Bob Gitzen, age 10, of St. Paul, Minn., for his question:

WHAT IS A MACH?

Mach numbers are used to describe the speed of planes flying near or above the speed of sound. A plane traveling at twice the speed of sound is flying at Mach 2.

Mach numbers are used because the speed of sound in the air is not always the same. The speed depends on altitude and the temperature of the air. At sea level and at 32 degrees Fahrenheit, sound travels at about 740 miles per hour. At 40,000 feet, sound normally travels at about 660 miles per hour.

The measure was named after Austrian physicist Ernst Mach ((February 18, 1838 – February 19, 1916) Mach 0.5 is half the speed of sound, or subsonic. Mach 9 is the speed of sound, or transonic. Mach 2 is twice the speed of sound, or supersonic.