Thanks to the cloud, websites and apps around the world can tap into vast, remote stores of data and computing power.

And thanks to the cloud, one good blow to one of those vast, remote storage centers can take down websites and apps around the world.

That’s what happened this past weekend. A ferocious lightning storm in Northern Virginia took down Netflix, Instagram, Pinterest, Heroku and more — not because any of those companies are based in Northern Virginia, but because they all apparently rely heavily on Amazon’s Elastic Compute Cloud facility there. Amazon said the storm, for reasons not immediately explained, took out both its main power supply and its backup generator.

The outage brought to mind a similar incident a year ago, in which an outage at the same Amazon facility felled Reddit, Quora and several other sites.

Does the ability of one local weather pattern to affect Web users around the globe point to a fundamental flaw in the cloud? Have we entered a world in which Internet users in Palo Alto, Johannesburg and Taipei must watch the weather report for Northern Virginia?

Not necessarily, but it does highlight the reality that the cloud is as much a physical system as it is a virtual one. And as with most physical systems, redundancy is essential to reliability.