But radiosondes and balloons come under an exemption, so can be flown pretty much anywhere. I think the exemption allows up to 2kg payloads. Radiosondes (the new Vaisala ones) are around 120g, and it's pretty cool technology.
if you're going to launch any kind of something >400', you should do the responsible thing and contact ATC. if they say you're fine and no further contact, then great. they'll ask the size of your balloon, the size of the payload, the expected altitude, expected flight duration, and if you have any radio beacons they can listen to, and similar question. they didn't even ask me my purpose. it's a really simple phone call to make. the last thing you want to see happen is some sort of incident involving your balloon.