with these clouds known as cirrocumulus undulatus.

Whilst not always the case with old folklore, many phrases of weather lore for mackerel skies do actually hold some truth, such as ‘mackerel sky, not twenty-four hours dry’ and ‘mackerel sky, mackerel sky, never long wet and never long dry’.

Mackerel skies are usually indicative that the weather is about to change. This is because these high-level clouds are influenced by shifting upper wind directions, typical of an advancing warm front or low pressure system.

As the clouds begin to pick up moisture, you’ll notice them thickening too.

On Sunday, when Charles sent in this image, you may have noticed it was a mainly bright and sunny day. Yet today, it has clouded over with outbreaks of rain pushing in, owing to the arrival of weather fronts sweeping eastwards across the British Isles.

Check out the other clouds explained so far in the 'What's that cloud?' series here.