The point of cooling a camera is to reduce the noise from the CCD sensor, this increases as the temperature increases. The best operation is when it's cooler.
The CCD temperature needs to be the same for all the exposures, both all image frames and all the calibration frames - dark frames at least - so that the noise in all the frames has the same magnitude. That's the same absolute temperature.
The camera tries to do this by cooling to a fixed, controlled, temperature but there's a limit to what it can do, this is the delta T that Jared mentions.
If it's hot and your camera can't cool enough then set a higher set point temperature but you will also have to collect a new set of dark frames at this temperature. Personally I run my ATK8300L at -15C because I can reach that all year round in the UK.
Chris