Section   20.2Air Masses

1. (Paired Exercises 3 and 4) How long does it take an average cold front to move from the West Coast of the United States to the East Coast?

A cold front travels at a speed of about 500 mi/d, and because it is about 3000 mi across the United States, we can find the time of travel by dividing the distance by the speed.

t = d / v = 3000 mi / 500 mi/d = 6 d

2. (Paired exercises 5 and 6) A bright flash of lightning is followed by a clap of thunder that occurs 7.0 s after you observe the flash. How far away, in mi and km, is the storm center that produced the lightning? Sound travels through air at about 0.20 mi/s, so the distance to the storm center can be calculated by multiplying this speed by the time delay observed in hearing the thunder.

d = vt = (0.20 mi/s)(7.0 s) = 1.4 mi

The storm is thus 1.4 mi away, or if we want to express this distance in kilometers:

d = (1.4 mi)(1.6 km / 1.0 mi) = 2.2 km

Return to Previous Page