The Weather and Folklore of Altocumulus Clouds

altocumulus sky sunset
An altocumulus sky. John B. Meuller Photography/Getty Images

An altocumulus cloud is a middle-level cloud that lives between 6,500 to 20,00 feet above ground and is made of water. It's name comes from the Latin Altus meaning "high" + Cumulus meaning "heaped."

More: How are clouds named?

(Ever wondered why the Latin in their name translates to "high" but altocumulus are classified as mid-level clouds? In this case, the "alto-" tells you that they are high forming liquid-based clouds.)  

Altocumulus clouds are of the stratocumuliform cloud family (physical form) and are one of the 10 basic cloud types.

There are four species of cloud underneath the altocumulus genus:

  • altocumulus lenticularis (stationary lens-shaped clouds that are often mistaken for UFOs)
  • altocumulus castellanus (altocumulus with tower-like sproutings that billow upwards)
  • altocumulus stratiformis (altocumulus in sheets or relatively flat patches)
  • altocumulus floccus (altocumulus with scattered tufts and fringy lower parts)

The abbreviation for altocumulus clouds is (Ac).

Cotton Balls in the Sky

Altocumulus are commonly seen on warm spring and summer mornings. They're some of the simplest clouds to identify, especially since they look like balls of cotton stuck into the blue background of the sky. They're often white or gray in color and are arranged in patches of wavy, rounded masses or rolls.

Altocumulus clouds are often called "sheepback" or "mackerel sky" because they resemble the wool of sheep and scales of mackerel fish.

Bellwethers of Bad Weather

Altocumulus clouds that appear on a clear humid morning can indicate the development of thunderstorms later in the day. That's because altocumulus clouds often precede cold fronts of low pressure systems. As such, they also sometimes signal the onset of cooler temperatures.

While they are not clouds from which precipitation falls, their presence signals convection and instability at mid levels of the troposphere.

Altocumulus in Weather Folklore

  • Mackerel sky, mackerel sky. Never long wet and never long dry. (Or as another version goes: Mackerel sky, not twenty-four hours dry.)
  • Mackerel scales and mares' tails make lofty ships carry low sails.

If you're a fan of weather folkore, you've likely heard the above sayings, both of which are true.

The first piece of lore warns that if altocumulus clouds are seen and air pressure begins to fall, the weather won't be dry for much longer because it may start raining within 6 hours' time. But once the rain does come, it won't be wet for long because as the warm front passes, so too will the precipitation.

The second rhyme warns ships to lower and take in their sails for the same reason -- a storm may be approaching soon and the sails should be lowered to protect them from the accompanying high winds. (The "mares' tails" in the rhyme above are wispy cirrus clouds. Like altocumulus, they also arrive in advance of frontal systems and indicate the arrival of precipitation and deteriorating weather conditions.)

Edited by Tiffany Means

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Oblack, Rachelle. "The Weather and Folklore of Altocumulus Clouds." ThoughtCo, Mar. 28, 2016, thoughtco.com/altocumulus-cloud-overview-3444135. Oblack, Rachelle. (2016, March 28). The Weather and Folklore of Altocumulus Clouds. Retrieved from https://www.thoughtco.com/altocumulus-cloud-overview-3444135 Oblack, Rachelle. "The Weather and Folklore of Altocumulus Clouds." ThoughtCo. https://www.thoughtco.com/altocumulus-cloud-overview-3444135 (accessed November 22, 2017).