How does high pressure affect indicated altitude on an altimeter?

High pressure affects the indicated altitude on an altimeter by causing the indicated altitude to be lower than the actual altitude. Altimeters measure atmospheric pressure and translate that into an altitude reading. When a high-pressure system is present, the air pressure at the surface is greater than standard atmospheric pressure.

If the altimeter is not adjusted for the current pressure settings, it will "think" the aircraft is at a lower altitude than it actually is because it is calibrated based on standard pressure conditions. Thus, in high pressure conditions, the altimeter will read a lesser altitude than the true altitude of the aircraft above sea level, leading to a potentially dangerous situation if a pilot is relying solely on that instrument for navigation or flight safety.

It's crucial for pilots to constantly monitor and adjust altimeter settings to account for changes in barometric pressure to ensure safety and accuracy during flight operations.