Stop 3g Data From Turning on While WIFI is on

[flyinonice4]

I have my phone setup so that my wifi is on even when I turn the screen off. However, after an hour or so the 3g data will turn on as well. Not only does this decrease my battery life by even more, I am never sure if I am using the 3g data or wifi when getting online. Obviously wifi will be faster so I would prefer to manually turn on 3g if I need it rather than it coming on randomly.

Can anyone explain to me why my 3g data is turning on and how to stop it from happening? I never get far enough from my home network to trigger the 3g data to start and I currently don't have email being pushed to my phone. The only scheduled data services that I know of would be updating the weather on my phone. Other than that, I have no idea what might cause it.