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SUMMARY. Replicated temperature data from passively heated high tunnels are lacking, especially in the southwestern United States. Field studies were conducted over three seasons in two locations in New Mexico--a southern site in Las Cruces and a northern site in Alcalde--to characterize the crop environment in three hightunnel designs during the winter growing season (October-March). High tunnels were 16 · 32 ft and oriented with the long edge running east to west. Heavyweight woven plastic covered the single-layer (SL) high-tunnel design. Double-layer designs (DL) were covered with a lightweight woven plastic on the bottom, followed by a second layer of the heavyweight plastic inflated with a fan. A heat sink was created using 16 55-gal barrels painted black, filled with water, and aligned along the north side of the double layer for the DLDB design. Soil temperature (3 inches deep) and air temperature (1 ft above the soil surface) were recorded inside the high tunnel, inside the high tunnel under a floating rowcover, and outside the high tunnel. In addition, photosynthetically active radiation (PAR) was recorded inside and outside the high tunnels during or near the winter solstice each year of the study. Daily air and soil temperature minimums were highest in the DLDB design and lowest in the SL design. Maximum air and soil temperatures did not significantly differ between high-tunnel designs, although the DLDB design measurements were consistently lower. During season 1, the SL design had significantly higher PAR transmission than the other two designs. In the northern location, the difference became insignificant during seasons 2 and 3, likely due to dust accumulation and plastic aging. In the southern location, the SL design maintained higher PAR transmission throughout the study, possibly due to plastic cleaning. Data collected in this study can help inform the decisions of high-tunnel growers and researchers in the region. [ABSTRACT FROM AUTHOR]