A stunning and rare sight occurred Monday evening as the sun set and storms rolled in. These pouch-like clouds are called mammatus clouds, and are not commonly seen in the Mitten State.
Several factors came together to get clouds like this. What we had on Monday was very dry air around the base of the clouds and below the clouds. Showers and thunderstorms were rolling across Lake Michigan at the time these pictures were taken. The best, most well-defined mammatus are usually found in the presence of anvil clouds, or they extend from the base of cumulonimbus clouds. They very commonly precede arriving strong to severe storms.
The dry air allowed for the ice in the showers/storms to sublimate as it fell into the drier air. Sublimation is when ice changes phases directly from solid to gas. The air above West Michigan was very cold for this time of the year, meaning the clouds were likely made up of a significant amount of ice.
The water vapor added by the sublimation of the ice caused the base of the cloud became very cold and dense. The cold, dense air was colder than the air around it, meaning it began to sink and form these pocket looking clouds. It actually seems counter-intuitive. Cold, dense air that is sinking creating pouches or pockets, while warmer, lighter air is rising through and into the cloud structure. Mammatus clouds are typically associated with a very turbulent environment and deeply sheared...meaning winds coming in from different directions at various speeds.
The dry air below the clouds also allowed for the strong winds that came with these showers and storms. As the rain fell and evaporated in the dryness, the air became denser and rushed out of the clouds, leading to the strong gusts.