What happens to the readings of an uncalibrated altimeter during a climb?

During a climb, an uncalibrated altimeter may display a lower altitude than the actual altitude due to its reliance on atmospheric pressure changes. An altimeter measures altitude based on the ambient air pressure, and as an aircraft climbs, the atmospheric pressure decreases. An uncalibrated altimeter, which is not adjusted for current atmospheric conditions or the standard setting, may not account for this decrease in pressure correctly.

If the altimeter is set incorrectly or not calibrated to the proper pressure setting for the current weather conditions, it can under-read the altitude. This is particularly true if the altimeter is set to a pressure level that is higher than the actual pressure at the new altitude, which would lead to an indication of a lower altitude than what is truly being flown. Correct calibration and an understanding of the local altimeter setting (often referred to as QNH) are essential for ensuring precise altitude readings during flight.

Thus, if the altimeter is not properly calibrated, it can lead to inaccuracies as described, causing it to display a lower altitude than the actual altitude during a climb.