Fluorescent Lights and CFLs Take Longer to Startup in Cold
When turned off, the mercury in the fluorescent tube condenses on the inside surface of the tube.  In order to emit light, the mercury must evaporate and form a vapor as the conductive path between the ends of the tube.  The colder the bulb is to begin with, the longer it takes for all the mercury to evaporate, and for the light to reach full brightness.

This also explains why fluorescent lights get dimmer as they age: there is a small leakage of mercury from the tube during its lifetime, so there is less mercury vapor in the tube, so less light produced.