Noticed a pretty sharp rise in your data usage since upgrading to iOS 9?

I did.

After a bit of investigation and some Apple news-reading, I found a likely culprit.

With iOS 9, Apple brought a horde of sweet new features to the iPhone.

One such feature is labelled Wi-Fi Assist, and you can find it in Settings > Mobile Data. It's hidden away at the very bottom of the screen.

Wi-Fi Assist on iOS 9

Wi-Fi assist is a pretty cool concept, but for those of us on plans with very little data (I'm working my way down from 2GB to 1GB a month to save some coin), it can be costly.

As you can imagine, it automatically switches over to your cellular data connection when Wi-Fi connectivity is shit poor.

There are some caveats, as Apple's own support page points out: doesn't switch during roaming, thank goodness; doesn't work for background downloads; doesn't support data-heavy use like emails, streaming, etc.

Still, if you think it's screwing you out of some data, disable it by sliding over to white in the Settings app.

Sing your song in the comments :-)