Flash, bang… but why the rum­ble?

The Western Star - - WEATHER - Chief Me­te­o­rol­o­gist Cindy Day

I love a good storm. Grow­ing up, I was never afraid of thun­der and light­ning. I’m not sure how that hap­pened; both mom and Grandma were!

Ev­ery sum­mer, pow­er­ful storms rolled down the St. Lawrence val­ley re­gion. In school, there were al­ways a few kids who were fright­ened by the crash­ing of thun­der. I re­mem­ber Mrs. Flaro telling us there was noth­ing to fear… the Lord was bowl­ing.

The other day I re­ceived an email from a man who wanted to know why thun­der rum­bles.

I have to start by say­ing that thun­der doesn’t make a sound.

Thun­der is caused by light­ning, which is a stream of elec­trons flow­ing be­tween clouds or be­tween a cloud and the ground. The air sur­round­ing the elec­tron stream can get as hot as 27,000 de­grees Cel­sius. How hot is that? It’s three times hot­ter than the sur­face of the sun!

The sud­den heat­ing causes the air to ex­pand as the flash passes through the air and im­me­di­ate cool­ing con­tracts the air again. As the air cools, it pro­duces a res­onat­ing tube around the light­ning’s path.

Now to the rum­bling. Typ­i­cally, a light­ning bolt can be sev­eral kilo­me­tres long. Let’s say the near­est part of the bolt is 1 kilo­me­tre away. Sound trav­els about 1 km in 3 sec­onds so you will hear the first part – the clap of the light­ning – 3 sec­onds af­ter the flash. If the far­thest part of the bolt is 5 km away, it will take 15 sec­onds to hear that part of the bolt. From 3 sec­onds to 15 sec­onds af­ter the flash you will hear ev­ery dif­fer­ent clap in be­tween, re­sult­ing in a rum­bling sound. The end of the rum­ble is the far­thest part of the bolt.

Some­times un­der­stand­ing some­thing can make it a lit­tle less scary.

On a fi­nal note, did you know that nearly 1,800 thun­der­storms are hap­pen­ing at any mo­ment around the world? That’s 16 mil­lion a year!

Dar­rell Cole snapped this great photo of a roll cloud - de­tached from the main thun­der­storm cloud. It was taken last month over Amherst, N.S. Five to ten min­utes later, he saw sev­eral bolts of light­ning but no rain.

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