Why are compact florescent lamps dim when they first start up?

Basically, the gas in the tube and the tube has to get warmed to the operating temperature so the pressure is at the design temperature. The gas is a very thin (almost a vacuum) plasma so it isn't really hot to the touch (warm) but it has a high specific temperature and with so much area that it can lose heat quickly. I have a compact fluorescent on my porch which stays dim down near freezing even enclosed in a dome and the long tube fluorescent bulbs in my unheated shop not only stay dim but flicker when it is really cold.

Some people complain about the initial brightness and that might be important when you have to plunge into a closet or down stairs just after turning one on, but using a higher rated bulb will still save electricity over incandescent, and I really like waking up in the dark and not having the full glare instantly.

The gas inside of them takes a while to warm up. Some CFLs don't take very long, if at all, to warm up and instantly light up.