Though they may be associated with severe thunderstorms, mammatus clouds, like the ones in this popular Youtube video, are not necessarily an indication of severe weather.
Mammatus clouds form when air sinks, unlike most clouds which form due to rising air. This results in the pouches of condensation that give these clouds their bumpy appearance.
The turbulent atmosphere that creates these "upside down" clouds can be conducive to thunderstorms. Generally, a severe thunderstorm is eminent if mammatus clouds form on the underside of a cumulonimbus cloud. If they form beneath an anvil cloud, however, they are typically not threatening.
Asselin said that the video was taken around 8 p.m. on July 22, and AccuWeather.com Expert Senior Meterologist Paul Walker said that coincides with a cold front that moved through the area. The high for that day around Iron Mountain, Mich., was 85. The day was also marked with high humidity. When the cold front moved through, it pushed down from atop the warm air and created the pockets in the clouds.
There were some thunderstorms in the area, but the temperatures evaporated much of the precipitation, leaving no measurable rainfall.