Is there any way to determine cloud tops other than PIREPS? Last week I put the first few hours of hard IFR into my logbook. During the first leg of the trip we were in IMC the whole time. I could catch glimpses of blue skies above every so often so we must have been skimming the tops. Sure I could have requested higher, but it would have been nice to know tops before I filed. There has to be a reason clouds stop forming at this magic altitude. Is there any guesstimate type formula, anything to get an idea?