Yesterday’s blog was about an intriguing and relatively new cloud type that was spotted in our region Friday - the undulatus asperatus. I wouldn’t normally blog about clouds 2 days in a row, but I have to make an exception today.

I’ve been getting so many photos and comments regarding the ominous clouds that billowed over Hants and Colchester counties yesterday morning that I felt like I really should address it.

The cloud was a shelf cloud.  A shelf cloud is a low-hanging, well-defined, sometimes wedge-shaped formation that occurs along the leading edge of an intense line of thunderstorm. Shelf clouds most often form in a gust front – a rush of air from the top of the cloud.  Cool, sinking air from a storm cloud's downdraft spreads out across the land surface, with the leading edge called a gust front. This outflow cuts under warm air being drawn into the storm's updraft.  As the lower cooler air lifts the warm moist air, its water condenses, creating a cloud which often rolls with the different winds above and below.  You don’t normally find persistent rotation on a vertical axis with this cloud.

Shelf clouds can be very scary-looking since they are usually low-hanging; they can sit only a couple hundred meters above the ground. There are two other phenomena that might resemble tornadoes or funnel clouds but are not:  dark rain shafts and the white color of a hail shaft that could create a light-dark contrast with surrounding rain, resulting in what might appear to be a funnel cloud or a tornado to the untrained eye.

Remember, the presence of a shelf cloud is a sign of some powerful thunderstorms in the area; take shelter and stay safe if you happen to spot one!


Chief Meteorologist

Cindy Day