I saw some cloud pouches hanging down from another cloud. What are they?

Those are mammatus clouds, which are often, though not always, associated with thunderstorms. Mammatus often extend from the bottom of the anvil cloud of a thunderstorm, also called a cumulonimbus cloud, and indicate an intense storm is near-by. Mammatus clouds may have a very ominous appearance; however, they are usually seen after the worst weather has passed.

The widely accepted cloud classification system used today is based on the scheme introduced by Luke Howard in 1803. This cloud naming convention uses Latin roots to describe the appearance of different cloud types. The Latin ‘mamma’ means udder or breast, and refers to the clouds’ characteristic udder-shaped protuberances.

Mammatus occur frequently in the Midwest during summer, and are less frequent in the West. We do not fully understand why they form and so there are a few hypotheses for their occurrence. Unlike most clouds, which form in rising air, mammatus are associated with sinking air in the upper parts of a thunderstorm. We also know that they require rapid changes in the temperature, moisture and wind between the bottom of the anvil and the cloud free air below. This rapid change in wind is a hazard to aviation and so pilots avoid mammatus, and thunderstorms in general. We sometimes see mammatus clouds hanging from contrails forming behind jet aircraft.