Wyoming Shields

Excerpted from "Rechard, Paul A., and Tsong C. Wei, Performance Assessments of Precipitation Gages for Snow Measurement, 1980." (WRRI Series #76)
This design was developed by Wyo WRRI from an idea to put snow fences around a gage site to reduce the effects of site eddies on the catch of a precipitation gage (Larson, 1971). Later, the fences were redesigned to reduce the size of the enclosed area around the gage. The primary configuration of the shield was determined by an airflow study conducted in the University of Wyoming aerospace laboratory wind tunnel (Larson, 1971). After several years of testing the shield in the field, it was improved to its present configuration. The dimensions and specifications of the Wyoming shield are given in Figures A-9 through A-11 and in Table A-1.

The shield consists of two snow fences. Twelve 8 ft, 6 in (2.59m) pipe posts are placed 10 ft (3.05m) from the center of the precipitation gage to form a polygon with 5 ft, 4.75 in (1.63m) sides. Each pipe post has an angle attached to its bottom for support and is pulled outward by a cable anchored securely in the ground, thus preventing the fence from being blown down by strong winds. Fastened to the top of each pipe post is a wood support 10 ft (3.05m) long that inclines inward to make a 30° angle for panels that also incline inward at a 30° angle. Each panel is made of lathes 4 ft (1.22m) long and 1-3/8 in (3.5 cm) wide that are equally spaced to cover 50 percent of the space and are connected to each other by wire at their tops and bottoms. The bottoms of the lathes are 5 ft (1.52m) above the ground.


Figure A-9 Wyoming shield configuration (Click to Enlarge)


Figure A-10 Wyoming shield configuration (Click to Enlarge)


Figure A-11 Wyoming shield configuration (Click to Enlarge)

TABLE A-l
MATERIALS AND ASSEMBLY FOR WYOMING SHIELD

Materials for Wyoming Shield

Note: Sizes of angles, pipes and other materials can vary depending on the availability of said materials.

Assembly Procedure for Wyoming Shield

  1. Establish radius point (see Fig. A-11, detail "C") lay-out outside ring.
  2. Weld outside ring and vertical pipe.
  3. Lay-out inside ring.
  4. Weld inside ring and vertical pipe.
  5. Cut 3/8" x 1" slot in top of pipe (see Fig. A-11, detail "B").
  6. Set pipe in rings.
  7. String 1/4" cable through 2" x 4" and pipe. Attach turn buckle to end of cable. DON'T tighten.
  8. Wire end of 2" x 4" to pipe.
  9. String 1/4" cable through other end of 2" x 4". Attach turn buckle to end of cable. DON'T tighten.
  10. Position 2" x 4" at a 45° angle from inside pipe.
  11. Attach #9 guy to bottom of 2" x 4" and inside pipe (see Fig. A-10).
  12. Guy 2" x 4" No. 1, 2, 3, 4 (see Fig. A-9) to radius point.
  13. Tighten turn buckles.
  14. Guy top of inside pipe to bottom outside ring.
  15. Repeat steps 5, 6 on outside ring.
  16. Drill 1/2" holes in both ends of 2" x 4" x 10'.
  17. String 1/4" cable through pipe and 2" x 4" x 10', attach turn buckle to end of cable. DON'T tighten.
  18. Attach 2" x 4" x 10' to bottom inside ring (see Fig. A-11, details "A" and "B").
  19. Guy outside ring (see Fig. A-9) but leave slack.
  20. Tighten top cable and guys, keeping pipes plumb.
  21. Attach snow fence.
  22. Note: In high wind area, it is advisable to guy bottom outside snow fence to outside pipe (see Fig. A-10).

The inner fence is an octagon measuring 4 ft, 2 in (1.24m) on a side whose pipe posts are placed only 5 ft (1.52m) away from the center of the precipitation gage. The fence is constructed in the same manner as the outer fence, but the vertical angle is 45° and the bottoms of the lathes are only 4 ft, 6 in (1.37 m) above the ground. In the center of the fenced enclosure, the precipitation gage is installed on a 4 ft, 6 in (1.37m) stand.

Figure A-12 Raised Wyoming shielded gage. (Note the use of fiberglass strip fencing.) (Click to Enlarge)

The fence descriptions and dimensions given above are for the shield known as the Raised Wyoming shield (see Figure A-12). This is a later refinement of the configuration known as the Low Wyoming shield (see Figure A-13). All the dimensions of the Low Wyoming shield are identical to those of the Raised Wyoming shield except that the bottom of the fence and instrument stand are only 2 ft and 1.5 ft (1.07 m) high, respectively. According to previous test data, the Low Wyoming shield is influenced by air turbulence near the ground surface (Rechard et al., 1973b). Thus, it was recommended that the entire setup be raised to the height described previously.

Figure A-13 Low Wyoming-shielded gage. (Note the use of wood laths. Also, note the Alter shield which was used in the first design tests of the Wyoming shield in 1971. This additional shielding was found to be of no significant value in earlier tests and was not used in the tests that form the basis of this report.) (Click to Enlarge)

In order to simplify construction of the fence, fiberglass strip fencing was used to replace the wood lathes at some of the sites. Instead of connecting the lathes vertically with wire, the fiberglass strips are horizontally arranged and directly fixed to the supporting post with the space covered remaining the same—50 percent. No effort was made to compare the effects of the two different materials and strip arrangements on the precipitation measurements. The fiberglass considerably reduces difficulties in construction and transportation of materials to the site and works especially well in mountainous terrain and severe climatic conditions.

Wyoming State Climatologist

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