Sometimes the temperature does not decrease with height in the troposphere, but increases. Such a situation is known as a temperature inversion. Temperature inversions limit or prevent the vertical mixing of air.
Middle range machines can cost on average $400 and are often multi-function with copier and laser printer capabilities. Finally, high-end machines which offer high speeds, high capacity and high resolution come at a higher price-expect to pay $1000 - $2500 for such a fax machine.
Between 1973 and 1983, the number of fax machines in the United States increased from 30,000 to 300,000, but by 1989 the number had jumped to four million. By the late 1980s, compact fax machines had revolutionized everyday communications around the world.