Mariner,
Thanks, I understand that altimeters read pressure. Pressure, air temperature and density are related though no?
That is the bit you are getting wrong Speedbird. An altimeter does not measure density altitude, it measures pressure. Pressure at a given altitude will be higher at a hotter temperature, not lower as you suggest.
Increased temperature results in lower pressure air which would result in a higher altimeter reading (hence takeoff performance etc is worse on hotter days - less dense air, lower pressure - if I am wrong here then sorry for wasting everyone's time). So if you are sat on the ground and the temperature rises by 25 degrees (for the sake of example) and you do not change your altimeter setting, your altimeter will give you a HIGHER reading i.e. your plane will be LOWER than your altimeter reading. Is this not correct?
Pontius - thanks. I understand that I am somewhat overthinking these and it would be more useful to think about them in a vacuum. I would just like to reconcile the two. From looking a little deeper, it would seem that the impact of temperature on rate of change is significantly greater than it is on altimeter reading at ground level.