Section 1. Preparation and Siting

H

10H

Height of tree (T)

10T

FIGURE 1.3-1. Effect of Structure on Wind Flow

1.4 Determining True North for Wind Vane Orientation

Magnetic declination, or other methods to find True North, should be determined prior to installing the weather station. True North is usually found by reading a magnetic compass and applying the correction for magnetic declination*; where magnetic declination is the number of degrees between True North and Magnetic North. Magnetic declination for a specific site can be obtained from a USFA map, local airport, or through an internet service called NSSDC CGM (Section 1.4.1). A general map showing magnetic declination for the contiguous United States is shown in Figure 1.4-1.

Declination angles east of True North are considered negative, and are subtracted from 0 degrees to get True North as shown Figure 1.4-2. Declination angles west of True North are con-sidered positive, and are added to 0 degrees to get True North as shown in Figure 1.4-3. For example, the declination for Logan, Utah is 14.5° East. True North is 360° - 14.5° , or 345.5° as read on a compass.

*Other methods employ observations using the North Star or the sun, and

are discussed in the Quality Assurance Handbook for Air Pollution Measurement Systems, Volume IV - Meteorological Measurements4.

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