Ground temperature measurements at a deep borehole (several measurement positions down to 30 m depth) and four shallow boreholes (one measurement position at 0.1 m depth) in mountain permafrost at the Kitzsteinhorn (Hohe Tauern mountain range, Central Eastern Alps (Austria)). Temperatures inside the deep borehole were measured with Pt100 thermistors with an accuracy of ± 0.03°C between -50 °C and 400 °C (Platinum Resistance Temperature Detector L220, 1/10 B, Heraeus Sensor Technology®, Germany) twelve times per day and averaged to derive daily mean temperature values. Temperature inside each shallow borehole was measured with iButton® miniature temperature sensors (model DS1922L, Maxim Integrated Products®, USA) eight times per day from which daily mean values were calculated. According to the manufacturer the accuracy is ±0.5 °C within the temperature range -10 to 65 °C. Temperature data from the deep borehole were spline-interpolated (Δz = 10 cm) for better depth resolution between measurements. From the measured and interpolated depth- and time-dependent temperature data, the depth z (m) and time t (day) of the year in which the temperature T(z,t) > 0 °C were estimated and daily thaw intensities for estimating active layer thickness were derived from that. We would like to thank the Gletscherbahnen Kaprun AG (project “Open-Air-Lab Kitzsteinhorn”) and the Austrian ResearchPromotion Agency (FFG) (project “MOREXPERT”) for their support.

Ground temperature measurements at a deep borehole (several measurement positions down to 30 m depth) and four shallow boreholes (one measurement position at 0.1 m depth) in mountain permafrost at the Kitzsteinhorn (Hohe Tauern mountain range, Central Eastern Alps (Austria)). Temperatures inside the deep borehole were measured with Pt100 thermistors with an accuracy of ± 0.03°C between -50 °C and 400 °C (Platinum Resistance Temperature Detector L220, 1/10 B, Heraeus Sensor Technology®, Germany) twelve times per day and averaged to derive daily mean temperature values. Temperature inside each shallow borehole was measured with iButton® miniature temperature sensors (model DS1922L, Maxim Integrated Products®, USA) eight times per day from which daily mean values were calculated. According to the manufacturer the accuracy is ±0.5 °C within the temperature range -10 to 65 °C. Temperature data from the deep borehole were spline-interpolated (Δz = 10 cm) for better depth resolution between measurements. From the measured and interpolated depth- and time-dependent temperature data, the depth z (m) and time t (day) of the year in which the temperature T(z,t) > 0 °C were estimated and daily thaw intensities for estimating active layer thickness were derived from that. We would like to thank the Gletscherbahnen Kaprun AG (project “Open-Air-Lab Kitzsteinhorn”) and the Austrian ResearchPromotion Agency (FFG) (project “MOREXPERT”) for their support.