Waterhorse

Adding to Sigwx’s excellent answer

Hot temperatures also have an effect on the aircraft’s altitude - this works both in cold weather and hot temperature. Air density is, as Sigwx detailed, dependent upon temperature. In cold weather a normal altimeter will under-read as they are all calibrated for ISA sea level conditions and the actual air is colder and therefore more dense. In reality unless it is very cold (we start to get interested at ISA -15) we can largely ignore this, but when it is cold there is a correction added to our Decision Alt. So for a standard Cat 1 ILS approach a normal DA would be 200ft for a sea level airfield. At ISA -15 we would add about 20feet to the DA and use 220feet to take into account the lower density altitude.

The opposite effect in the summer can be quite noticeable - so on the approach into Alicante in the summer with a temp of 35C or ISA +20 we would not lower the DA, but a glide slope check alt at 5500ft could easily show as 5900ft on the altimeter. With any approach where there is a physical glide slope beam like an ILS this is not really important as you can predict it and make an allowance - the actual slope of the approach does not change. Should the approach be one without a physical beam rather a glide slope predicated on distance (such as a VOR or ADF, or some of the RNAV type approaches) then that glide slope becomes steeper, and hence a little more tricky to fly.

Interestingly the A350 can fly an approach based on Satellite derived position AND height, which makes the temperature issue on the approach far less important and it is hoped that this type of approach will soon be certified for Low Vis Ops, just like the ILS with its physical beams.