Wind Rose

Wind speed determines the travel time of a pollutant from its source to a receptor and accounts for the amount of pollutant diffusion in the windward direction. Therefore, the concentration of pollutant at any receptor is inversely proportional to the wind speed. Wind direction determines in what direction a pollutant travels and what receptor is affected at a given time. Wind direction is normally defined by a wind rose, a graphic display of the distribution of wind direction at a location during a defined period. The characteristic patterns can be presented in either tabular or graphic forms.

Wind speed is usually measured by an anemometer, which consists of three or four hemispherical cups arranged around a vertical axis. The faster the rotation of the cups, the higher the speed of the wind. A wind vane indicates wind direction. Although wind is three-dimensional in its movement, generally only the horizontal component is denoted because the vertical component is much smaller.

Wind Circulation

Wind Circulation A. If the earth did not turn, air would circulate in a fixed pattern.

Polar Front

■ Polar Easterlies

Horse Latitudes

Doldrums

Wind Circulation A. If the earth did not turn, air would circulate in a fixed pattern.

Polar Front

Horse Latitudes

Doldrums

Prevailing Westerlies

Northeast Trades

Southeast Trades

Prevailing Westerlies

Polar Easterlies

B. The earth turns, creating variable wind patterns.

FIG. 5.6.2 Global wind patterns. (Reprinted, with permission, from the American Lung Association.)

Prevailing Westerlies

Northeast Trades

Southeast Trades

Prevailing Westerlies

Polar Easterlies

B. The earth turns, creating variable wind patterns.

FIG. 5.6.2 Global wind patterns. (Reprinted, with permission, from the American Lung Association.)

A wind rose is a set of wind statistics that describes the frequency, direction, force, and speed (see Figure 5.6.3). In this plot, the average wind direction is shown as one of the sixteen compass points, each separated by 22.5° measured from true north. The length of the bar for a direction indicates the percent of time the wind came from that direction. Since the direction is constantly changing, the time percentage for a compass point includes those times for wind direction at 11.25° on either side of the point. The percentage of time for a velocity is shown by the thickness of the direction bar. Figure 5.6.3 shows that the average wind direction from the southwest direction is 19% of the time and 7% of the time the southwesterly wind velocity is 16-30 mph.

Figure 5.6.4 shows the particulate fallout around an emission source and a wind rose based on the same time period.

The wind rose is imprecise in describing a point in a study region because the data are collected at one location in the region and not at each location. The data are often a seasonal or yearly average and therefore not accurate in describing any point in time in an ideal representation of atmospheric diffusion. A final limitation of wind rose is that the wind is only measured in the horizontal plane and is assumed identical at any height above the earth's surface. (Note: wind speed generally increases with height in lower levels due to the decrease of the frictional drag effect of the underlying ground surface features.)

FIG. 5.6.4 Wind rose and corresponding particulate fallout pattern.

FIG. 5.6.4 Wind rose and corresponding particulate fallout pattern.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment