Field Guide / Clouds & Weather Signs
Mammatus Clouds
Pouch-like lobes that hang from the underside of a large storm cloud. They show a strong storm nearby but are not a sign of a tornado.
- Height
- Special & Ground-Level
- Abbreviation
- mam
- Typical altitude
- Hanging from the base of a thunderstorm anvil, high in the sky
- Usually means
- Storms possible
Safety: There is no safe place outdoors when a thunderstorm is nearby. If you can hear thunder, go to a substantial building or a hard-topped vehicle with the windows up, and wait 30 minutes after the last thunder before you go out again. Tents, open fields, ridgelines, open water, and isolated trees are not safe places to be.
How to recognize it
- Rounded, bulging pouches hanging from the base of a cloud.
- Most often under the anvil of a cumulonimbus.
- Often glowing in golden light near sunset.
What it usually means
- A strong thunderstorm is nearby or has just passed.
- Mammatus clouds do not signal that a tornado is coming, despite the myth.
- They may appear after the worst of the storm has moved on.
What to do outdoors
- Do not assume the danger is over. Check radar and alerts.
- Wait for thunder to stop for 30 minutes before heading back out.
Often confused with
- Cumulonimbus: the storm cloud that produces them.
Field notes
Mammatus clouds form where cold, moist air sinks from the base of an anvil. They are striking but not dangerous by themselves.
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.