What happens to the aircraft's altitude in cold weather when an altimeter is not adjusted?

When an altimeter is not adjusted in cold weather, the instrument tends to indicate an altitude that is lower than the actual altitude. This phenomenon occurs because the altimeter is calibrated based on the standard atmospheric conditions, which assume a specific temperature and pressure. Cold air is denser than warm air, and when the air temperature is lower than standard, the altimeter will not compensate for this increased density, leading it to register a lower altitude than the aircraft's true position.

In cold conditions, the air pressure at a given height is actually lower than it would be under standard temperature conditions. As a result, if a pilot flies into colder air without adjusting the altimeter, they may encounter a situation where the altimeter shows a lower altitude than they are actually flying. This can lead to hazardous situations, especially when flying in mountainous areas or during instrument flight rules (IFR) operations, as it could result in flying at a dangerous altitude without realizing it.