Wind direction

Cooling Towers. The cooling tower location relative to the prevailing wind direction should be such that the wind hits the short side or the side perpendicular to the inlet louvers. This helps balance the air flow to the two inlet sides. 

Plant layout and noise suppression material are two general noise abatement methods. Plant layout does not affect noise levels at any given point however, noise can be abated by screening off a section of the plant. An example of this is to orient cooling towers with their closed faces toward the critical location. This method must also consider wind direction to balance air draft. Tankage can be located to act as a noise screen. 

Wind Direction and Speed Wind direc tion is measured at the height at which the pollutant is released, and the mean direction will indicate the direc tion of travel of the pollutants. In meteorology, it is conventional to consider the wind direction as the direction from which the wind blows therefore, a northwest wind will move pollutants to the southeast of the source. 

Provide wind direction indication visible from inside the building / control room... 

In addition to short-term emission estimates, normally for hourly periods, the meteorological data include hourly wind direction, wind speed, and Pasquill stability class. Although of secondary importance, the hourly data also include temperature (only important if buoyant plume rise needs to be calculated from any sources) and mixing height. 

Generalized Mesoscale Windflow Patterns Associated with Different Combinations of Wind Direction and Ridgeline Orientation... 

Wind direction relative to ridgeline Time of day Ridgeline orientation... 

Fig. 18-2. Surface chart for 06Z Friday, November 20, 1981. Contours are isobars of atmospheric pressure 12 is 1012 mb. Line with triangles, cold front line with semicircles, warm front line with both triangles and semicircles, an occluded front (a cold front that has caught up with a warm front). Wind direction is with the arrow wind speed is 10 knots for 1 barb, 5 knots for one-half barb. Small station circles indicate calm. H, center of high pressure L, center of low pressure.

In such emergencies, it is most important to know the local wind direchon at the accident site, so that the area that should be immediately evacuated can be determined. The next important factor is the wind speed, so that the travel hme to various areas can be determined, again primarily for evacuation purposes. Both of these can be estimated on-site by simple means such as watching the drift of cigarette smoke. It would be well to keep in mind that wind speeds are higher above ground and that wind direction is usually different. 

The initial direction of transport of pollutants from their source is determined by the wind direction at the source. Air pollutant concentrations from point sources are probably more sensitive to wind direction than any other parameter. If the wind is blowing directly toward a receptor (a location receiving transported pollutants), a shift in direction of as little as 5° (the approximate accuracy of a wind direction measurement) causes concentrations at the receptor to drop about 10% under unstable conditions, about 50% under neutral conditions, and about 90% under stable conditions. The direction of plume transport is very important in source impact assessment where there are sensitive receptors or two or more sources and in trying to assess the performance of a model through comparison of measured air quality with model estimates. 

The manifestation of turbulent eddies is gustiness and is displayed in the fluctuations seen on a continuous record of wind or temperature. Figure 19-3 displays wind direction traces during (a) mechanical and (b) thermal turbulence. Fluctuations due to mechanical turbulence tend to be quite regular that is, eddies of nearly constant size are generated. The eddies generated by thermal turbulence are both larger and more variable in size than those due to mechanical turbulence. 

Fig. 19-3. Examples of turbulence on wind direction records (a) mechanical, (b) thermal. Source From Smith (2).

This latter axis system is convenient in assessing the total concentration at a receptor from more than one source provided that the wind direction can be assumed to be the same over the area containing the receptor and the sources of interest. 

Where specialized fluctuation data are not available, estimates of horizontal spreading can be approximated from convential wind direction traces. A method suggested by Smith (2) and Singer and Smith (10) uses classificahon of the wind direction trace to determine the turbulence characteristics of the atmosphere, which are then used to infer the dispersion. Five turbulence classes are determined from inspection of the analog record of wind direction over a period of 1 h. These classes are defined in Table 19-1. The atmosphere is classified as A, B2, Bj, C, or D. At Brookhaven National Laboratory, where the system was devised, the most unstable category. A, occurs infrequently enough that insufficient information is available to estimate its dispersion parameters. For the other four classes, the equations, coefficients, and exponents for the dispersion parameters are given in Table 19-2, where the source to receptor distance x is in meters. 

