How does temperature affect the altimeter?

Colder than standard temperatures can cause the altimeter to read higher than the true altitude, meaning the aircraft is actually lower than indicated. Conversely, warmer temperatures can cause the altimeter to read lower than the true altitude, indicating the aircraft is higher than it actually is. This occurs because colder air is denser, causing the vertical pressure levels to contract and be spaced more closely together, resulting in the altimeter indicating a higher altitude for the same pressure level.