Temporal and spatial fluctuations in ground cover surface temperature at a Northern New England site

Ground cover surface temperature at a field site in South Royalton, Vermont, is calculated from 30-minute averages of upwelling longwave (3–50 μm) radiation measured with a pyrgeometer, resulting in 48 estimations of surface temperature each day. The ground covers are a seasonal sequence of (1) dormant grass prior to snowfall, (2) snow cover, (3) an irregular distribution of thatch, exposed soil and new-growth grass following final snowmelt and (4) lush, continuous grass cover. Diurnal variation in ground cover surface temperature and also seasonal differences in temperature spread and rate of temperature change are evident. Daily ranges (maximum-minimum) in ground cover surface temperature are 9–13°C (dormant grass), 1–12°C (midwinter snow cover), 22–25°C (late winter snow cover), 21–45°C (grass-thatch-soil), and 23–37°C (lush grass). The associated maximum rates of change of ground cover surface temperature are 5°C/h, 3°C/h, 10°C/h, 20°C/h and 10°C/h, respectively. ication of relative spatial uniformity of surface temperature for these ground covers is obtained by monitoring the surface with a second instrument, a passive infrared sensor system that responds to differential changes in thermal radiance from the ground cover. The snow cover is a thermally uniform background (on the scale of field of view of the passive infrared sensor system), and the grass-thatch-soil is thermally the most diverse, while the lush grass is thermally heterogeneous when sunlit grass blades blow in the wind. The use of such a passive infrared sensor system provides information on the variability of ground cover surface temperature, and by implication, on changes in radiant-energy loading and heat exchange processes, on a spatial scale larger than that of a standard ground-based pyrgeometer.