as defined in the WFCM and ASCE7-10 is as follows: "Urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of singe family dwellings or larger."
as defined in the WFCM and ASCE7-10 is as follows: "Open terrain with scattered obstructions including surface undulations or other irregularities having height generally less than 30 feet extending more than 1500 feet from the building site in any quadrant. Exposure C extends into adjacent Exposure B type terrain in the downwind direction for the distance of 1500 feet or 10 times the height of the building or structure, whichever is greater. This category includes open country and grasslands, and open water exposure for less than 1 mile."
Exposure D as defined in the WFCM is as follows: "Flat unobstructed areas exposed to wind flowing over open water for a distance of a least 1 mile. This exposure shall apply only to those buildings and other structures exposed to the wind coming from over the water. Exposure D extends inland from the shoreline a distance of 1500 feet or 10 times the height of the building or structure, whichever is greater." The ASCE7-10 definition is similar.
Exposure D is outside the scope of Chapter 3 of the WFCM. It would require either design using WFCM Chapter 2 or ASCE 7. Conversion factors for Exposure D are available in WFCM Table 188.8.131.52 and their applicability is noted in Chapter 2 Table footnotes.