R1Tamer,
The atmosphere is not vertically constrained, and will expand and contract in accordance with temperature variation. The International Standard Atmosphere (ISA) assigns altitudes for the various pressure levels assuming standatd ISA temperatures throughout. The Altimeter is calibrated for this.
On a day hotter than ISA, the vertical distance between pressure levels increases, and conversely, on a day colder than ISA, the vertical distance between pressure levels decreases. These pressure levels we read as Indicated Altitude. The degree of expansion or contraction is directly proportional to the ratio of the Mean Temperature of the column of air beneath the aircraft to the ISA Mean Temperature of the column of air (All temperatures are, of course, in degrees Absolute / Kelvin).
An example for 10,000 feet Indicated Altitude. On an ISA day, 10,000 feet Indicated is indeed 10,000 feet of True Altitude. On an ISA-15°C day, 10,000 feet indicated is only 9,461 feet of True Altitude - UNSAFE!. On an ISA+15°C day, 10,000 feet indicated is 10,539 feet of True Altitude - SAFE!
Regards,
Old Smokey