What effect does temperature have on altimeter readings?

In cold air the altimeter will over read. In warm air the altimeter will under read. Remember that standard temperature is 15 degrees Celsius.

From hot to cold, don’t be bold. When flying from an area of warm air to an area of cold air be careful as your altimeter will likely be over reading. Your altimeter will be telling you that you are at a higher altitude than you actually are.