250 225 Year 2 200 175 150 125 100 75 50 Year 1 25 22 25 28 31 3 6 9 12 15 18 21 24 27 30 July August Date Visibility rating

Figure shows summer air visibility measurements for Denver, Colorado. The acceptable visibility standard is 100, with readings above 100 indicating clean air and good visibility, and readings below 100 indicating temperature inversions caused by forest fires, volcanic eruptions, or collisions with comets.
a. Is a trend evident in the data? Which time-series techniques might be appropriate for estimating the average of these data?
b. A medical center for asthma and respiratory diseases located in Denver has great demand for its services when air quality is poor. If you were in charge of developing a short-term (say, 3-day) forecast of visibility, which causal factor(s) would you analyze? In other words, which external factors hold the potential to significantly affect visibility in the short term?
c. Tourism, an important factor in Denver’s economy, is affected by the city’s image. Air quality, as measured by visibility, affects the city’s image. If you were responsible for development of tourism, which causal factor(s) would you analyze to forecast visibility for the medium term (say, the next two summers)?