The Science Behind the Weather: Is there really a calm before the storm?

Friday, October 17, 2014, 12:40 PM - We've all heard the phrase, very likely about things not having anything whatsoever to do with weather, but is there really such a thing as 'a calm before the storm'?

It can happen in the moments before a thunderstorm, or you can see it hours ahead of a powerful hurricane (such as with Hurricane Arthur, in the video above). That strange, almost eerie calm that settles over the area as the storm is approaching.

While it doesn't happen all the time, it's common enough that it's become a cliché for the quiet period before any kind of upheaval. While it's very likely just the period of psychological preparation for what's to come in the non-weather cases, there's some solid science behind why it happens with storms.

All storms - from airmass thunderstorms to supercells to immense tropical cyclones - work on the same principle. They get the majority of their energy from drawing warm, moist air up through their base, which rises up to the top of the clouds, cooling and condensing out the water vapour. Once that now cool and dry flow of air reaches the top of the storm, though, where does it go? For a tropical cyclone, some of that air is sucked down through the eye due to the extreme low pressure there, while for all storms much of that dry air flows out to the edges and then sinks towards the surface.

Whereas the rising air inside the storm clouds causes instability and strong gusts of wind, the sinking air around it (or inside the hurricane eye) has the opposite effect. As the air sinks it stifles cloud formation, which produces clear skies, it warms and absorbs water vapour, raising its humidity and storing away energy. While plenty of that stored energy will get used up as the air is drawn back into the storm, while it's still outside the storm (or in the eye) all of this combines together to calm everything down. The winds die to a mere breeze (if that) and everything goes quiet and still, as if nature is taking a 'time out' to prepare for what's coming.

To see the 'calm' in action, this webcam view, showing the Royal Naval Dockyard in Port Bermuda, will be the first to witness it, just before Hurricane Gonzalo unleashes its fury on the islands on Friday.