Simpler is easier, and less likely to induce mistakes.
If a chap years ago looked up and estimated that the clouds were 600 feet above him, it would be easier to report a 600 foot cloud base.
If he had to add the elevation of the field, that would be math - introducing the possibility of an error.
If a chap years ago was about to land at an airport and the tower reported 600 foot cloud base, it would be easier to know if that was above the minimums.
If he had to subtract the field elevation, that would be math - introducing the possibility of an error.
Since the cloud base is really more important for landing than for flying, it makes sense to have the least amount of calculations be at the time of landing.
I don't see how you really care what the cloud bases are if your flying anyway. I mean, if you're IFR, you're flying at a safe altitude. And if you're VFR it's below you anyway (but that's really not a good idea to be over a cloud deck VFR). Or maybe you're thinking of scud running, but then that's not a good idea if the bases are that low, and you want to know how much room you have under the clouds to fly safely, so again, you want AGL.
Additionally, hearing that the bases are at 3500 MSL, might lead you to think that the cloud bases are somehow stable at that height. 600 AGL over one airport is often close to 600 AGL over another nearby airport even if that airport is a different elevation. Clouds often follow the terrain, and they also often slope, so MSL would kind of confuse the information.
Last edited by darkroomsource; 28th Jun 2012 at 15:39.