What effect does a warm day have on a pressure altimeter?

On a warm day, the air density decreases due to the increase in temperature. This reduction in density occurs because warm air is less dense than cool air. Pressure altimeters are designed to measure pressure, which is influenced by the weight of the air above a certain altitude. Under warmer conditions, the actual pressure at a given altitude is lower than what is typically expected for standard temperatures. As a result, the pressure altimeter will interpret this lower pressure as indicating a higher altitude than the actual altitude. Consequently, it will show a lower altitude than the true altitude you are flying at. Thus, a warm day causes the pressure altimeter to read lower than the actual altitude, leading to potential misjudgment of the aircraft's height above terrain or obstacles.