Passage IV

Scientists studied the speed of the wind and the size of sand grains at 4 sites on a sand dune (see diagram). At each site, 4 anemometers (devices for measuring wind speed) were attached to a vertical pole at different heights above the ground , and a sand trap was placed at the base of the pole. Each trap could collect only windblown sand grains having diameters from 0.0625 mm to 0.5 mm.

Study

Over a particular 45 min period, wind speed was mea­sured multiple times by each anemometer and sand was collected at all 4 sites

The average wind speed at each anemometer for the 45 min period was calculated . Then each average wind speed was converted to a relative wind speed. (The relative wind speed at an anemometer equaled the average wind speed at the anemometer divided by the average wind speed 12.0 m above the ground at the dune crest site for the same period.) The results are shown in Figure 1.

The mass of the sand collected at each site was mea­sured. Then the sand from each site was sifted through a stack of 12 screens to separate the grains into 13 portions according to diameter. (Each screen had uniform openings of a smaller diameter than the openings in the screen above.) The percent of the sand's mass from grains making up each of the 13 portions was determined (see Figure 2).