Final answer:
If ambient pressure is lower than standard, the true altitude is higher than indicated altitude because an altimeter assumes standard pressure at sea level. When the ambient pressure is lower, the altimeter reads an altitude that is less than the actual altitude.
Step-by-step explanation:
If ambient pressure is lower than standard, the true altitude is higher than indicated altitude. This is because an altimeter is calibrated to atmospheric pressure at sea level under standard conditions (1013.25 hPa or 29.92 inHg). When the ambient pressure is lower than the pressure the altimeter is set to (the standard pressure), it gives a reading that is lower than the actual altitude. One way to illustrate this is by using a hot-air balloon scenario. Assuming the pressure of the gas in the hot-air balloon is equal to the atmospheric pressure outside the balloon, if that atmospheric pressure decreases, the altitude recorded by instrumentation that assumes standard pressure would be less than the true altitude.
As for negative pressure, this term generally refers to pressure that is lower than atmospheric pressure, while positive pressure is greater than atmospheric pressure. And as the atmospheric pressure decreases with altitude, so does the ambient pressure around the balloon or aircraft, leading to a true altitude that is higher than what's indicated.