St. Swithin's Day
"Mares' tails and mackerel scales make lofty ships to carry low sails" ~ Traditional weather proverb
A "mackerel sky" also referred to as a "buttermilk sky" is a common term for a sky with rows of cirrocumulus or altocumulus clouds displaying an undulating, rippling pattern similar in appearance to fish scales. This distinctive pattern appears almost exclusively some way ahead of a warm front and is a reliable forecaster that the weather is about to change! St. Swithin's Day is traditionally a weather prediction day! Supposedly, if it rains on St. Swithin's Day, you should expect 40 more days of rain! May you get the rain or dry weather you prefer. 🌧️☀️
"St Swithun's day if thou dost rain
For forty days it will remain
St Swithun's day if thou be fair
For forty days 'twill rain nae mare"
There is thought to be a scientific basis to the weather pattern behind the legend of St. Swithun's day. Around the middle of July, the jet stream settles into a pattern which, in the majority of years, holds reasonably steady until the end of August. When the jet stream lies north of the British Isles then continental high pressure is able to move in; when it lies across or south of the British Isles, Arctic air and Atlantic weather systems predominate.
By designer Carol A.L. Martin, the Mackerel Sky tartan evokes the colors of a sunrise buttermilk sky.
A mackerel sky is a common term for a sky with rows of cirrocumulus or altocumulus clouds displaying an undulating, rippling pattern similar in appearance to fish scales; this is caused by high altitude atmospheric waves.
Cirrocumulus appears almost exclusively with cirrus some way ahead of a warm front and is a reliable forecaster that the weather is about to change.
The old rhyme:
"Mackerel sky, not twenty-four hours dry" and
"Mares' tails and mackerel scales make lofty ships to carry low sails"
both refer to this long-recognized phenomenon.
This cloud pattern is also sometimes known as a buttermilk sky, particularly the early cirrocumulus stage, in reference to the clouds' "curdled" appearance.
In France it is sometimes called a ciel moutonné (fleecy sky); in Spain a cielo empedrado (cobbled sky); and in Germany it is known as Schäfchenwolken (sheep clouds).
Click the picture below for more spectacular photography from Chip Phillips Photography including this mackerel sky reflected in Lake Trillium, Oregon, at sunrise.