The sci­ence be­hind stormy skies

Truro Daily News - - WEATHER - Cindy Day is Saltwire Net­work’s Chief Me­te­o­rol­o­gist.

I’d like to be­gin by thank­ing my many Face­book friends for tak­ing time to post such fab­u­lous pho­tos. If you don’t fol­low me on Face­book, you don’t know what you’re miss­ing. It’s a fan page, so just “like” one of my posts, and we’re friends.

Last week, I came across a se­ries of dra­matic pho­tos taken by Jordy Leighton. Jordy lives on Grand Manan Is­land, N.B. He was out just be­fore sun­set Wednesday and couldn’t help but reach for this phone and snap pho­tos of the omi­nous sky.

Jordy wants to know what was go­ing on…

Well Jordy, last Wednesday evening a cold front was mov­ing across the re­gion. As the colder air gushed down, it cre­ated a gust front that caught your at­ten­tion.

A gust front is the lead­ing edge of cool air rush­ing down and out from a thun­der­storm. The swirling ef­fect of cold air push­ing down and the warm air ris­ing above it can pro­duce an omi­nous-look­ing shelf cloud.

So what causes the air to flow out of some thun­der­storms so rapidly?

The pri­mary rea­son is the pres­ence of low hu­mid­ity in the lower at­mos­phere. This dry air causes some of the rain fall­ing through it to evap­o­rate which, in turn, cools the air. Since cool air is heav­ier than warm air, it sinks, and this causes a down­rush of air that spreads out at the ground. The edge of the rapidly spread­ing pool of cool air is the gust front.

If the wind fol­low­ing the gust front is in­tense and damaging, the wind­storm is known as a down­burst.

Thanks for be­ing cu­ri­ous Jordy; I hope this an­swers your ques­tion.

I was blessed with many great teach­ers over the years. I re­mem­ber my Grade 11 bi­ol­ogy teacher say­ing: “If you don’t ask, how will you know?”

I’ve never for­got­ten that.

Jordy Leighton watched as the sky grew dark and the bot­tom of the cloud came to life. He snapped this photo on Grand Manan Is­land, N.B., last Wednesday.

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