Mach numbers are not the same thing as airspeed. Actually, there are a lot of different types of airspeed, but we'll stick with true and mach. True airspeed is how fast you are moving through the air. You're mach number is the true airspeed relative to the speed of sound. As Dwcace said, it depends on your altitude. At sea level, the speed of sound is 776 mph. As you go up in altitude, the air gets thinner, and the speed of sound drops. Something like 640 mph at 30000 ft. Therefore, if you are traveling at the same true airspeed, your moach will be higher at altitude then at the ground. It really doesn't matter whether your flying level or climbing or descending, the instantaneous mach will be the same, though it will tend to get lower as you descend, higher as you climb. Etc. Did you get that, or do I need to rephrase myself? :D