treasure valley road dust study: final report - ResearchGate

A.1 Wind speed
The cumulative frequency distribution of wind speed is shown in Figure B-3. Winds in
the 4-8 mph range were most common, occurring 56% of the time. Calms occurred about 8% of
the hours. Sixteen percent exceeded 10 mph and 5% exceeded 14 mph. The maximum recorded
wind speed for the period was 30 mph.
Cumulative frequency (%)
100
90
80
70
60
50
40
30
20
10
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Wind Speed (mph)
Figure B-3. Cumulative frequency distribution of wind speed (mph).
Average wind speed by month is shown in Figure B-4. Average wind speeds are highest
in the spring (March-May) and lowest in December and January.
Average wind speed by time of day (averaged over all months) is shown in Figure B-5.
Wind speeds are low from about 9 pm through 8 am. They increase steadily from 8 am to 1 pm
and then more slowly to 4 pm. From 5 to 9 pm winds subside steadily. At night, surface-based
inversions cause light winds by decoupling the surface air from winds aloft. This results in
stable atmospheric conditions overnight. As the mixing layer deepens during the day, higher
momentum air is mixed down from aloft, increasing the surface wind speed. Most of the growth
in the mixed layer depth and increase in wind speed occurs by early afternoon. Later in the
afternoon, the mixed layer depth and surface wind speed increase more slowly. Wind speed
decreases later in the day as the inversion sets in. Days are shorter in winter; therefore,
compared to the average shown in Figure B-5 wind speeds decrease earlier in the evening and
remain calm later into the morning.
B-5