Mach number is the speed of __________ for altitude.

The Mach number is defined as the ratio of an object's speed to the speed of sound in the surrounding medium. This means that it is a dimensionless number that indicates how fast an object is moving compared to the speed of sound, which is influenced by factors like temperature and altitude.

At sea level, the speed of sound is approximately 343 meters per second, but it decreases with altitude due to lower temperatures. Therefore, when discussing the Mach number in the context of altitude, it is specifically the speed of sound that serves as the benchmark for comparison. When an aircraft reaches a Mach number of 1, it is traveling at the speed of sound, while speeds greater than 1 indicate supersonic travel, and speeds less than 1 indicate subsonic travel.

For the other options, while they relate to speed in different contexts, they do not serve as the reference point for calculating the Mach number.