Yet another cloud caper: a roll cloud

One of my recent weather columns featured a solo, but spectacular, lenticular cloud photographed from Dingwall and Cape North, N.S.

The next day, Kay Boucher shared a few spectacular cloud photos that she took at Cape Jack Beach in Antigonish County. She couldn't believe her eyes when she spotted this unique cloud. She says it was amazing and wondered if it might be a shelf cloud.

You're close, Kay. The cloud you saw is a roll cloud. Roll clouds and shelf clouds are the two main types of arcus clouds.

Roll clouds are rare but by no means unheard of; they can form on the leading edge of cold fronts with just the right amount of humidity present. They tend to occur when there's not enough moisture to trigger rain or thunderstorms but just enough water vapour to condense into an opaque, oblong cloud.

Roll clouds are triggered by a downdraft that causes moist warm air to rise, cool below its dew point and form a cloud.

When this happens uniformly along an extended front, a roll cloud may form. They are not, and do not, produce tornadoes.

I checked the hourly weather records for the Havre Boucher area that day; there was no frontal passage so was this indeed a roll cloud? Yes, roll clouds can also develop along coastal regions as a result of a sea breeze.

Sea breeze circulations occur because the sun heats the land and sea surfaces at different rates, creating an onshore flow during the day and offshore flow at night. Sea breeze roll clouds tend to develop when humidity values are high and when two air masses of different temperatures clash.

Kay was at the right place at the right time. These roll clouds are so rare that you could go your whole life without ever seeing one. Thanks for sharing this special moment with all of us, Kay.

WEATHER BY DAY

DID YOU KNOW:

While roll clouds can occur in Germany, Canada, South Africa, Brazil, Uruguay and even Florida, they are regionally known as "Morning Glory" clouds along the north Australian coast. The clouds get their name from their early morning appearance; these phenomenal clouds can be up to 800 km in length!

Cindy Day is the chief meteorologist for Saltwire Network.

CONTRIBUTED You might know Cape Jack Beach in Antigonish County, but you may not recognize it looking like this! Recently, a rare roll cloud stopped Kay Boucher in her tracks. She said it was very impressive and extended much farther to the right, well beyond the edge of her great photo.