Thanks for the prompt answer Mark. I knew I was missing something. I think I now know how to do it properly.

Why do I use global variables? I use them to store some global information like wind direction which is calculated in an expression based on a helper object's (an arrow) rotation. My particle system and fields uses then this information extensively. I got this idea from the Secrets Of The Pros book. It's true I could use custom attributes directly but I prefer it this way (at least for this kind of implementation). I don't know if it's any faster but at least it keeps the channel boxes clean