Field Guide / Clouds & Weather Signs
Altocumulus
sheep's back
Patchy rows or rolls of white and gray puffs at mid-level. Often harmless, but on a warm, humid morning they can mean thunderstorms later in the day.
- Height
- Mid-Level Clouds
- Abbreviation
- Ac
- Typical altitude
- About 6,500–20,000 ft (2,000–6,000 m)
- Usually means
- Weather changing
How to recognize it
- White or gray rounded masses, in rows, patches, or layers, with shaded undersides.
- Each cloudlet is about one to three fingers wide at arm's length.
- Gaps of blue sky between the rolls.
What it usually means
- Usually fair weather, and common on many days.
- On a warm, humid morning, altocumulus with turret-like tops or ragged tufts can be an early sign of afternoon thunderstorms.
- A sky of altocumulus that thickens and lowers through the day can mean a front is coming.
What to do outdoors
- On a warm, humid morning with altocumulus, plan to be off exposed ground by early afternoon and keep an eye on the sky.
- Check the forecast for storms.
Often confused with
- Cirrocumulus: smaller cloudlets and no shading.
- Stratocumulus: lower and with larger, heavier rolls.
Field notes
Altocumulus is common and often means nothing at all. The cases that matter are on warm, sticky mornings, when it can be the first sign of unstable air.
Learn to tell it from the small cloudlets of cirrocumulus and the bigger rolls of stratocumulus.
Related guides
Cloud signs are rough guides, not forecasts. Always check the National Weather Service forecast at weather.gov before and during a trip, carry a way to receive alerts, and treat any watch or warning as a reason to change your plans. If you can hear thunder, you are close enough to be struck by lightning.