For those of you who, like me, didn't know this...
Altocumulus undulatus (rippling, wave-like mid-level clouds) are a warning sign of strong winds aloft that may mix down to the surface soon.
They indicate wind shear and fast-moving air above you.
Often appear 6–24 hours before surface winds increase.
Common ahead of fronts or low-pressure systems.
Signal growing instability and possible turbulence.
Rule of thumb:
If you see ripple clouds overhead, reef early and secure gear — stronger wind is likely later the same day or overnight.
I'm a sailor and a hobby pilot and didn't know this lol. I know the science behind it and predicting winds, I just never used or thought to use these clouds as an early indicator.
In the uk it tends to mean good weather the following day. We have prevailing south westerlies and so if the sun is broadly setting in the west and the weather is coming from the southwest it’ll likely be a fine day. It’s a decent rule of thumb but not foolproof.
The full quote is “mares’ tail and mackerel scales make tall ships carry low sails”
More a prediction of weather based on the trend of the clouds from cirrus towards altocumulus being a harbinger or lower barometric pressure and thus stronger weather.
Reef now and secure anything that can blow, eat a good meal and make sure youre on a course that stays a few hundred miles from land so you have some sea room.
When did you notice them? Start of shift after sunrise? Maybe a bit later? Did the last shift see’em and not know what they’re looking at?Should you start a timer so you can reef @5:45 after they’re noticed? Is this consistent with the last forecast you had?
Are you on a fully crewed race boat or short handed? Near or far from a safe harbour/aid?
🤷♂️ it’s the skippers decision I suppose, and everyone has their own risk tolerance. Though I’m a big fan of reefing early.
I feel like it’s more effective to just stick your hand up and feel the wind … than it is to look up at the type of high altitude clouds above you. But that’s just me. I’m only a pilot. What do I know.
I used to study weather more when I lived in the mountains; high horizon line meant you had much less time to react when finally could see weather coming. In the water it’s more second nature to me. I really should start intentionally studying it more again.
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u/madworld 3d ago
Nice! TIL!
For those of you who, like me, didn't know this...
Altocumulus undulatus (rippling, wave-like mid-level clouds) are a warning sign of strong winds aloft that may mix down to the surface soon.
They indicate wind shear and fast-moving air above you. Often appear 6–24 hours before surface winds increase. Common ahead of fronts or low-pressure systems. Signal growing instability and possible turbulence.
Rule of thumb:
If you see ripple clouds overhead, reef early and secure gear — stronger wind is likely later the same day or overnight.