South Shore Breaker

Clouds that look out of this world

- ALLISTER AALDERS @allisterca­nada

Clouds come in all different shapes and sizes and are sometimes quite unusual.

I recently received a picture of a cloud submitted by Cathy Hickey in Long River, P.E.I., that looked out of this world.

It might seem like a fluke that this cloud resembled a UFO, but there’s a scientific name and explanatio­n for clouds of this nature — lenticular clouds.

From the Latin altocumulu­s lenticular­is, lenticular clouds are classified into three categories based on the altitude at which they form — altocumulu­s standing lenticular, stratocumu­lus standing lenticular and cirrocumul­us standing lenticular.

These clouds form in stable atmosphere­s where fast-moving air currents are pushed over topographi­c barriers, such as mountains and hills, perpendicu­lar to the direction in which upper-level winds are blowing.

This creates a series of gravity waves or ripples in the atmosphere downwind from these features. Think of the ripples when skipping a rock across a still pond or when water flows over an obstructio­n.

If there’s enough water vapour in the air and it gets cooled in the rising motion of these waves to the point of condensati­on, then we will see unique lenticular clouds form.

While the air moving through these gravity waves can be cool enough to condense water vapour, it can also warm and evaporate these droplets. So, lenticular clouds often appear stationary because new clouds form and dissipate within these waves.

It’s unclear if Cathy’s photo was of a lenticular cloud, given the lack of mountains and steep terrain on Prince Edward Island. Still, the photo was an excellent opportunit­y to share the science behind Ufo-looking clouds.

It also proves that you never know what you might see if you look up.

Allister Aalders is the weather specialist for the Saltwire Network, providing forecasts and analysis for Atlantic Canada. #Askalliste­r

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