Brookhaven Gustiness Classes (Based on Variations of Horizontal Wind Direction over 1 Hr at the Height of Release)... 

The vane can be used for both average wind direction and the fluctuation statistic determined over hourly intervals. The vane should have a distance constant of less than 5 m and a damping ratio greater than or equal to 0.4 to have a proper response. Relative accuracy should be 1° and absolute accuracy should be 5°. In order to estimate accurately, the direction should be sampled at intervals of 1-5 sec. This can best be accom-... 

Rather than using separate systems for horizontal and vertical wind measurements, a u-v-w anemometer system Fig. 19-9) sensing wind along three orthogonal axes, with proper processing to give average wind direction and (Tj, from the combination of the u and v components and w and o-, from the lO component may be used. 

A tracer experiment includes sampling on an arc at 1000 m from the source. If the horizontal spread is expected to result in a a- between 120 and 150 m at this distance, and if the wind direction is within 15° azimuth of that forecast, how many samplers should be deployed and what should be that spacing It is desirable to have above seven measurements within 2fTj, of the plume centerline and at least one sample on each side of the plume. 

Wind direction, tens of degrees a2imuth, two digits"... 

Numerous analyses of data routinely collected in the United States have been performed by the U.S. National Climatic Center, results of these analyses are available at reasonable cost. The joint frequency of Pasquill stability class, wind direction class (primarily to 16 compass points), and wind speed class (in six classes) has been determined for various periods of record for over 200 observation stations in the United States from either hourly or 3-hourly data. A computer program called STAR (STability ARray) estimates the Pasquill class from the elevation of the sun (approximated from the hour and time of year), wind speed, cloud cover, and ceiling height. STAR output for seasons and the entire period of record can be obtained from the Center. Table 21-2 is similar in format to the standard output. This table gives the frequencies for D stability, based on a total of 100 for all stabilities. 

The use of a measurement generally dictates the circumstances of data collection. For example, to provide a best estimate of plume transport direction, hour by hour, of a release from a 75-m stack, a wind vane at the 100-m level of a tower will probably provide more representative wind direction measurements than a vane at 10 m above ground. If the release has buoyancy so that it rises appreciably before leveling off, even the 100-m measurement may not be totally adequate. 

Data for one full year (1964) for Nashville, Tennessee, and Knoxville, Tennessee, 265 km (165 mi) apart, were compared to determine the extent to which the frequencies of various parameters were similar. Knoxville is located in an area with mountainous ridges oriented southwest-northeast Nashville is situated in a comparahvely flat area. The data available are the number of hours during which each of 36 wind directions (every 10° azimuth) occurred, the average wind speed for each direction, the number of hours of each Pasquill stability class for each direchon, and the mean annual wind speed. 

The maximum number of hours of each stability class in a single wind direction is given in Table 21-5. The total hours for A, C, and D stabilities are nearly the same. The maximum number of hours of B stability, with winds from a single direction, is about 50% higher at Knoxville. For all three stable cases, E, F, and (G), the maximum number of hours at Knoxville is about two- thirds that at Nashville. 

Fig. 21-10. Average concentrations of hydrocarbons and sulfur dioxide for each wind direction and wind direction frequency in two classes (0-7 mi hr and greater than 7 mi hr ), Philadelphia, 1963. Source U.S. Department of Health, Education and Welfare (8).

An example of frequencies of wind direction when the concentration exceeds a particular value is shown in Fig. 21-12. For this example, the concentration threshold is 0.1 ppm (262 fig m ). Although the maximum frequency from any one direction is only about 1%, this can be significant,. Munn (9) is careful to point out that "The diagram suggests but, of course, does not prove that a major source of SOj is situated between the sampling stations" (p. 109). 

Fig. 21-12. Frequency of wind direction when sulfur dioxide exceeds 0.1 ppm near Parkersburg, West Virginia, Source Munn (9).

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