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WEATHER UNIVERSITY: Uncommon clouds over Amherst

Annette Belliveau looked up and spotted this intriguing cloud at 7 p.m. Sunday, in Amherst, N.S.
Annette Belliveau looked up and spotted this intriguing cloud at 7 p.m. Sunday, in Amherst, N.S. - Contributed

I don’t always check my Facebook messages on the weekend, but I did last Sunday evening. I expected to see a few photos of people enjoying the great weather… and I did. 

But I was also treated to photos of something a little less common: roll clouds!  Morgan Naylor and Annette Belliveau both posted photos taken over Amherst, N.S., Sept. 23. In both cases, beautiful examples of roll clouds!  

A roll cloud or solitary wave has a single crest and travels without change in speed or motion. They are often associated with cold fronts in the absence of thunderstorms.  On Sunday, a cold front pushed across Nova Scotia. In Amherst, where the photos were taken, it was 17 degrees by 2 p.m. and the wind was warm, from the southwest.  By 7 p.m. the upper wind behind the front had shifted and the colder, drier air was working its way to the ground.  The warm and moist air that sat at the surface was pushed up by the sinking cold air.  The moisture condensed, forming clouds.  The final ingredient was a westerly breeze a few hundred metres up that gave the cloud its unique shape.  Roll clouds often make their appearance in coastal areas, forming as a result of the circulation of sea winds. 

These roll clouds can last for many hours and extend far into the distance, depending on atmospheric conditions. 

Just after 7 p.m. Sunday evening, Megan Naylor noticed this solitary cloud over her house in Amherst, N.S.
Just after 7 p.m. Sunday evening, Megan Naylor noticed this solitary cloud over her house in Amherst, N.S.

Cindy Day is the chief meteorologist for SaltWire Network.

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